Formula for computer use bi-focals

Greetings,

What is the typical or ideal difference in diopter power between the
reading portion of the bi-focal lens & the computer screen (upper)
portion of the lens? I think I read somewhere that it should be a 1.00
diopter drop. For instance, I use +2.50 diopters to read with (about 16
inches distance). Would +1.50 diopters be ideal for the computer screen
(about 24 to 26 inches distance)?
Second question: the optometrist determined that +0.5 diopters was
adequate for distance correction for me(progressive lenses). I later
realized that in situations like sporting events or church, I see
distant detail best through the narrow middle of the progression &
slightly blurry through the top (+0.5) & more blurry through the bottom
(+2.5). Furthermore, I have found that while lying in bed watching a TV
about 15 feet away, an old +1.0 diopter pair of reading glasses are
perfect for seeing the screen. Is this a case of the tendency of a
patient to over correct when judging the proper strength of lenses?

All insights are welcome.

Thanks,

Rich

 

The difference between computer & near is usually around 0.75D but could
be anything from 0.50D to 1.00D or easily more, depending on the individual.

You mean is it a case of the patient giving the wrong answers in the eye
test? It's more likely a case of the optometrist not doing enough to
ensure your accommodation was fully relaxed during the testing process -
or, if the eye test was some time back, your eyes simply may have
changed since then.

Dom

 

Could also depend on where the book is held, and how far out the
computer display is.

Say, for a book at 15 inches, and a computer screen at 25 inches,
taking 40 inches as equivalent to 1 meter:

1/(15/40) - 1/(25/40) ~ 1.0 diopter

which is to say that you would need about 1.0 diopter more of
spherical correction for the book at 15 inches than for the
computer screen at 25 inches.

Of course, if you cannot do simple arithmetic, you will need to
consult a Dr. for a subjective opinion.

OP, who appears to be ~ 0.5 diopter farsighted, could find a pair
of Walgreens readers for every non-distant visual task, even if
totally hyperopic. Or he can get everything rolled together in
a single pair of $400+ fancy eyeglasses (which he might not even
like).