Difference between eye testing machine and eyeglass lenses?

What is the physical difference between the optics you look through
during the "which is better" objective part of an eye exam and the
optics used in eyeglass lenses?

Am I correct in assuming that multiple lenses are used in the testing
machine?

 

....

 

Dear Rishi,

Mike is right. Measurements in a darkened room looking
through the small aperture of a phoropter can produce
profoundly different results that the results
you get if you use a trial-lens kit and a Snellen
with room illumination -- under DMV test conditions.

I have talked to one man on "i-see" who could pass
the DMV with 20/30, yet some "over-prescriber" had
given him a -1.75 diopter lens. Fortunately he
rejected this powerful minus lens, and always
passed the DMV.

Yes, you should check your own "free space" Snellen,
for fear of getting "over prescribed".

Thanks for the advice and suggestion, Mike.


Best,

Otis