Eye Test

Using this at the opticians

Sit where you normally would. Laptop at your usual working distance, phone where you'd normally hold it.
Keep browser zoom at 100% (Ctrl 0 or ⌘ 0). Use A− / A+ here instead, so the sizes stay comparable.
Test one eye at a time. Cover the other eye, or let the optician do it with the trial frame.
Compare lenses on the same tab. Old lens, then new lens. Note the smallest line you can read comfortably, not just the smallest you can guess.
Hit Shuffle between goes so you're reading, not remembering.

What each test shows

Reading	Everyday text at real screen sizes. Your main acid test.
Letters	Overall sharpness: the smallest row you can read.
Red / Green	Whether the lens strength is balanced. Tell the optician which side looks sharper.
Dial	Astigmatism: lines in one direction look darker or sharper than the rest.
Lines	Blur in one direction only. Another way to spot astigmatism.
Contrast	Faint detail. Blur shows up in pale text before it shows in black text.
Terminal	Small monospace text on a dark background, the way you work every day.

This is a comparison tool, not a medical chart. Sizes are in screen pixels and depend on your screen and distance. Keep the screen, distance and settings the same and compare like with like.

Read down until the text stops being comfortable. Try each typeface.

Read each row left to right. Note the smallest row you get fully right.

Look at the black letters and rings. Which side looks sharper and blacker: red, green, or the same?

"The same" is the target. As a rough guide, red sharper usually means slightly under-corrected, and green sharper slightly over-corrected. Just tell the optician which side wins.

Do all the lines look equally dark and sharp? If not, note which ones stand out, e.g. "the 2 to 8 line".

All four squares should look equally crisp. If one looks smudged or greyer than the others, note which.

Wavy patterns or shimmer come from the screen, not your eyes. Try a coarser setting.

Same size, fading out. Note the faintest row you can still read.

Monospace at the sizes terminals and editors actually use. Watch for 0/O, 1/l/I and rn/m.