I just thought I'd share a few simple tips on getting the best prescription on your next eye exam.

(1) The doctor will usually isolate a single line of letters for you to read.  Make sure that line is somewhat difficult to read.  It's much harder to tell the difference when you can easily make out each letter.  As the testing continues you may need to switch to a more difficult line.  Search for particular parts of letters that are more difficult to see than others and use those to tell subtle differences.

(2) Take your time!  As they try to figure out your prescription they'll break it down into simple choices "This one or this one".  If it takes you five or six tries, take five or six tries.  You'll only be in the chair a few minutes extra but you'll probably be wearing those glasses for years.

(3) Blink and look around.  While you're repeatedly going back and forth between those two choices, your eyes will dry out.  If you stare in the same direction for a long time your eyes' geometry will change slightly.  Between each "This one or this one", blink a couple times quickly and glance around only moving your eyes.

Assuming you don't have unusual issues with your retinas or other parts of your eye, you should have no problem at all reaching 20/20 and I you'll probably end up more toward 20/15 or 20/13.