Mirror and Prism Degradation in old SLRS

[Yashinoff]

My question is this: is it possible to re-silver old pentaprisms? I've seen people get good results re-silvering old mirrors, but that is a flat plane of glass. Further is the quality that is fine for a mirror good enough for a camera pentaprism?

I ask because I've noted degradation of prisms on several Edixas and Prakticas I have, which results in a hazy, hard to focus image. When a good prism was substituted the finder image became sharp again, so I know it is not a problem with the focus screen or eyepiece. I also have one Praktica which has acquired an odd greenish tint to the reflex mirror which causes a similar issue but also makes the viewfinder image much dimmer.

On a couple of the Edixas there appeared to be a fungus growing on the prisms:


These prisms all gave hazy viewfinder images.

So how about it? Can prisms be re-silvered? Is it worth the bother? (ie. will the quality be high enough for use in a camera?)

Thank you in advance for any useful insights.

[E. von Hoegh]

I'm pretty sure all or almost all of these were "silvered" by vacuum depositing aluminium onto the surfaces. I helped a friend do this on a telescope mirror, and it's doable at home if you have the equipment - a vacuum pump, big enough bell jar, and a high current power source to evaporate the aluminium. Look around the DIY astronomy websites.

I have two prisms I'll be redoing at some point.

[Chan Tran]

I guess that they do silver the prism but as I understand the prism by its nature would reflect light without the silver isn't it right?

[Ian Grant]

Yes you can get mirrors and presumably pentaprsims re-silvered. Vacuum Coatings Ltd in London are good, I've recently had a new front surface mirror made for a pre WWII quater plate SLR, relatively inxepensive at £16 inc postage.

You'd need to remove the old coating, this is usually done with acid which doesn't affect the glass itself, then after cleaning the item is re-silvered, usually these days it's an aluminium layer but just as good if not better than silver. The two surfaces entry/exit would need to be masked off.

Once you got it back you'd need to paint the coated srfaces of prism here I've not thought what would be ideal yet.

Ian

[Ian Grant]

I didn't have time to post a link last night to Vacuum Coatings Ltd / Csientific Mirrors. They do list camera prism restoration. I'm going to get a new front surface mirror for a sceond SLR shortly - a pre WWII Ensign Speed Roll film reflex and if I get time may get them to refurbish the mirror and prism from an old Exa II.

You'd need to check whether the'll strip a prism before re-silvering, they will strip & clean mirrors although I had them make a new one for my Ruby Reflex. Their prices are very reasonable and they are extremely helpful.

Ian

[Diapositivo]

A laboratory specializing in optical constructions should certainly be able to do the re-silvering job.

People like these guys:

http://www.adrianololli.com/index.asp

[Yashinoff]

Thanks for the answers. I'll keep looking to see if I can find a U.S. service for recoating... but if not posting a couple of prisms isn't too big of a deal.