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Introduction

We've talked recently about the roadmaps for high refresh rate displays and panels in the monitor market, spanning across various panel technologies and screen sizes. One interesting sector was in the TN Film market where high refresh rate screens of 120Hz and 144Hz have been available for many years, originally being the only choice in the LCD monitor market for high frame rate gaming. While other panel technologies like IPS and VA have more recently ventured in to the 120Hz/144Hz refresh rate market themselves, TN Film is again pushing the boundaries this year with the arrival of native 240Hz refresh rate support. This doubles the potential frame rate support from the original 120Hz panels from many years ago, going back to 2009 in fact. If you refer to the previously mentioned roadmap article you will see that it is AU Optronics who, as a panel manufacturer, are pushing the refresh rate for this new generation of TN Film panel, with options being produced in 24.5" and 27" sizes. The first to go in to production was the 24.5" panel variants which have quickly been adopted by some of the main gaming display manufacturers. Acer, Asus, BenQ and AOC have all got equivalent 24.5" models (often referenced as 25" in size for ease) coming out to compete in the ever-popular gamers sector.

We have with us now the first European sample of AOC's version of this screen, their AGON AG251FZ display. This is part of their AGON branded gaming line-up of screens and this 24.5" model offers some impressive gaming features including the aforementioned 240Hz native refresh rate, along with a 1ms G2G response time and AMD FreeSync support. The other models from Acer, Asus and BenQ are all NVIDIA G-sync compatible so at the time of writing this is the only FreeSync option using this new panel.

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Specifications and Features

The following table gives detailed information about the specs of the screen:

The AG251FZ offers a very good range of connectivity options with DisplayPort 1.2a, 2x HDMI (one with MHL), 1x Dual-link DVI and 1x VGA connections offered. DisplayPort or HDMI are needed to support the refresh rate up to 240Hz, including FreeSync support from compatible AMD graphics cards via DP (48 - 240Hz range). The digital interfaces are HDCP certified for encrypted content and the video cables are provided in the box for all four video connections.

The screen has an external power supply and comes packaged with the power cable and power brick you need. There are also 4x USB 3.0 ports available with 2 located on the back of the screen with the video connections, and 2 on the right hand side of the screen for easy access - with one have fast charging capabilities as well. A headphone jack and mic input are also provided on the right hand side of the screen, while the back of the screen also has an audio output and an alternative mic input. There are integrated 2x 3W stereo speakers on this model as well.

Below is a summary of the features and connections of the screen:

OSD Menu

Above: OSD control buttons on the bottom right hand edge of the screen. Click for larger version

Power Consumption

In terms of power consumption the manufacturer lists typical 'on' usage as TBC but do at least list 0.5W usage in standby. We carried out our normal tests to establish its power consumption ourselves.

We tested this ourselves and found that out of the box the screen used 24.4W at the default 90% brightness setting. Once calibrated the screen reached 17.2W consumption, and in standby it used only 1.4W. We have plotted these results below compared with other screens we have tested. The consumption is comparable to the other screens in this 24 - 25" size range we have tested as you might expect (comparing the calibrated states).

Panel and Backlighting

Panel Part and Colour Depth

The screen features an AU Optronics M250HTN01.0 TN Film technology panel which is capable of producing 16.7 million colours. This is achieved through a 6-bit colour depth with additional Frame Rate Control (FRC) added. The panel part is confirmed when dismantling the screen as shown below:

Screen Coating

The screen coating is a medium anti-glare (AG) offering. It isn't a semi-glossy coating, and isn't as light as some modern IPS type panels either. It's in keeping with other TN Film panels we've tested. Thankfully it isn't a heavily grainy coating like some old IPS panels feature, although there is some graininess noticeable. It retains its anti-glare properties to avoid too many unwanted reflections of a full glossy coating, but does not produce an too grainy or dirty an image that some thicker AG coatings can. There were some slight cross-hatching patterns visible on the coating as well but only if you looked very closely.

Backlight Type and Colour Gamut

The screen uses a White-LED (W-LED) backlight unit which is standard in today's market. This helps reduce power consumption compared with older CCFL backlight units and brings about some environmental benefits as well. The W-LED unit offers a standard colour gamut which is approximately equal to the sRGB colour space. Anyone wanting to work with wider colour spaces would need to consider wide gamut CCFL screens or the newer range of GB-r-LED type (and similar) displays available now. If you want to read more about colour spaces and gamut then please have a read of our detailed article.

Backlight Dimming and Flicker

We tested the screen to establish the methods used to control backlight dimming. Our in depth article talks in more details about a common method used for this which is called Pulse Width Modulation (PWM). This in itself gives cause for concern to some users who have experienced eye strain, headaches and other symptoms as a result of the flickering backlight caused by this technology. We use a photosensor + oscilloscope system to measure backlight dimming control with a high level of accuracy and ease. These tests allow us to establish

1) Whether PWM is being used to control the backlight
2) The frequency and other characteristics at which this operates, if it is used
3) Whether a flicker may be introduced or potentially noticeable at certain settings

If PWM is used for backlight dimming, the higher the frequency, the less likely you are to see artefacts and flicker. The duty cycle (the time for which the backlight is on) is also important and the shorter the duty cycle, the more potential there is that you may see flicker. The other factor which can influence flicker is the amplitude of the PWM, measuring the difference in brightness output between the 'on' and 'off' states. Please remember that not every user would notice a flicker from a backlight using PWM, but it is something to be wary of. It is also a hard thing to quantify as it is very subjective when talking about whether a user may or may not experience the side effects.

