Two different problems
People often use “motion blur” and “ghosting” as if they meant the same thing. They look similar but they come from different causes, and sorting them out is the first step in testing.
- Motion blur is a general softening of moving objects. Edges lose their sharpness while they move, and the image looks sharp again when it stops.
- Ghosting is a trail or faint copy that follows a moving object, often brighter or darker than the background. It is often called a trailing artifact.
The two can occur together. A screen can show both smear from the way it holds frames and trails from slow pixels.
Why moving images smear
Most current flat screens are sample-and-hold displays. They show one frame and hold it until the next frame replaces it. Meanwhile, your eyes follow a moving object smoothly. Because the image stays still while your eye keeps moving, each frame is smeared across your retina. The longer each frame is held, the more smear you see. This happens even if the pixels change instantly.
A hypothetical example with plain numbers: suppose an object moves across the screen at 960 pixels per second, and the screen shows 60 frames per second, holding each frame for the whole frame time. During one hold, your eye moves 960 divided by 60, which is 16 pixels. The edge is spread over about 16 pixels. At 120 frames per second, the same motion gives 960 divided by 120, which is 8 pixels. So a higher refresh rate tends to reduce this smear, because each frame is held for less time. These figures are illustrative only; real perception depends on the content and the viewer.
Another way to cut hold time is to flash the backlight briefly, or insert black frames, so the image is visible for only part of each frame. This lowers blur, but it can reduce brightness and may introduce flicker, a trade-off covered in the article on screen flicker and PWM dimming.
Where pixel response comes in
Pixels do not change color instantly. The time a pixel needs to move from one level to another is the response time. If it is slow compared with the frame time, the old image lingers, and you see a trail behind a moving object. This is the ghosting.
Response time depends on the starting and ending levels, so the figure on a spec sheet reflects one kind of transition. Different transitions take different times, which is why the article on response time and what the numbers leave out is worth reading.
Some panels use a technique that drives pixels harder to speed up transitions, commonly called overdrive. Too little leaves trails. Too much overshoots, producing a bright or dark halo ahead of the moving edge, called overshoot or inverse ghosting. A setting that clears one problem may cause the other, so testing at different settings is useful.
The type of panel also matters. Different LCD technologies and OLED screens generally have different transition behavior, as described in the panel technology articles on VA panels and IPS panels, though results vary by model.
Simple visual tests
You can learn a lot without instruments, using a moving test image from any reputable test page or a short video you create yourself.
- Set the screen to its native settings, including its highest standard refresh rate. The article on refresh rate explains how to check this setting in the system.
- Use a moving object with clear edges, such as a bright square or a line of text moving across a dark or mid-gray background.
- Watch the trailing edge. A faint smear or a duplicate image behind the object suggests slow transitions.
- Watch the leading edge. A bright or dark halo ahead of the object suggests overshoot.
- Try different backgrounds. Transitions between dark shades are often the slowest on LCDs, so a dark-gray background can reveal trails that a white one hides.
- Compare on and off. Turn the overdrive or motion-clearing options through their settings and see what changes.
- Check readability. Scroll a page of text and see whether letters stay legible while moving.
A good habit is to track the object with your eyes, because that is what produces the smear that sample-and-hold causes. If you keep your eyes fixed and the object moves past, you see ghosting trails but less of the eye-tracking blur.
Photographic and instrumented tests
For a more objective result, people use a camera. A common approach is to photograph a moving object with a known exposure time and compare the blur width in the photo. Another uses an instrument that records how quickly a pixel changes level. Some methods pan a camera along with a moving test pattern to mimic the eye.
These approaches are more repeatable, but their results depend on the method, and different methods can give different numbers. Treat a quoted blur figure as meaningful only if you know how it was made.
Things that can be mistaken for display blur
Not every smear comes from the panel. Possible other causes include:
- Content frame rate. A film at a low frame rate looks juddery or smeared because of the source, not the screen.
- Compression. Streaming or video compression can soften motion.
- Input lag or processing. Extra picture processing can smooth or blur motion, and the delay itself is covered in the article on input lag. The picture modes in the on-screen menu often control this processing.
- Screen tearing. A horizontal break in the image is a different artifact, discussed under screen tearing.
- Eye and fatigue. Tired eyes can also see doubling.
Testing with a different source, or on a different screen, helps isolate the cause.
Who cares most
Motion clarity matters most in fast content such as sports, action games and scrolling text. Competitive players often care about it, which is discussed in the article on esports displays. For office work and still images it matters much less. As with other display qualities, the right standard depends on the use, and a good result on one test does not guarantee a good result on all content.