Input Lag of Monitors
A monitor’s input lag is the amount of time that elapses between a picture being generated by a source and that image appearing onscreen. When gaming, you’ll experience this as the time between making an input and seeing the reaction appear onscreen. With a mouse, it's the time between moving your mouse and the same movement being made with the cursor.
We record the lowest input lag time of which a monitor is capable, the amount of lag a monitor has when using non-native resolutions, the amount of lag present when the variable refresh rate feature is enabled, and the amount of lag a monitor has when displaying HDR content.
Note: Do not confuse the input lag time with the response time. The response time is the time it takes a pixel to shift from one color to another, which is often significantly shorter than the input lag time. Response time is related to motion blur.
If you want to see our test for TVs, check our "Input Lag of TVs" article.
When it matters
Since most of the time spent using PC monitors is with direct interaction using a mouse, a low input lag is valuable in almost all cases in order to have a responsive experience.
Some people might have experienced it in the form of a cheap wireless mouse, where the additional latency can make the mouse feel "floaty" and delayed. Even for productivity tasks, a high input lag slows you down, and in cases where precise movements are necessary, it can be quite a pain. Gaming is also another aspect where input lag is important, even more so than general usage. Fast-paced games require quick reflexes.
Input lag at native resolution
This input lag test represents the lowest lag a monitor is capable of achieving while using its native resolution. This is the amount of lag that is best for both general use of the mouse and for most gamers.
We use our own input lag tool to perform this test, as it provides an accurate, continuous measurement of a monitor’s input lag as well as the spread in which the speed varies. To get the lowest amount of lag on some monitors, it’s necessary to enable some sort of "game" or "instant" mode.
Input lag at a non-native resolution
Lowest input lag possible at the center of the screen, when the monitor is displaying an alternative resolution at its native refresh rate. The non-native resolution tested depends on the native resolution of the monitor, following this pattern unless otherwise specified in the Input Lag text:
|Native Resolution||Non-Native Resolution Tested|
This input lag test represents the lowest lag a monitor is capable of achieving while using a non-native resolution. This type of input lag is important for more competitive gamers that might use a lower resolution to achieve higher framerates or for those that use older devices incapable of supporting higher resolutions.
We use the same testing process as for the first test, only we use the next standard resolution down from the native one (2560x1440 for UHD, 1366x768 for 1080p).
Input lag at native resolution at 60 Hz
This input lag test represents the lowest lag a monitor is capable of achieving while using its native resolution and a 60 Hz refresh rate. This is the amount of lag that is most important for those planning to use their monitor with gaming consoles that cannot output a signal higher than 60 Hz.
We use the same testing process as for the first test, but we set the monitor to a standard 60 Hz refresh rate.
Input lag with Variable Refresh Rate
Our ‘Variable Refresh Rate’ measurement represents the amount of input lag that is present when a monitor has its variable refresh rate feature enabled (FreeSync, G-Sync). This feature is useful for most gamers that plan on using adaptive-sync to reduce stutters and remove tearing caused by fluctuations in framerate.
We use the same testing process as for the first test, only we enable the respective variable refresh rate feature for the monitor.
Input lag with HDR
Our ‘HDR’ measurement represents the amount of input lag that is present when a monitor is displaying HDR content. This number is important for modern HDR games, but also for those that plan on using their monitor for HDR content production.
We use the same testing process as for the first test, except we transmit a 10-bit HDR signal instead of the usual SDR signal.
Input lag with BFI
Our ‘BFI’ (Black Frame Insertion) measurement represents the amount of input lag that is present when a monitor has its black frame insertion (ULMB, Lightboost, Backlight Flicker) feature enabled. This number is important for people that use the BFI feature of their monitor to reduce blur and enhance motion clarity
We use the same testing process as for the first test, except we enable the monitor's BFI in its on-screen menu.
How input lag is measured
Input lag is not an official specification because it depends on two varying factors: the type of source and the settings of the monitor. The easiest way you can measure it is by connecting a computer to two different monitors and displaying the same timer on both screens. One of them has to be a known quantity, so you either have a measurement on hand or use a virtually instant CRT monitor as a baseline. You can find a timer here. Then, if you take a picture of both screens, the time difference will be your input lag. This is, however, an approximation, because your computer does not necessarily output both signals at the same time. In this example image, an input lag of 40ms (1:06:260 – 1:06:220) is indicated.
How we measure input lag
In our tests, we measure input lag using our own dedicated device we've designed just for this purpose, which sends an image on-screen and records the time it takes for the image to appear in its sensor (learn more about our tool here). This is a lot more accurate than the two screens method. When measuring the input lag using the two screen method, your precision is entirely limited by the frame rate your camera is capable of capturing, introducing a fair margin of error and inconsistency in your measurement.