100%                                                  50%                                                  0%
 
Above scale = 1 horizontal grid = 5ms

At 100% brightness a constant voltage is applied to the backlight. As you reduce the brightness setting to dim the backlight a Direct Current (DC) method is used, as opposed to any form of PWM. This applies to all brightness settings from 100% down to 0%. The screen is flicker free as a result which is great news.

Contrast Stability and Brightness

We conducted these tests in the default settings. The brightness control gave us a very good range of adjustment. At the top end the maximum luminance reached 366 cd/m² which was a little shy of the specified maximum brightness of 400 cd/m² from the manufacturer but still more than adequate we're sure. There was a good 307 cd/m² adjustment range in total, and so at the minimum setting you could reach down to a low luminance of 58 cd/m². This should be adequate for those wanting to work in darkened room conditions with low ambient light. A setting of 23 in the OSD menu should return you a luminance of around 120 cd/m² at default settings in this preset mode. It should be noted that the brightness regulation is controlled without the need for Pulse Width Modulation, using a Direct Current (DC) method for all brightness settings between 100 and 0% and so the screen is flicker free.

We have plotted the luminance trend on the graph above. The screen behaves as it should in this regard, with a reduction in the luminance output of the screen controlled by the reduction in the OSD brightness setting. This is not a linear relationship as adjustments between 100 and 90 control a steeper luminance range, but this becomes linear from 90 to 0.

The average contrast ratio of the screen was good for a TN Film panel at 938:1. This was pretty stable across the brightness adjustment range as shown above.

Testing Methodology

An important thing to consider for most users is how a screen will perform out of the box and with some basic manual adjustments. Since most users won't have access to hardware colorimeter tools, it is important to understand how the screen is going to perform in terms of colour accuracy for the average user.

We restored our graphics card to default settings and disabled any previously active ICC profiles and gamma corrections. The screen was tested at default factory settings using our new X-rite i1 Pro 2 Spectrophotometer combined with LaCie's Blue Eye Pro software suite. An X-rite i1 Display Pro colorimeter was also used to verify the black point and contrast ratio since the i1 Pro 2 spectrophotometer is less reliable at the darker end.

Targets for these tests are as follows:

• CIE Diagram - validates the colour space covered by the monitors backlighting in a 2D view, with the black triangle representing the displays gamut, and other reference colour spaces shown for comparison

• Gamma - we aim for 2.2 which is the default for computer monitors

• Colour temperature / white point - we aim for 6500k which is the temperature of daylight

• Luminance - we aim for 120 cd/m², which is the recommended luminance for LCD monitors in normal lighting conditions

• Black depth - we aim for as low as possible to maximise shadow detail and to offer us the best contrast ratio

• Contrast ratio - we aim for as high as possible. Any dynamic contrast ratio controls are turned off here if present

• dE average / maximum - as low as possible. If DeltaE >3, the color displayed is significantly different from the theoretical one, meaning that the difference will be perceptible to the viewer. If DeltaE <2, LaCie considers the calibration a success; there remains a slight difference, but it is barely undetectable. If DeltaE < 1, the color fidelity is excellent.

Default Performance and Setup

Default settings of the screen were as follows:

AOC AGON AG251FZ- Default Settings, Gamma mode 1

   

Initially out of the box the screen was set in the default gamma mode 1 and with the game preset modes turned off. With a very high default 90% brightness setting out of the box the screen was overly bright and uncomfortable to use, so you will definitely need to turn that down. You could tell the screen was using a standard gamut backlight as well with the naked eye, and the colour balance and temperature felt pretty good, although the image did appeared quite washed out, as if gamma was a long way off the usual 2.2 target.

We went ahead and measured the default state with the i1 Pro 2. The CIE diagram on the left of the image confirms that the monitors colour gamut (black triangle) is fairly equal to the sRGB colour space. There is some modest over-coverage in greens but not by anything too significant. Default gamma was recorded at 1.7 average in this gamma 1 mode, leaving it with a significant 23% deviance from the target which was a potential issue. Remember this is a gaming screen, where a low gamma like this is often desirable for gaming uses. There are a couple of other gamma settings in the OSD menu which we will test in a moment, so don't be too worried. White point was measured at an accurate 6568k, being only 1% out from the 6500k we'd ideally want for desktop use. Note that the default 'color temp' preset was set to 'warm' here which was actually about right for our targets.

Luminance was recorded at a very bright 324 cd/m² which is far too high for prolonged general use. The screen was set at a default 90% brightness in the OSD menu but that is easy to change of course to reach a more comfortable setting without impacting any other aspect of the setup. The black depth was 0.35 cd/m² at this default brightness setting, giving us a decent (for an TN Film panel) static contrast ratio of 924:1. Colour accuracy was pretty very poor out of the box because of the very low gamma here, with an average dE of 5.4 measured. Testing the screen with colour gradients showed smooth transitions in all shades, with some slight gradation evident in darker tones.

Overall this default setup was aimed at gaming, where the low gamma was desirable. There was at least a reliable white point but you would certainly need to turn the brightness down a lot to be comfortable. Let us test the other gamma modes to see if they can return a better setup for more general day to day use, away from gaming.

