I bet she couldn't write him a formula to calculate the length of a piece
of string, either...

My wife was a chemist.  Her boss was also.  He wanted a formula to
convert volume into weight.  She couldn't do that.

Yes but your two "methods" don't really address her question, do they?

She is asking why you can't predict VA from refraction.

Is a cycloplegic necessary to do retinoscopy? Can't I use lenses with
cycloplegic?

Perhaps because they don't measure the same thing.

And it's possible you don't know what you're talking about.

Not a whit.

Pick either of your "methods," then find two people who measure the same.
Why does one see better than the other without glasses?

Hint: Answers include pupil size, higher-order aberrations, accommodative
spasm and erroneous refraction, plus neurology, and psychology, and there's
also the possibility that measured VA might NOT vary quite as much as she
thinks it does.

She wasn't asking for opinions. She was asking why subjective acuity varies
from one person to another your refractive STATE is the same.

-MT

Dear Katy,

In fact there are two DIFFERENT methods of judging
a person's refractive STATE.

1.  Use a Snellen and a trial lens kit.  (The so-called
"subjective" measurement).  In this method you
have the person read the snellen at say 20/50.  You
then use stronger minus lenses until the 20/20
line is cleared, with say a -3/4 diopter lens.)

2.  Measure with a retinoscope/cycloplegic (drops),
and determine a person's refractive STATE.

These two methods often do not agree -- and
can be profoundly different.

In fact it is possible to have 20/20 and a reafractive
STATE (cycloplegic) of -3/4 diopters.

This my explain some of the difference between
your refractive measurement and your current
visual acuity.

It is also true as suggested by the ODs on
sci.med.vision, that as you get older your
refractive STATE will change in a positive
direction, and your Snellen will clear.

That might also explain why you can
get along with no minus lens.

Obviously, opinions vary concerning your
distant visual acuity.

Best,

Otis

Is there a scientific explanation for why two people with the same
prescription appear to have different levels of vision?  For example, I've
read somewhere that two people who are -3 won't necessarily see the same
without glasses, one might see better/worse than the other.  What factors
influence this?