The rangefinder on my Century Graphic had the prizm go black. What happenend, and where can I get a new prizm without having to get a new rangefinder?
I spent quite a while getting this one to work with an 80mm lens and don't want to re-adjust another whole rangefinder for a while.

Thanks for any help.

Dave

Have you looked at the prism? Is it still where the factory put it?

Yes, it is still there. I had it working great last week. I took the cover off today to see why I wasn't getting any split-image. Now, there is no light reflecting through the prizm. A small flashlight beam will not shine through it, nor can I see anything through it when looking down through just the prizm. It's like the mirror has been covered up with the prizm still in place.

Dave

I decided to take the prizm out and examine it (I was an assistant camera repairman for four years back in the early eighties, so this isn't difficult thing for me to do). There appears to be a layer of moisture between the prizm silverd surface and the rectangular back plate. I carefully dabbed this with a cotton ball and reassembled everything. It works now, but is still a bit dimmer then it was. I've no idea where the moisture came from, though it has been quite humid for a few days.

Dave

Very curious, and useful to know. I suppose a layer of water interferes with the "total internal reflection" at the hypotenuse of the prism. I'd have thought the silvering would prevent that, but it does deteriorate over time.

We really ought to explore some of the shops that offer vacuum aluminizing services to the owners of reflecting telescopes. I also wonder whether the few outfits that advertise lens polishing, coating and recementing services can reapply the coating to beamsplitters.

With all the humidity and rain we've had this summer, I wouldn't be surprised to find water anywhere!