AOC AGON AG251FZ- Default Settings, gamma mode 3

A simple change in the OSD menu to gamma mode 3 returned a much better setup for every day non-gaming uses. Note also that the gamma mode 2 showed very little change to the measured gamma from mode 1 (measuring 1.7 average, 21% deviance). In gamma mode 3, the average gamma was now 2.1 with a small 4% deviance from our target. White point remained reliable, and now the colour accuracy was much better with an average dE of 2.4 measured now. Again, you need to turn down the brightness, and you have a small drop in contrast ratio because of the gamma adjustment, but this mode represents a much better setup for normal uses. Thankfully AOC have provided a good gamma adjustment via the OSD as otherwise that can be very tricky to correct without a hardware calibration device.

Calibration

We used the X-rite i1 Pro 2 Spectrophotometer combined with the LaCie Blue Eye Pro software package to achieve these results and reports. An X-rite i1 Display Pro colorimeter was used to validate the black depth and contrast ratios due to lower end limitations of the i1 Pro device.

AOC AGON AG251FZ - Calibrated Settings

   

We stuck to the gamma mode 3 which had delivered a more reliable gamma close to 2.2 out of the box, and then also switched to the 'user' color temp mode. Although we know the default 'warm' preset was very reliable, we wanted to be able to tweak the RGB channels as necessary during the calibration process. We adjusted the RGB channels and brightness setting as shown in the table above. All these OSD changes allowed us to obtain an optimal hardware starting point and setup before software level changes would be made at the graphics card level. We left the  LaCie software to calibrate to "max" brightness which would just retain the luminance of whatever brightness we'd set the screen to, and would not in any way try and alter the luminance at the graphics card level, which can reduce contrast ratio. These adjustments before profiling the screen would help preserve tonal values and limit banding issues. After this we let the software carry out the LUT adjustments and create an ICC profile.

Average gamma was now corrected to 2.2 average with a 0% deviance, correcting the minor 4% deviance we'd seen out of the box in this gamma mode 3. The white point had now been corrected to 6469k, which largely corrected the minor 1% deviance we'd seen out of the box. Luminance had been improved thanks to the adjustment to the brightness control and was now being measured at 119 cd/m². This left us a black depth of 0.128 cd/m² and maintained a decent static contrast ratio (for a TN Film panel) of 931:1. Colour accuracy of the resulting profile was excellent, with dE average of 0.6 and maximum of 1.1. LaCie would consider colour fidelity to be very good overall. Testing the screen with various colour gradients showed smooth transitions. There was some slight gradation in darker tones and some very minor banding introduced due to the adjustments to the graphics card LUT from the profilation of the screen. You can use our settings and try our calibrated ICC profile if you wish, which are available in our ICC profile database. Keep in mind that results will vary from one screen to another and from one computer / graphics card to another.

Calibration Performance Comparisons

The comparisons made in this section try to give you a better view of how each screen performs, particularly out of the box which is what is going to matter to most consumers. When comparing the default factory settings for each monitor it is important to take into account several measurement areas - gamma, white point and colour accuracy. There's no point having a low dE colour accuracy figure if the gamma curve is way off for instance. A good factory calibration requires all 3 to be well set up. We have deliberately not included luminance in this comparison since this is normally far too high by default on every screen. However, that is very easily controlled through the brightness setting (on most screens) and should not impact the other areas being measured anyway. It is easy enough to obtain a suitable luminance for your working conditions and individual preferences, but a reliable factory setup in gamma, white point and colour accuracy is important and not as easy to change accurately without a calibration tool.

From these comparisons we can also compare the calibrated colour accuracy, black depth and contrast ratio. After a calibration the gamma, white point and luminance should all be at their desired targets.

Default setup of the screen out of the box was very much aimed at gaming requirements and so it's a little unfair perhaps to judge the screen on this basis. Out of the box, the gamma mode 1 delivered an average 1.7 gamma which skewed the gamma curve a long way from our desired 2.2, and had a knock on effect to the dE colour accuracy. The white point was at least very reliable, and contrast ratio good for a TN Film panel. Thankfully it's very simple to improve the setup with a change to the gamma mode 3, which returned a more accurate 2.1 gamma and lower dE of 2.4. It's very common for the TN Film gaming screens to be set up out of the box with a lower gamma, and you can see this if you look at the Acer Predator XG270HU (1.8), Asus MG248Q (1.9) and BenQ XL2730Z (1.9) for example. With the better day to day set up being easily accessible to users via the gamma mode 3, we don't need to penalise the screen here for its gaming gamma curve at default settings. Had AOC not provided gamma controls in the menu it would be another story, as the gamma curve can be very tricky to correct otherwise without access to a hardware calibration device.

The display was good when it came to static contrast ratio for a TN Film panel, at 931:1 after calibration. This was similar to some other high end gaming TN Film screens like the Asus MG248Q (1005:1) and BenQ XL2730Z (917:1) for instance. Of course none of these TN Film panels can compete with VA panel types which can reach over 2000:1 easily, and commonly up to 3000:1 (e.g. Acer Predator Z35) or even near 5000:1 (Eizo Foris FG2421).

Viewing Angles

Above: Viewing angles shown from front and side, and  from above and below. Click for larger image

Viewing angles of the screen were as you might expect from a TN Film panel. Unfortunately this panel technology is inherently poor in this field, and so viewing angles are more restrictive than other competing technologies like IPS and VA variants. Although the manufacturer will quote a viewing angle of 170 / 160 (a classic indication that a TN Film panel is being used by the way if in doubt), in practice there are some obvious contrast and colour tone shifts horizontally, and especially vertically.

