External finder problem

[Towermax]

I just got a decent Canon 35mm finder in a batch of odds n' ends. I've been looking for one to go with the 35/2.8 on my IIF2. It's an older finder without the auto-parallax foot.

This one will clean up well, but has one problem--there's a roughly oval-shaped 1/2 inch multi-colored ring on the front surface of the front lens. It's one of those rainbow effects that looks like separation or a coating problem. It's still there after careful cleaning of both sides of the lens.

I've attached a photo taken through a magnifying glass.

Does anyone have any idea what this might be? Is it something that commonly occurs with these finders? Any possible solutions?

 

[peterm1]

This finder is very easy to disassemble and clean. The front chrome ring screws off and then if memory serves me correct, inside there is a black metal frame cut out to give the correct angle of view. Remove that (it slips out- there is only a tab top and bottom to keep it placed correctly and stop it from rotating) and that gives access to the front lens. The lens will drop out. From memory when I got mine several years ago it had similar grunge - looks a little like oil on the glass. Mine cleaned up with nothing more than a little spirit. I think I lost some coating but it still works just fine. Then again yours looks in a little worse condition. Good luck

 

[Bill58]

On all VF glass surfaces, use some lab grade acetone (not fingernail polish remover!) on a Q-tip TWICE to clean it up, then follow up w/ a couple of Q-tips w/ distilled water. Rotate the wet Q-tip end w/ the other end each time you use the acetone and water. Same procedure works great on lenses too.

Note:they are easy to disasemble.

 

[Towermax]

Thanks for the replies. Your memory is spot on, Peter--it is easy to disassemble, exactly as you stated. I took it apart last night and lightly cleaned both sides of the lens, but didn't want to use any strong chemicals until I got some feedback.

Bill58, I don't have any acetone, so what about naphtha (lighter fluid)? I've got a can of 100% pure naphtha. (If it needs to be acetone, I'll try to find some.)

 

[Bill58]

Nope--naptha leaves a slight oily residue.

 

[Towermax]

Well, I tried acetone and distilled water with no success. The problem area looks like it's just below the surface, though that may be an illusion.

Does anyone know the construction of this lens--is it made from more than one piece of glass?

 

[Bill58]

Sorry my suggestion didn't work. If it's just the front glass that is flat and has no VF lines on it, why don't you just replace it? Any optical store or maybe Edmund Scientific (cut to fit) should be able to furnish you w/ a new one.

 

[Towermax]

If it's just the front glass that is flat and has no VF lines on it, why don't you just replace it? Any optical store or maybe Edmund Scientific (cut to fit) should be able to furnish you w/ a new one.

The front glass has a strongly concave rear surface and appears to be one piece. It's possible that it is two pieces cemented together, but I would need definitive info before trying to take it apart.

The finder still works just fine--the rainbow effect only shows up under strong light at a specific angle.