separating diopter lens pieces

[gregoryzim]

Hi

The shortage of parts means I need to make a new finder diopter lens.

The one on my Bronica finder is made of two pieces of glass. There is the lens (convex on surface facing finder - concave on the back surface) which is adhered to a piece of plain glass where one surface is ground out to a convex shape for the concave lens to sit in.

The end result is a square block of glass flat on the front surface facing the eye and convex on the back surfaces.

How do glass makers join view through items like this? Sure there is no adhesive where they fit together? (that would cause blurr wouldn't it)

If they do literally glue them together why isn't it blurred?

Either way - any idea how to separate them? (these 2 pieces as a single block do have adhesive smeared over their edge surfaces - so maybe they are just fixed at the very edges?)

Any experience on multiple layer optic glass joining methods and how to separate without damage much appreciated thanks.

 

[retinax]

Yes there probably is an adhesive that joins them, it's called an optical cement. Usually a special epoxy since the sixties, natural tree resins before that afaik. You do not want to separate them. The new lens would have to have the surface to be cemented ground to exactly the same curvature.
Most cameras just add an extra lens for dioptre correction rather than exchanging the whole viewfinder eyepiece lens. Hopefully someone can tell you if and add-on dioptre correction lens from another camera fits, if not, make one yourself from your old glasses.

 

[peterm1]

I had an old pair of WW1 binoculars in which the balsam in the lens elements had crazed. I soaked them in methylated spirits (alcohol). It dissolved the balsam and I was able to separate the lenses in the element. You can try this.

 

[gregoryzim]

Tks - I will try it. If I'm making a new one - what is balsam sold as or where is it sold? Do they still use it in optics or is there some new chemical product in the 21st century? 🙂

 

[peterm1]

Tks - I will try it. If I'm making a new one - what is balsam sold as or where is it sold? Do they still use it in optics or is there some new chemical product in the 21st century? 🙂

Optical balsam does show up from time to time on eBay but is not cheap I guess because it has to be highly refined and is highly specialized. https://www.ebay.com/itm/Canada-Bal...m3ad8577ba9:g:-qoAAOxycmBS3Yut&frcectupt=true

There are some more modern alternatives I understand. This appears to be one such alternative but there is no real description of its uses but it seems to be the sort of thing used. https://www.ebay.com/itm/NEW-Astro-...m3f7ccf2808:g:XvMAAOSwY3BZHdkM&frcectupt=true

You can Google this subject and I am pretty sure you will find how the process is carried out - I seem to recall seeing such posts at some time. Use this search phrase "alternatives to balsam for optics" and "optical lens glue"

Example http://forum.mflenses.com/re-cementing-doublet-elements-with-canadian-balsam-t34467.html

In the event that you are ever working on an existing balsamed element an alternative if yours is not too far gone is to gently heat the lens group having first removed it from the lens barrel, cleaned it etc then when the balsam melts under heat to push the elements together. And I believe the google search will find posts where people have discussed how they have done this.

 

[Timmyjoe]

Not completely clear on what you are asking, but if you have a camera that you would like to add a diopter to, I wouldn't mess with the optics currently in the viewfinder, I would add an optic on the "eye side" of the optical viewfinder. I've made these in the past (where there were none available for purchase) out of drugstore bought reading glasses, the ones with plastic lenses. I pop out one of the lenses and carefully machine it to fit the existing eyepiece of the camera. Works surprisingly well.

Best,
-Tim

 

[gregoryzim]

Very useful replies everyone - thank you. I have added a lens from a pharmacy $5 lens cut to size - works perfectly 🙂 Rgds Greg