As you move your head from side to side in a horizontal plane, there is a contrast shift and the image becomes more pale and introduces a yellow hue. As you move to a wider angle the image can become more washed out as well and a slight pink hue is introduced. Vertically the fields of view are more restrictive still. From above the image becomes pale and washed out, while from below there is a characteristic TN Film darkening of the image. Unfortunately vertically the viewing angles will introduce noticeable shifts in the contrast and colour tone of the image which mean that for any colour critical work it is not really very well suited. TN Film panels have long suffered from these restrictive viewing angles due to the nature of their pixel structure. They are still fine for a single user for general use and certainly the TN Film panels offer their advantages when it comes to pixel response times and refresh rate for gaming. If however, you were hoping to do any colour critical or photography work you may find these shifts in the appearance of the image difficult. An IPS-type panel would probably be a wiser choice if you were looking for a screen with much wider viewing angles but having said that you are probably mainly interested in gaming if you are considering this screen. Remember, this screen is specifically designed for gaming, and so you will have to live with some of the sacrifices of TN Film to get the kind of gaming performance and features offered here. There are some high refresh rate gaming IPS panels available now in larger sizes as well which can offer better viewing angles than TN Film models, although they are normally priced higher and have some other characteristic differences, and so TN Film models like this still have their place for many users.

Above: View of an all black screen from the side. Click for larger version

On a black image there is a moderate pale grey tint introduced to the image when viewed from a wide angle. This isn't too severe and shouldn't present any real problems in practice. Certainly not the obvious white glow you get from most modern IPS-type panels in similar situations and fairly standard for a TN Film panel. Very similar to what we have seen from other recent gaming TN Film screens like the Asus MG248Q, Asus ROG Swift PG278Q and BenQ XL2730Z. The glow you see from most modern IPS panels can put off some users. So on the one hand, those IPS models have much better general viewing angles than the TN Film models, but they do show more glow which some people find an issue.

Panel Uniformity

We wanted to test here how uniform the brightness was across the screen, as well as identify any leakage from the backlight in dark lighting conditions. Measurements of the luminance were taken at 36 points across the panel on a pure white background. The measurements for luminance were taken using BasICColor's calibration software package, combined with an X-rite i1 Display Pro colorimeter with a central point on the screen calibrated to 120 cd/m². The below uniformity diagram shows the difference, as a percentage, between the measurement recorded at each point on the screen, as compared with the central reference point.

It is worth noting that panel uniformity can vary from one screen to another, and can depend on manufacturing lines, screen transport and other local factors. This is only a guide of the uniformity of the sample screen we have for review.
 

Uniformity of Luminance

The luminance uniformity of the screen was reasonably good overall. The screen was brighter towards the bottom edge than at the top, so there was a fair difference if you compare the two extremes, but it was more subtle between the top/bottom and the central area of the panel. In the worst case the luminance dropped to 110 cd/m² (-9%). Actually, 100% of the screen was within a 10% deviance from the centrally calibrated point which was decent.

Backlight Leakage

Above: All black screen in a darkened room. Click for larger version

We also tested the screen with an all black image and in a darkened room. A camera was used to capture the result. The camera showed there was no real backlight bleed evident, but a little clouding from the bottom edge of the screen where we know the luminance was a little higher than the upper areas.

Note: if you want to test your own screen for backlight bleed and uniformity problems at any point you need to ensure you have suitable testing conditions. Set the monitor to a sensible day to day brightness level, preferably as close to 120 cd/m² as you can get it (our tests are once the screen is calibrated to this luminance). Don't just take a photo at the default brightness which is almost always far too high and not a realistic usage condition. You need to take the photo from about 1.5 - 2m back to avoid capturing viewing angle characteristics, especially on IPS-type panels where off-angle glow can come in to play easily. Photos should be taken in a darkened room at a shutter speed which captures what you see reliably and doesn't over-expose the image. A shutter speed of 1/8 second will probably be suitable for this.

General and Office Applications

With a 1920 x 1080 resolution, the desktop real estate  of the AG251FZ feels a step down compared with all the high resolution panels we've tested, and the 27" 2560 x 1440 models we are used to using day to day. You do lose a large amount of desktop space, and although side by side split screen working is possible, it's not as easy due to the more limited resolution and space. With a 0.2825mm pixel pitch, text is comfortable and easy to read natively, providing a sharp and crisp image. It is not as sharp as the 1440p panels we've become accustomed to, or of course any ultra HD/4K resolutions where scaling is used, but it is perfectly adequate. For this size screen, 1920 x 1080 is about your limit of sensible resolution without needing to use operating system scaling options.

The moderate AG coating of the TN Film panel could be considered a bit grainy, especially on white office backgrounds to a lot of people. It's not as clear as modern IPS coatings or any semi-glossy solution. Still, it's not as grainy as old IPS panels and is on par with other TN Film matrices we've tested. Perhaps the main issue with this panel technology though is the restrictive viewing angles, making contrast and colour tone shifts a bit of a problem when it comes to colour critical work. They are the same here as other TN Film panels, being restrictive especially vertically. The screen is fine when viewed head on though really for office and text work, but for colour critical work or photo editing etc you'd be better off with an IPS-type panel. The default setup of the screen was a bit restrictive for normal uses, as the gamma is set up more for gaming, but thankfully this was very easy to adjust without a calibration tool via a simple change in the OSD menu. That provided a good default setup then for day to day office work, once you've turned the brightness setting down. There are 3 blue light filter modes (weak, medium and strong) offered in the menu if you want to add further eye care protection and might be worth experimenting with for prolonged office use or text reading.

