For more than 100 years, Optometrists have used a typical eye chart in order to test your vision and find your most accurate prescription. To the patient, all eye charts must look about the same, but to me, there are vast differences.
A brief history of the eye chart: the typical “Snellen Chart” is named after a Dutch Ophthalmologist who designed the chart in 1862 and is comprised of block letters. These letters have a standard and specific geometry, with each black line being the same thickness as each space within the letter, and the height being five times the size of each block. Originally, only the letters C, D, E, F, L, N, O, P, T, Z were used. These are the wall mounted charts that we see in all doctors offices, school nurse stations and and health clinics across America.
Newer charts have a slightly different arrangement, and range from perfect block letters to fonts that even an eagle-eye couldn’t differentiate. The whole idea of the eye chart is to make it confusing and difficult to discern if your vision is blurry, but crisp if you can see clearly.
For instance, an F and a P look very similar if there is any slight blur. This is also true for a C or an O. Only the small openings at the far right differentiate these letters. It’s much easier to tell an A from a V even if your vision is a little blurry because of the letter form and the point facing either up or down.
One curiosity to me as an eye doctor is how many people guess Q during the exam. The only difference between an O and a Q is the ‘descender’ (the downward vertical tail, I looked it up!). It’s actually funny to me that mostly adults guess Q– a child will never guess the letter. I figure that kids don’t expect such trickery during their exam and don’t over-analyze.
In my exam room, I have three different letter configurations for each line vision. I still hear the same guesses and mistakes, and the same letters differentiate good vision with a particular lens versus blurry vision. The only advice I can give my patients is to never guess Q.