Mamiya M645 finder prism damage caused by degrading foam

[cinejerk]

I sure wish I had found out about the prism foam damage issue before I bought my camera.
I did find some info buried in a thread about the Mamiya M645 foam damage issue but it took a
lot of searching.
Maybe this thread will alert prospective purchasers as to what questions to ask the sellers.
I don't know if anyone has found a reasonable place to re-coat the prisms. I did find someone
who coats telescope mirrors but at $50 it probably isn't going to happen.

 

[cinejerk]

I guess these things are just too old to care about.

 

[benjiboy]

I would just buy another prism finder.

 

[cinejerk]

That's exactly what I'm going to have to do Ben.

Repairing them is not very cost effective.

These aren't being made anymore and from what I hear they could all have this problem.

Like a ticking time bomb !!

 

[wblynch]

Think of the future when they are not easily purchased. We will have to repair them. I would hope people don't chuck them in the bin today. Pretty soon they will be precious and then "worth it" to get them fixed.

 

[ic-racer]

I'm curious about this (I don't have a M645 prism to check). I have not seen the goop from a bad SLR mirror-bump foam cause a problem with the mirrored surface. The goop has always come off with solvent, leaving the silvered front mirror OK.

 

[John Koehrer]

On the prisms I've seen the reflecting surface looks like it's given a protective(?) coat of lacquer or enamel. THat may be the weakness.

 

[cinejerk]

The guy I talked to who does telescope mirror's said that when he does a prism like that he does paint the back with lacquer.
Maybe you've got something there. It doesn't seem the foam residue would attack straight aluminum.
But it doesn't seem it would penetrate lacquer either.
I don't know.

 

[MattKing]

I always understood that the problem with the prisms was predominantly one of element separation, not damage from degrading foam?

 

[chriscrawfordphoto]

Some Olympus 35mm SLRs had the same issue, degrading foam released chemicals that ate into the prism. Which 645 prisms are we talking about? I have a 645 Super with non-metered prism. Is this something I need to watch for on mine, or was it only older 645 models whose prisms had this?

 

[wblynch]

Good point. There are several different ones even for the M645. I have two of the metered prisms and they show no signs of foam degradation.

 

[cinejerk]

Yes they have separation problems too. The problem I'm talking about is from foam degradation under the hold down clamp. It shows up as a rectangle in the viewing area. I'm not sure if the newer prisms have that problem or not.
That's the main problem. There doesn't seem to be a whole lot of information around.
Here is an example of the problem.

http://www.pbase.com/pganzel/image/71821169

 

[ic-racer]

Thank you for the picture. Clearly a problem there.

 

[cinejerk]

I finally got a response from the guy that was repairing the prism in the above link. He said when he got the silvering cleaned off he cut a front surface mirror and just taped it on.
I wonder how that compared optically to the original. I suppose that doesn't matter all that much because it doesn't show up in your final image. Unless it impedes your ability to focus in the first place.