This is a satellite post to that page with the test.
What is it?
The term itself is basically a “how good your vision is”, or how well you can distinguish small details. Much more small details than I can see dare to describe here you can find on Wikipedia.
What about the test?
The chart itself is a LogMAR chart, the abbreviation standing for (decimal) Logarithm of the Minimum Angle of Resolution. This is probably the most geometric visual acuity metric. Good acuity is thought to correspond to the ability of resolving details as small as 1 minute of visual angle, or LogMAR = 0.0. In the chart, the numbers to the right of the letter rows are LogMARs.
Now, latin letters have quite a few details, so a person with zero LogMAR shold be able to distinguish letters subtending 5 minutes at the eye. Half of this value, 2.5’, constitutes an acute angle (at the eye) in the right triangle, whose sides are half-height of the letter and the distance to the letter. Knowing LogMAR (basically, the angle), the distance and basic trigonometry, we can calculate the height of the letter that a person with this LogMAR would be able to distinguish. If you didn’t understand a thing, here’s an optometrists’ explanation of visual acuity with examples and pictures, more detailed and more coherent.
As for the technical realization of my chart, it uses absolute CSS units (mm) to set the font sizes for different LogMARs depending on the distance entered. In fact, only one size (for LogMAR = 1.0, the highest row) is calculated on-the-fly with JS as
fontSizeInMm = 2 * Math.tan((Math.pow(10, 1.0) * 2.5) / 60 / 180 * Math.PI) * (distanceInCm * 10);This fontSizeInMm is taken as 1em in the chart container, and the coefficients setting the sizes of the other letters in CSS were (for reasons unknown) generated in Python:
logmar = lambda x : 2 * math.tan((10**x * 2.5) / 60 / 180 * math.pi)
logmar_multipliers = [logmar(i/10) / logmar(1.0) for i in range(10, -4, -1)]
for i, lm in enumerate(logmar_multipliers):
print(f'.sloan-row:nth-of-type({i + 1}) {{')
print(f' font-size: {lm}em;')
print('}\n')Why did I take 70 cm as the default distance? Well, it was loosely based on the assumption of the distance between you and the screen being about the length of an outstretched hand. The latter, in turn, was based on some anthropometric data, which you can check out as well if you ever wondered what is the average length of different body parts. I’ll save you the disappointment, though: there is no that body part in those tables 😅.
The letters in the rows were taken from some real vision test chart. The font I used is called Sloan and is a special font intended for vision testing. It is distributed by a CC license and is available from Denis G. Pelli’s repo, where a description of the font can also be found.
Disclaimer
I guess I don’t need to reiterate (but I’m still doing it) that my test is not medical and not diagnostic. If your vision is poor (mine is), you don’t need a random webpage to tell you so (I don’t). Besides, if you’re curious, there are better tests out there. For example, you can take a look at this one by Zeiss.
Please also be aware of the limitations of modern technology: not all digital screens are expected to be able to render the low letter sizes well, especially if the distance entered is small.
Other references
More about acuity tests:
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Ricci F, Cedrone C, Cerulli L. Standardized measurement of visual acuity. Ophthalmic Epidemiol. 1998 Mar;5(1):41-53. doi: 10.1076/opep.5.1.41.1499. PMID: 9575537.