# Optical Forums > Ophthalmic Optics >  Eye power and lens power

## Alteaon

Hello,

I am (easily) confused, so please humor me. 
I was reading an article that stated that a patient who is using a +1.50 add is actually using +2.50 of power, +1.00 of that coming from the eye's accomodative reserve. ( This is at 16 inches ).

Even at twice the distance (32 inches), that half the add is used, it still states that +1.00 is used from the eye itself.

The confusion is at:

A +1.75D add wearer gets +1.75 from the add, +0.75D from the eyes reserve.

Why is it not +1.00?

How is it determined how much 'accomodative reserve' power the eye is using? 

I'm sure this is probably a simple answer and I'm making it much more involved. 
This is almost too embarassing to ask, but hey...it's another thing to learn..

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## drk

Good questions...

The term "accommodative reserve" is goofy.  Think of it as "accommodation ability".

Example:

Nice 10-year-old reading at 33 cm.  Call her "Lettie".  1/.33 M = 3 diopters of accommodation needed.  Plenty of "accommodative reserve" for this kid to see clearly.

Sourpuss 65-year-old man-- let's call him "Chip"-- reading at 50 cm = 2 diopters of accommodation needed.  But Gramps has only 1/2 diopter of accommodation left, so he needs +1.50 from the lens.

There is no way in a story problem to know accommodative reserve.  It has to be measured in reality, or given to you in the problem.

In the +1.75 example, I think they mean:
If the person is reading at 40 cm, and the add is +1.75, that means the eye must be using +0.75D accommodation.

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## Alteaon

Thanks for the enlightenment. I see now where they got thier numbers and where I got derailed. 

Thanks so much.

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## OpticianVlad

From what I understand in order for your eye to focus at 16 inches'your eye must provide +2.50D of accomodation.  If your near Rx is +1.00 this means your eye is providing the other +1.50 on it's own "the accomodative reserve".

If your near point is within the 16" range then you don't need reading glasses.
which means your accomodative reserve is more than +2.50 D.

You can test your near point by taking a PD ruler and placing it between your eyes and pull out slowly until you can focus on the numbers clearly. the distance at which your number look clear is your near point.  You can calculate your accomodative reserve by converting your near point to a dioptric power of a lens of the same focal length as your near point.  
At 16" that's +2.50 D, if I remember correctly.

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## chip anderson

DRK: 

 Ole Chip ain't got no accomodative reserve left but he do got two and a half cylinder against de rule so he can read some pretty close, see about 20/25 at distance too.
 Now ain't neither one as good as with dem trifocals, but I can get by and drive, and read for short periods of time without correction.

Chip;)

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