The brightness range of the screen was very good, with the ability to offer a luminance between 366 and 58 cd/m². This should mean the screen is perfectly useable in a wide variety of ambient light conditions, including darkened rooms. A setting of ~23 in the OSD brightness control should return you a luminance close to 120 cd/m² out of the box. Otherwise you might want to try the settings from our calibration section. On another positive note, the brightness regulation is controlled without the need for the use of the now infamous Pulse-Width Modulation (PWM), and so those who suffer from eye fatigue or headaches associated with flickering backlights need not worry. There was no audible noise or buzzing from the screen, even when specifically looking for it using test images with a large amount of text at once. The screen also remains cool even during prolonged use.

There are quite a few extra features on this screen for office environments, even though it's primarily a gaming screen. There are 4 USB ports, including 2 easy access on the right hand side of the screen and one featuring fast charging support. There's a mic, headphone and audio out connection along with the integrated stereo speakers which should be fine for the odd YouTube clip or mp3. There are no ambient light sensor, card reader, motion sensor or anything else provided which can sometimes be useful in office environments. There was a good range of ergonomic adjustments available from the stand allowing you to obtain a comfortable position for a wide variety of angles. The VESA mounting support may also be useful to some people as well.

 
Responsiveness and Gaming

The AG251FZ is firmly a gamers screen, with key features including the native 240Hz refresh rate, 1ms G2G response time and support for AMD FreeSync. You will need to keep in mind the demands on your system and graphics card to power a screen like this, as there's quite a drain on resources to run at 240Hz! The resolution is 'only' 1920 x 1080 so that is at least a bit easier than if this was a 1440p or 4k resolution screen. For systems which can't manage the 240Hz reliably or frequently, FreeSync is supported for variable refresh rate control, helping to eliminate tearing and stuttering and operating in a nice wide range from 48 to 240Hz. AOC have provided a wide range of additional extras for gaming that we will talk about in a moment.

The AGON AG251FZ is rated by AOC as having a 1ms G2G response time which indicate the panel uses overdrive / response time compensation (RTC) technology to boost pixel transitions across grey to grey changes. The part being used is the AU Optronics M250HTN01.0 TN Film technology panel. Have a read about response time in our specs section if you need additional information about this measurement. As a reminder, this is the first native 240Hz refresh rate panel of any size and type we have seen.

We will first test the screen using our thorough response time testing method. This uses an oscilloscope and photosensor to measure the pixel response times across a series of different transitions, in the full range from 0 (black) to 255 (white). This will give us a realistic view of how the monitor performs in real life, as opposed to being reliant only on a manufacturers spec. We can work out the response times for changing between many different shades, calculate the maximum, minimum and average grey to grey (G2G) response times, and provide an evaluation of any overshoot present on the monitor.

We use an ETC M526 oscilloscope for these measurements along with a custom photosensor device. Have a read of our response time measurement article for a full explanation of the testing methodology and reported data.
 

Overdrive Setting

The 'overdrive' control is available via the 'game setting' section of the OSD menu as shown above. We will test all four modes to see which is optimal first of all. Note that these results were basically the same from an NVIDIA and AMD test system. For now we have taken these measurements at the maximum 240Hz refresh rate, but we will look at the implications of the refresh rate in a moment once we've established the behaviour of the response times under this setting.

In the 'off' setting the response times were mixed, with some around the advertised 1ms G2G figure, but many being much slower up around 9 - 13ms. There was no overshoot as you would expect with overdrive turned off, but the response times were slower than we would like from a TN Film panel. Turning the overdrive up to 'weak' brought about some minor improvements, reducing the average figure from 8.3ms to 7.2ms. There was again an improvement as you moved up to 'medium', dropping the average response time to 5.9ms, and still without any sign of overshoot. At the maximum 'strong' setting, response times were improved significantly, down to 2.7ms G2G now. Some moderate overshoot was starting to creep in, but on par with most fast TN Film gaming screens we've seen, so nothing too severe. The optimal overdrive setting will be between medium and strong, and we will consider which is best when we look at the refresh rate impact in a moment.

Refresh Rate and FreeSync

The AG251FZ supports a refresh rate of up to 240Hz natively, and as we've discussed earlier this is the first panel supporting such a high refresh rate on the market. You can quickly and easily select this refresh rate in Windows as shown above, although we did find we had to use the provided DisplayPort cable rather than our regular day to day cable, so perhaps it is a little fussy on the cabling choice. Just stick with the one provided and you will be fine. When enabled, and from a compatible system, FreeSync is also available which operates in a range between 48 and 240Hz. There are improvements in perceived motion clarity as you increase the refresh rate, and the 240Hz certainly feels more fluid and faster than 120 / 144Hz. The improvement is not as noticeable as when you move from 60 to 120Hz for the first time, but the ability to deliver even higher frame rates from 144 - 240Hz is certainly welcome.

From an AMD test system we found stable performance without any frames being dropped at all refresh rates from 60Hz to 240Hz which was good news. From an NVIDIA system we had some issues at the maximum 240Hz refresh rate (not at anything lower), where some frames were being dropped. This display is clearly aimed at AMD users anyway and so that might explain the incompatibility there for NVIDIA when you push the boundaries of the refresh rate. We've asked AOC for comment nonetheless.

Refresh Rate Impact, Overdrive Mode = Strong

We stuck to the 'strong' overdrive setting for now and we wanted to test the response times at a range of refresh rates to see if that influences the pixel transitions. It's quite common for the overdrive impulse to be dynamically controlled across a wide refresh rate range like this. The overshoot can also be impacted we have seen in the past.

