The test is simple in theory: you need to look at an image and move away from the screen until you no longer see a certain pattern. For this to work, it is crucial that the image is displayed properly. Every single pixel in the image must end up on the screen either as a single pixel or a square group of pixels with no blending. If the image is downscaled by any factor, or upscaled by a non-integer factor, the test will be useless. The following section allows to check whether the image will be displayed correctly or not.
The test cannot work if the test image is distorted by your computer, smartphone, tablet, projector, … (In case you are unable to correct any distortion should it occur, you can still fall back to a paper print-out.) Before proceeding, take some precautions to improve your chances for success:
Once you've gone through the above checklist, we can do a first verification. The following image is not the acuity test, but a custom image that easily reveals if it is not displayed pixel-by-pixel.
This image must fit on your screen and give an overall impression of a plain uniform grey area. Only when you look very closely (without needing a magnifying glass), you should see alternating black and white lines in a concentric pattern. If this is OK, then move on to the test.
If you see anything else as illustrated below, like an obvious ‘X’ across the image, blots, patterns, or banding, then the image is being rescaled and you must rectify this first.
If on the other hand the image does look like a uniform gray area but you can only see the separate lines with a magnifying glass, your display is either of too high resolution, or, well, there's no other way to say it: you have very poor visual acuity. Don't worry, there is still a good chance the test can work if you try the following workarounds.
The first thing you can try in case of problems, is to open the verification image separately. If this causes it to be displayed fine, remember to open the actual test image separately as well, and move on to the test. A second thing you can try, especially if you cannot manage to see individual lines, is open either a 2× upscaled version of the image, or a 3× upscaled version of the image. This might help to display it correctly on so-called ‘retina’ displays. Again, if this solves the problem, remember to do the same with the actual test image and move on to the test.
If you are trying this on a smartphone or tablet and cannot convince it to display the image correctly, there are two things you can try. The first is hitting this magic button: which will attempt a workaround that ought to work for many devices. Re-check the verification image. If it is OK, then the test image will also be OK and you can move on to the test.
If the magic button failed, you can try some manual fiddling. First measure the actual width of the visible part of your screen (in portrait orientation) and the verification image (if it is wider than your screen, measure the height, or try to zoom out). Enter the values here:
This will bring both the verification and test images close the the correct size, but you need to do the final fine-tuning yourself. Try using the ‘Size-/+’ buttons below the verification image to nudge the dimensions until it looks perfect. Mind that if your browser is zoomed to any custom zoom level or misbehaves otherwise, it will likely be impossible to get it right.
There can be situations wherein this verification may appear OK while the test image below is not displayed correctly. The only way to be absolutely certain is to look at the test image such that you can see the individual pixels (e.g. use a magnifying glass, walk up to the projection screen, or take a macro photo) and check that there are only pure black and white pixels, and that all lines are either exactly one, two, three, or any other integer number of pixels wide.
In case you cannot get the image to display right no matter what, you can either find a regular desktop PC or laptop with an old-fashioned monitor or a TV, or use a paper print-out of the test image, although this also is not entirely fool-proof.
Once you have checked that the test image will be displayed fine or found a suitable workaround, you can start with the true test. The test setup is simple. All you need is some kind of tape ruler or any other reasonably accurate measuring device. If the above verification image checked out right, use the image below. If you had to open the verification image separately to make it work, do the same for the test image now. Similarly, here are the 2× upscaled and 3× upscaled versions of the test image should this be what you need. Or, use your paper version if necessary.
The image consists of two areas. One area has vertical alternating stripes of black and white pixels, the other has horizontal stripes. To help avoid bias in the test, the online versions of the test are animated: the two areas swap places every two seconds. If you are using the paper version, you can avoid knowing beforehand which area contains the horizontal stripes by asking someone else to hang the page on the wall in a random orientation.
Proceed as follows: move away from the screen or paper until you can no longer discern which part of the image contains the horizontal stripes, and which the vertical. The image must look like two perfectly uniform gray zones divided by a line. Do not rely on subtle differences in intensity between the two areas or other tricks: this test is all about seeing the actual horizontal and vertical patterns. Then, slowly come closer again while keeping on focusing on the center of the image, around the dividing line. When you have just reached the point where you can tell which area contains the horizontal pattern and which one the vertical, measure the distance between your eye and the screen. Call this distance D. Next, measure the width or height of the test image (the grey area). Call this value W.
You can now obtain your visual acuity limit angle α with the formula α = arctan(W/(320⋅D)), or this calculator:
It does not matter what unit you use for D and W, only that the units for both values are the same (e.g. if you use centimetres for D, you must also use centimetres for W). The lower limit for α is about 0.5 arc minutes. If you get something much smaller, something is wrong with your test setup.