For example, a camera capable of recording at 240 frames per second will record an image every 4.17 ms (this is the frame time). This means that you can't measure any monitor with a lower input lag than 4 ms accurately.
However, that only ever applies in perfect conditions. Since LCD screens take time to switch from one frame to another, it's difficult to figure out which part of the cycle you are capturing with a picture. You will probably be seeing parts of frames that were either before or after the input appeared on-screen instead of the exact moment of its appearance. It is possible to take a lot of pictures to create a more accurate average, but it would take significantly more time than with a dedicated device while possibly still not being as accurate. In comparison, our tool takes a sample multiple thousand times a second and is synchronized with its own output, allowing us to precisely find the moment the input appeared on the screen very easily.
Why there is input lag on monitors
There are three main functions that delay the monitor: acquiring the source image, processing the image, and displaying it.
Acquisition of the image
The more time it takes for the monitor to receive the source image, the more input lag there will be. With PCs, this has never really been an issue since previous analog signals were virtually instant and current digital interfaces like DisplayPort and HDMI have next to no inherent latency. Other than from the display itself, the other major cause of input lag in the chain is the device the input was first acquired on. Some mice, usually wireless ones, can add a significant amount of input lag before the image even reaches the screen.
Once the image is in a format understandable by the monitor's video processor, it will apply at least some processing to alter the image in some way. A few examples:
- Adding overlays (like custom crosshairs)
- Adjusting the colors and brightness
- Scaling it (like 720p to 1080p, or 1080p to UHD)
The time this step takes is affected by the speed of the video processor and the amount of processing needed. Although you cannot control the speed of the processor, you can exercise some control over how many operations it needs to do by enabling and disabling settings. The more settings you enable, the more work the processor needs to achieve.
G-Sync monitors all have the same NVIDIA designed video processor which is made to keep input lag low, making them some of the quickest monitors available today.
Displaying the image
Once the monitor has processed the image, it is ready to be displayed on the screen. This is the step where the video processor sends the image to the screen. The screen cannot change its state instantly, and the amount of time it will take depends on the technology and components of the display. Some monitors feature an "Overdrive" setting that can add an adjustment to make this process faster. This can work well, but it can also introduce unwanted artifacts. We cover this in our motion blur tests.
How to get the best results
PC monitors tend to have a much lower input lag than the average TV, making input lag less of an issue for most people. More sensitive users can adjust settings on most monitors to reduce it even further. As a general rule, try the following (which is how we set up the displays in our tests):
- Set the monitor to Game or Instant Mode
- Disable all picture enhancement settings
Additionally, you can try different combinations of settings/modes/inputs until you arrive at whatever balance of features and input lag that you like.
- Some monitors will have a higher input lag when they have to upscale content. To avoid this, you can enable GPU scaling in your graphics card's drivers to offload this task to your computer. It might not produce as sharp of an image, but it should remove any additional input lag.
- The input lag varies slightly depending on the frame rate and refresh rate, so games played at 120 Hz tend to feel more responsive than those played at 30 Hz.
- The input lag also varies in time. On some monitors, it even varies +/- 5ms.
- The input lag of your monitor is not the only component in the chain. While the lag produced by your computer might be marginal, wireless mice, keyboards, and controllers tend to have significantly higher latency.
Input lag is the amount of time that elapses between performing an action with a source device and seeing the result onscreen. It’s important for general usage of a computer with a mouse and is particularly important for fast-paced, competitive games. Anything below 20 ms is not an issue for most people. We test to find the lowest amount of lag a monitor can have, as well as how much lag is present when a monitor is displaying a non-native resolution, or when it has features enabled such as HDR or Variable Refresh Rate.
Questions & Answers
Hey RTINGS Team,
I am struggling to decide if I should buy a 240hz or 144hz monitor. The question I am asking myself is, is there a difference in input lag (and maybe even motion blur) when using a 240hz native monitor in 144hz mode. I am considering to buy the BenQ xl2450. This would be important to now as I would like to switch to 144hz mode for the majority of games that don't reach the 240fps by far. I am talking about games like PUBG which I am currently enjoying a lot. I would only switch to 240hz mode when playing games like Fortnite or CS GO. From my understanding, this is how to do it. If there is a difference in input lag, would it be better to stay in 240hz mode? And for the case that I stay in 240hz mode and the game outputs about 140fps, would it require the monitor to duplicate the missing frames resulting in an increased latency?
Thanks for your concerns and keep up the good work. Your Tests are by far the best I have ever seen online :)
Thank you for your kind comments!
You won't benefit from a 240Hz monitor unless you are able to send a 240Hz signal (either by upgrading your PC or lowering graphics settings). Sending a 140 fps signal won't result in increased latency, but it isn't worth getting a higher refresh rate monitor if you can't take advantage of it. Please look at this article. Although it is meant for TVs, the same principles apply to monitors.
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