As you can see from these measurements the actual pixel transition times do not seem to really change that much overall as you change the refresh rate. We measured basically the same average response time of around 2.5 - 2.7ms G2G across each refresh rate from 60 to 240Hz, while sticking with the same 'strong' overdrive setting in the menu. What did change though was the levels of overshoot, which improved as you increased the refresh rate. So at 60Hz there were high levels of overshoot detected on a wide range of transitions, but as you reached 144Hz and then 240Hz a lot of the overshoot was eliminated or reduced. This is good news of course, but you need to be achieving a high refresh rate really to be able to experience the screen without the overshoot becoming an issue in this 'strong' mode.

We will look in a moment at the lower 'medium' setting to see how refresh rate impacts response times/overshoot there, as for those using the screen at lower refresh rates it might well be more optimal than this aggressive 'strong' mode.

Optimal Overdrive Setting at 240Hz = Strong

If we compare above the response time performance at 240Hz in the 'strong' mode, vs. the next step down in the 'medium' mode we can see that really to support the 240Hz refresh rate effectively, you need to stick to the 'strong' mode. To handle the high frame rate of 240Hz, response times needs to be reliably and consistently under 4.17ms (1000 / 240 = 4.17), otherwise you will get additional blurring and smearing where response times cannot keep up with the frame changes. In the 'medium' setting, the response times are too slow to really support 240Hz refresh rate, and in practice you will see some added smearing to a moving image. The 'strong' setting supported 240Hz well, with an average G2G figure of 2.7ms measured. As we discussed above, the overshoot at this max refresh rate was moderate but not too severe. So if you want to enjoy the screen at the maximum 240Hz setting, and can reliably achieve the frame rates to make this worthwhile, then stick to the 'strong' setting.

Optimal Overdrive Setting at Less than 240Hz = Medium

We know that the 'strong' overdrive mode is optimal if you're achieving the 240Hz maximum refresh rate of this screen, but at lower refresh rates the overshoot starts to become too distracting and severe. Thankfully, the 'medium' overdrive setting is far less aggressive and shows no overshoot at all at any refresh rate. The response times remains fairly consistent as well across the refresh rate range, actually getting a little slower as you increase the refresh. It looked from the 'strong' setting that maybe the overdrive impulse was being turned down a little as you increased refresh rate, which accounts for the improvements we saw in levels of overshoot. Maybe the same is true here, where the overdrive impulse is turned down ever so slightly as you increase refresh rate and so response times slow a little. It's not enough to make any real difference anyway, and in fact the improvements you get in perceived motion clarity from the increased frame rate easily out-weigh anything you might see from slightly slower pixel transition times.

So at all refresh rates there is no visible overshoot in this 'medium' overdrive mode. Given the average G2G response times are around 5.2 - 5.9ms G2G we can estimate that in theory, refresh rates up to around 175Hz are achievable in this overdrive mode, before additional smearing starts to become an issue. So if you're using FreeSync and managing to deliver in the 48 - 175Hz range, this 'medium' mode will be adequate and will avoid any noticeable overshoot. At the top end if you're achieving from 175 - 240Hz you'd probably be best to switch to the 'strong' mode, which brings response times down to avoid any additional smearing, but introduces some moderate (but manageable) overshoot. Obviously for any console gaming (60Hz) or fixed refresh rate from 60 - 144Hz away from FreeSync use, you're better with the 'medium' setting.

Detailed Response Time Measurements
Refresh Rate = 240Hz, Overdrive = Strong

We stuck with what we consider to be the optimal 'strong' response time setting at the maximum 240Hz refresh rate. The average G2G response time was measured at 2.6ms which was very good and represented very fast pixel transition times from this new TN Film panel. Some transitions reached as low as the advertised 1ms G2G as well.

If we evaluate the Response Time Compensation (RTC) overshoot then the results show moderate to high levels here in this 'strong' mode. That is not dissimilar to most fast TN Film gaming screens and it's not overly obvious in practice due to the high frame rate and smooth motion clarity. You can eliminate the overshoot completely if you revert to the 'medium' overdrive setting, but that will only really support refresh rates up to around 175Hz reliably without additional smearing starting to appear.

Display Comparisons

The above comparison table and graph shows you the lowest, average and highest G2G response time measurement for each screen we have tested with our oscilloscope system. There is also a colour coded mark next to each screen in the table to indicate the RTC overshoot error, as the response time figure alone doesn't tell the whole story.

As a reminder, these figures are at 240Hz refresh rate and with overdrive set to 'strong'. The response time performance of the AG251FZ was very impressive overall, with actually the fastest response times we've measured from any gaming screen at 2.6ms G2G. Other fast TN Film screens are very close of course, like the Asus ROG Swift PG278Q (2.9ms) and Dell S2716DG (3.1ms) for instance. There is a boost here on the AOC with frame rate support, pushing quite a long way beyond the 144Hz supported from those other models mentioned. If your system can support it, then the 240Hz does bring about obvious advantages in frame rate and motion clarity. There is moderate levels of lag on these ultra-fast TN Film screens which you have to live with to drive the response times as low as they are.

Pursuit Camera Tests (updated)

We've already tested above the actual pixel response times and other aspects of the screen's gaming performance. We wanted to carry out some pursuit camera tests as well to give an even more complete idea of the performance of this screen.

