What does it mean to have 20/20 vision?
In the US, visual acuity or sharpness is measured with the Snellen eye chart (that's the chart with the letters of varying sizes, usually topped with a big "E"). Each line on the chart has a corresponding number ranging from 10 to 400; the higher the number the bigger the letters on the chart. The test is reported as the test distance (20 feet) over the smallest row of numbers you can read at that distance. Normal vision is reported as 20/20. If your vision is 20/60, it means that you see at 20 feet what a normal sighted person sees at 60 feet. Picture it this way: if you are 20/60 and your friend is 20/20, the smallest line you can both read on the chart is the same -- except your friend is standing 40 feet behind you. There's no exact correlation with a glasses prescription, but if you're nearsighted you can guess that for every 0.25 diopters of prescription, your vision is one line worse.
Optical illusions are images that differ from the objective reality. But how do they work?
Basically, it's our minds trying to find the easiest way to look at things. At a first glance, we try to relate the image with the most basic and close interpretation of it, and only after a few seconds do we realize that separate details of the image don't even make sense. This element of surprise is also what gets us in the so-called perfectly timed pictures: first we see one thing, but then it can be deconstructed into something else.
We love our patients and we love the Giants. Naturally, we have extra special love for patients who love the Giants! Way to be a superfan, Erik!
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