Re: New Rx

On Jul 26, 9:40 pm, p.clar...@xxxxxxxxx wrote:
On Jul 26, 11:51 pm, "Ms.Brainy" <mikabra...@xxxxxxxxx> wrote:

Why? What's so magical about 20 feet?

20 feet is termed optical infinity. anything beyond 20 feet will be
properly focused with the same lens power that provided clear vision
at 20 ft.

I know that.

And 20/20 = 10/10 = 2/2 etc. They are all equivalent just like they
are equivalent mathematically.
these ratios simply mean-- what letter size you can read at X feet /
what letter size the population norm can read at X feet.

I cannot read at 20' what the population norm can (with or without
correction), but I can read at 1' approximately what a normal eye can
read at this distance, at least through my good natural L eye.

... to determine what is needed at any
distance closer than 20 feet is simply a calculation. Lens power (in
diopters) equals the 1/focal distance (in meters). Thus the lens
prescription to see an object at 12 feet (~ 4 meters) is the distance
prescription plus 1/4 diopters.

I didn't know that, thanks for the explanation. Does it mean that if
my distance Rx is -3D ( for 20' and beyond), then if I want my
sharpest vision to be at 12' the Rx should be -2.75D ? And for 3' (~1
meter) it should be (-3D + 1/1D) = -2D ? And for 18" reading it
should be (-3D + 1/.5) = -1D ?

With progressive lenses like you are getting, what the optometrist
does is measure the distance prescription, then specify the add power
which is usually set on the high end, like +2.50 or +2.75 (that number
is dependent on age, whether cataract surgery has been performed,
working habits, etc.).

Here is where I have the problem. Using you formula (as I did above)
gives a +2D add, yet both you and my doctor added +2.75D. Do all eyes
age in the same pace? Or is the +2.75D add designed for a shorter
reading distance?

The progressive portion of the lens will
therefore provide the proper lens power at all distances between
infinity and about 15 inches from your face depending upon how far
down in the lens you view the object (by holding it down or tilting
your head back). Determining the near add power is pretty simple for
progressives but more prone to error with lined bifocals since its
hard to know at what distance the patient does much of their near
work.

Are you saying that true progressives are really progressing gradually
and not just 3 zones with no lines? I have wondered about it and have
been told that the modern special computer glasses progress gradually,
whereas the "normal" distance progressives actually are tri-focals
with no lines.


I checked it with the following results, which I think are typical to
most people:

24" - 28" for computer
8' - 12' for TV viewing (in my case it's 10')
22" for relaxed reding
10" - 14" for reading & writing at the desk

All of these working distances can be easily achieved using a
progressive lens with a strong bifocal add. Your last measurement,
the reading and writing at a desk measurement, who cause me to want to
push your add power even higher to about +3.00D and even then you
would be more comfortable if you moved a little further back for
that-- say 13" - 16"

Well, desks height is pretty standard, chairs height can be somewhat
adjusted (with a limitation of sufficient room for legs under the
desk), but people come at various sizes, in addition to different
postures. I will have to sit uncomfortably erect in order to create a
working distance of more than 12", but I always have the option of
taking my glasses off, which is what I do now at such situaltions.

I am ready for a revolution!

Bull***-- many different progressive lens designs exist. Some are
optimized for larger reading and intermediate areas. Below is a link
to one such lens and I am not sure if it is even the best. A good
optician, such as Rob M who often posts in this NG, could tell you
many others and recommend the ones that are best.http://www.zeiss.com/4125682000258738/EmbedTitelIntern/SOLAMAX_Sell_S...

I wish Rob M would come in with his recommendation. I have checked
your link for Zeiss, and the optician I spoke with indeed recommended
Zeiss for my computer glasses. However, my comment of frustration was
regarding the HEIGHT (from top to bottom) of the mid zone, not the
width. Zeiss product has a wider mid zone, but not taller. Anyway,
if the lense really progresses gradually (not jumping from one zone to
the next, as in bi- or tri-focals), then the hieght is less important,
I presume.

BTW -- as your cat surgery was 4 weeks ago, then you should be healed
quite well and there shouldn't be further Rx changes to worry about.
Its time to go ahead with getting your new glasses.

Also, I would recommend getting a good anti-reflection coating on your
lenses with a no-stick teflon finish. Transitions would be a good add-
on if you are outside in direct sunlight a lot. They won't darken if
your are behind the windshield of a car however.

Thanks for all the advice, I appreciate it.

.

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