Pursuit cameras are used to capture motion blur as a user might experience it on a display. They are simply cameras which follow the on-screen motion and are extremely accurate at measuring motion blur, ghosting and overdrive artefacts of moving images. Since they simulate the eye tracking motion of moving eyes, they can be useful in giving an idea of how a moving image appears to the end user. It is the blurring caused by eye tracking on continuously-displayed refreshes (sample-and-hold) that we are keen to analyse with this new approach. This is not pixel persistence caused by response times; but a different cause of display motion blur which cannot be captured using static camera tests. Low response times do have a positive impact on motion blur, and higher refresh rates also help reduce blurring to a degree. It does not matter how low response times are, or how high refresh rates are, you will still see motion blur from LCD displays under normal operation to some extent and that is what this section is designed to measure. Further technologies specifically designed to reduce perceived motion blur are required to eliminate the blur seen on these type of sample-and-hold displays which we will also look at.

We used the Blurbusters.com Ghosting Motion Test which is designed to be used with pursuit camera setups. The pursuit camera method is explained at BlurBusters as well as covered in this research paper. We carried out the tests at various refresh rates, with and without Blur Reduction enabled. These UFO objects were moving horizontally at 960 pixels per second, at a frame rate matching refresh rate of the monitor.

Overdrive Setting = Medium

These tests capture the kind of blurring you would see with the naked eye when tracking moving objects across the screen. As you increase the refresh rate the perceived blurring is reduced, as refresh rate has a direct impact on motion blur. It is not eliminated entirely due to the nature of the sample-and-hold LCD display and the tracking of your eyes. No matter how fast the refresh rate and pixel response times are, you cannot eliminate the perceived motion blur without other methods like blur reduction strobing backlights, which this model does not feature.

As you can see, the perceived motion clarity improves significantly as you increase the refresh rate from 60 to 100 and 144Hz levels. The moving image becomes easier to track and appears sharper. With the overdrive setting at 'medium' here, at the maximum 240Hz refresh rate you will notice a slightly more smeared and blurry image. While the refresh rate is increased significantly, the response times in this setting are not quite fast enough to keep up with the frame rate demands as measured in our previous section. As a result, you get a little additional smearing introduced on the moving image. There is no overshoot at any of these refresh rates when using the 'medium' overdrive option.

Overdrive Setting = Strong

Switching up to the 'strong' overdrive setting showed similar results in terms of improved motion clarity as you increase the refresh rate. Again, the moving image became clearer and sharper. At the maximum 240Hz refresh rate here, the response times were fast enough to keep up with the high frame rate and so you don't get any added smearing like you do at the 'medium' overdrive setting. There is a large amount of overshoot at lower refresh rates as we've already discussed earlier in the review, but it becomes progressively less as you increase refresh rate. By the maximum 240Hz refresh rate the overshoot is only moderate and hard to notice in many cases. As we said earlier in the review, if you're achieving somewhere around 175 - 240Hz refresh rate then the 'strong' overdrive mode is optimal. For 48 - 175Hz then you'd probably be better sticking with the 'medium' mode.

Note: optimal overdrive settings used on each screen

We can also compare the pursuit camera tests at 60Hz and 144Hz compared with a couple of very fast and very popular gaming screens above. The performance is very comparable in actual perceived motion blur between all three in practice, with very little to separate them. The TN Film PG278Q and AG251FZ models feel ever so slightly more fluid we felt, thanks to the slightly faster response times.

The good thing about the AG251FZ is that you can then also boost the refresh rate up higher than 144Hz, offering additional motion clarity benefits and a massively increased frame rate. This does make a difference in practice.

Additional Gaming Features

Aspect Ratio Control - the AG251FZ has a wide range of options for aspect ratio control through the OSD 'extra' menu as shown above. There are options for (wait for it!): 1:1 pixel mapping, 17" (4:3), 19" (4:3), 19" (5:4), 19"W (16:10), 21.5"W (16:9), 22"W (16:10), 23"W (16:9), 23.6"W (16:9), 24" (16:9) and 'wide'. Each of the specified size options is designed to mimic the screen size visible from each, although we're not entirely sure for what purpose, unless you were maybe trying to match the screen size with another display? Anyway, there was a good range of options and importantly 1:1 is available which will cover many non-16:9 needs we are sure.

Shadow Control - There is a shadow control slider available in the 'game setting' menu and also via the quick access option from the OSD buttons/switch accessory (pressing the right arrow). This alters the gamma levels quickly and easily which might be useful for different gaming scenarios.

Game Color- This operates like a digital vibrance control, boosting the colours (or dulling them) which some users might like for their gaming.

Preset Modes - There are several specific game preset modes available from the 'game setting' menu. There are three preset modes for FPS, RTS and racing games. In those, you can change some settings yourself although some (including overdrive) are greyed out. There are then 3 gamer preset modes where you can change the settings manually for each in the 'game setting' menu (including overdrive now). Oddly you can't access the 'color setup' menu when you're using any of these gaming presets and also they seem to have a slightly boosted and accentuated sharpness added, for which there is no control in the OSD anyway! Still, they are useful if you want to set up different  options for different gaming scenarios including perhaps altering the 'game color' and 'shadow control' options. The 3 gamer preset modes are accessible quickly and easily via the switch accessory too which is useful.

Lag

For the full reviews of the models compared here and the dates they were written (and when screens were approximately released to the market), please see our full reviews index.

We have provided a comparison above against other models we have tested to give an indication between screens. The screens tested are split into two measurements which are based on our overall display lag tests (using SMTT) and half the average G2G response time, as measured by the oscilloscope. The response time is split from the overall display lag and shown on the graph as the green bar. From there, the signal processing (red bar) can be provided as a good estimation.

