Glasses - Near and Intermediate

[Peter Burke]

Most likely one for RDS I think.

I'm wondering if there would be any difference optically between glasses ordered as:

1. Intermediate with an ADD of +2 and PD specified as 63

and

1. Near with an ADD of +2 where the supplier adjusts the buyer's actual PD of 65 down to 63 to compensate for the inward angling of the eyes.

If different I thought it might be that intermediates could be ground as expecting the light from the vision source to be parallel whereas the nears may be ground angled inwards to compensate for the origin as a close-up point source.

Any thoughts?

[Tricky Dicky]

Seems you are looking for varifocal glasses where for close up work you look through the bottom of the glasses and other distances higher up the lens. Your eyes soon get accustomed to looking through the different part of the lens depending on what distance you want to focus on.

Richard

[Peter Burke]

Tricky Dicky [e-mail] wrote in news: [e-mail]:

Nope, the query is about single vision lenses. It's really a query about manufacturing and terminology.

In short I'd like to have the option to specify the effective PD required for the distance that I know I will be working at rather than have a one size fits all adjustment made that seems common when you request nears/readers.

[Michael Chare]

Why not ask an optician what they can have made for you. Youn perhaps need a good local optician rather than a branch of one of the chains.

[R D S]

Light only travels straight through the centre of a lens.

Having probably done some math in the past we tend to knock 1mm off intermediates. If the lenses aren't set at your distance PD and you are converging slightly you will be catching bent light and it will go straight into the eye.

In reality I doubt it would make any difference whichever way you do it, and would probably fall within acceptable tolerances anyway.

[Peter Burke]

R D S [e-mail] wrote in news:sihml4$q58$ [e-mail]:

Thanks, that's useful info.

[newshound]

Interesting, I have never thought about a possible effect for PD.

People who use binoculars and binocular microscopes tend to be able to lock in to stereo easily; others (like my wife) find it very difficult.

I suspect that the effect of theoretically incorrect pupil spacing is much the same as the effect of adjusting eyepiece spacing on microscopes or binoculars. While it is certainly more comfortable to get it right, there is a tolerance band. I have always assumed that the eye muscles are doing the necessary adjustment, although I guess the actual stereo creation is done in the brain so perhaps there is some adjustment there too.

I use varifocals, but plain lenses at +1.5 for computer work, and then add +1.5 or +3.0 clip-ons to those for close-up work. On reflection, these are less comfortable than using a low power stereo microscope which has eyepiece adjustment. My +3s give me a working distance of

15cm, so maybe that is close enough to justify a PD "correction".