"add "on eyeglass rx, "still confused"?

Question

okay, so if my rx is:
od -3.25 cyl. .50 axis 55
os -3.50 cyl.50 axis 55 add 4.00
so then if i'm understanding this right, the 4 is subtracted by my rx and my actual add on my glass would be 4.00 - "3.50"my lens power?? is that right or am i totally screwed up/!

Answer 1

umm no it is actually more simple then you think..... add power literally means adding the power on the "sphere" number (the first number) in your prescription.
Your astigmatism "cyl" and location of astigmatism "axis" correction remains the same because it is relative to the sphere number.

So your full reading correction for the reading portion in your bifocal/multifocal lenses would be :
OD +0.75 cyl -0.50 axis 055
OS +0.50 cyl -0.50 axis 055

They add the +4.00 to the sphere number.... thats about it!
Hopefully this helps you a bit :)

Answer 2

The first set of three numbers for each eye...
Sphere power,
Cylinder power (Astigmatism),
Cylinder axis (direction)
set up the eyes for distance vision, to infinity.
The numbers may be + or -

The addition (almost always the same in both eyes, and always +), adjusts the Rx to a closer range if the eyes won't comfortably do it for themselves, classically for people over 45.

It's dependent on distance. If you need a +4.00 add for close reading, you may well need a +1.75 add for a mid-range computer screen.

The final lens power is the sum of the basic prescription and the add, allowing for signs.

With your Rx being just over -3.50 you will be close to your reading power with no glasses on at all.

(-3.25, +4.00) => +0.75 /-0.50 ax 55
(-3.50, +4.00) => +0.50 /-0.50 ax 55

Answer 3

Look at it this way, sort of...

lenses bend light. windows do too, but not so much. lenses bend light so that the light going through either to focus at a point like a magnifying glass and the sun to make a little fire....or the lens bends light out as IF it came from a place closer than far away. Near sighted people are already focused at a distance. Their glasses bend light so when it hits the eye it is at an angle that lines up exactly to the nearsighted distance of that person.

The Power is equal to one over the distance (in meters) or P=1/d

So a lens that takes parallel light and focuses it at one meter away, that'd be a +1.00 lens.

If it focused it a half meter away, +2.00.

third of a meter +3.00, fourth of a meter +4.00

You and myopes or shortsighted or near sighted people like you have eyes that are TOO powerful. You are already focused at just under a third of a meter or about a foot in front of your eyes. To make you see distance, they give you an Rx that weakens your refractive power to zero. Those lenses bend light as if it came from a third of a meter in front of you. And you see that far just fine.

The little bit of astigmatism is the 0.50 thing which can be either in a + form which would mean that at 55 degrees for both eyes, the power is even slightly stronger, or a minus form or - form, which means it's a half diopter less strong at that axis.

So now that your powerful eyes have been adjusted to see at distance, you want to read. You either look over your glasses to read, or you use progressive lenses which get stronger as your gaze goes down or you use bifocals or trifocals that add +4.00 diopters more plus, than your distance Rx. That moves that focal plane in from infinity backwards how much?

right! a fourth of a meter.

P=1/d=1/onefourth= 25cm.

Do you really read at 1/4 of a meter in front of you? that's like 9 inches or so. Seems a bit close to me. Most are fairly unhappy with about 15 inches which is about the +3.00 readers you see at the drug store.

So now you know what the numbers mean. If you are age about 20, an add of about +1.50 would be enough (Optometrists please feel free here.....)

At age 40 or so, a +2.25 is usually sufficient as you do want to have a little bit of accomodative power to use so you can move the page in or out a little.

If you have more questions on this let me know. But with the opticians and optometrists that answer these things, you should be fairly well answered as eveyone sort of takes a different tac to try and explain this stuff...people seem to all say the same things but with different 'views'.
cool.