The screen features a 'low input lag' mode option in the menu. First of all we tested the screen with this turned off. This showed a total average display lag of 13.5 ms as measured with SMTT 2. Taking into account half the average G2G response time at 1.3ms, we can estimate that there is ~12.2ms of signal processing lag on this screen which pretty low anyway. Switching the low input lag mode on returned improved results, with an estimated 3.7ms of signal processing lag now instead. This represents a very low lag which shouldn't represent any problems in gaming.

One annoyance with this was when you're connected to a FreeSync system. As with most FreeSync screens we've tested, the screen seems to always think you are operating with FreeSync, even if you disable the option in the AMD graphics card software. We believe this is a known bug and in many cases it doesn't really cause any issue. You can tell that is happening here as in the OSD menu the vertical refresh rate is listed as "FreeSync", regardless of whether you've enabled the option in the graphics card control panel. The issue with this though is that when FreeSync is active (or even not active, but detected), the 'low input lag' option is greyed out and not accessible. So from a FreeSync system you will have to live with the slightly higher lag, although this is still very low really and shouldn't represent any major issues. We've asked AOC if there is a reason why this option is greyed out when using FreeSync and will update this section accordingly when we have more info.

Movies and Video

The following summarises the screens performance in video applications:

• 24.5" screen size makes it a reasonable option for an all-in-one multimedia screen, but being quite a lot smaller than most modern LCD TV's of course.

• 16:9 aspect ratio is more well suited to videos than a 16:10 format screen, leaving smaller borders on DVD's and wide screen content at the top and bottom.

• 1920 x 1080 resolution can support full 1080 HD resolution content

• Digital interfaces support HDCP for any encrypted and protected content

• Good range of connectivity options provided with DisplayPort, 2x HDMI, Dual-link DVI and VGA offered.

• Cables provided in the box for all video connections

• Moderate AG coating provides reasonably clear images with no major graininess, and without the unwanted reflections of a glossy solution. Some graininess apparent as with other TN Film panels, but shouldn't present a problem in movies.

• Wide brightness range adjustment possible from the display, including a maximum luminance of ~366 cd/m² and a decent minimum luminance of 58 cd/m². This should afford you good control for different lighting conditions. Contrast ratio remains stable across that adjustment range as well and is good for a TN Film panel. Brightness regulation is controlled without the need for PWM and so is flicker free for all brightness settings.

• Black depth and contrast ratio are good for a TN Film panel at 931:1 after calibration. Detail in darker scenes should not be lost as a result.

• There is no specific 'movie' preset mode available for movies or video but you could easily set up one of the 3 customisable gamer modes if you want for movie viewing

• Very good pixel responsiveness which will handle fast moving scenes in movies without issue. You will want to stick with the 'medium' overdrive mode to avoid any overshoot issues present in the 'strong' mode. Certainly when using an external Blu-ray player operating at 60Hz.

• Viewing angles are limited due to the use of TN Film panel technology. May cause issues with gamma and contrast shift if you change your line of sight or have several people trying to see the screen at once. Not really an ideal technology for movies as a result of this viewing angle limitation.

• Very good and mostly easy to use range of ergonomic adjustments available from the stand, so should be easy to obtain a comfortable position if you want to sit further away from the screen for movie viewing.

• No particularly noticeable backlight leakage, and none from the edges which is good. This type of leakage may prove an issue when watching movies where black borders are present but it is not a problem here.

• 2x 3W integrated stereo speakers on this model for the occasional movie clip and YouTube video, but probably not suitable for movie viewing. There is also an audio output and headphone jack connection if needed.

• Good range of hardware aspect ratio options including various defined screen sizes and aspect ratios, along with 'wide' and 1:1 pixel mapping which should be fine for most uses. The screen is natively 16:9 which should be fine for external devices anyway

• Picture in picture (PiP) and Picture By Picture (PbP) are not available.
   

Conclusion

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The AG251FZ was the first 240Hz native refresh rate screen we've tested and it delivered very well when it came to its primary focus of gaming. The additional boost in frame rate and motion clarity were decent and extends the possibilities for high frame rate gaming from LCD screens by a significant amount. Thankfully the TN Film panel offers very fast response times which can keep up with the refresh rate demands, unlike some other panel technologies we've seen where they struggle -especially where overclocked refresh rates are introduced. The addition of AMD FreeSync will be very useful for those who can't power the 240Hz at 1080p consistently and this is so far the only FreeSync option in this space with 240Hz support. There was a very good range of additional gaming features and options in the menu, and we liked the included switch accessory which made changing options much simpler and quicker.

There is a useful low input lag option as well which worked well at reducing the already pretty low lag of the screen. Unfortunately this didn't seem to be available when using a FreeSync system which was a shame, but don't be put off as the lag is still not bad even without it. We would have liked to see an added blur reduction backlight option if we were being picky, but that would have presumably added to the retail cost, and most people will probably want to use FreeSync anyway given the wide refresh rate range and demands on your system pushing anywhere near the 240Hz.

In other areas the screen had a decent out of the box setup once you made a simple change to the gamma mode, and we were pleased with the flicker free backlight and wide backlight adjustment range. You will have to live with some of the inherent limitations of TN Film technology, most notably the restrictive viewing angles so just be aware of that. There is a very good range of connections and stand features, and the additional easy access connections on the side of the screen were handy. It's a well thought out screen with plenty of extras and features, and certainly a very interesting option for gamers.