Mamiya Prism

thuggins · Nov 30, 2020

So I got the new (to me) 645 kit and am really enjoying it. After the first couple of frames, handling the "box" became intuitive and the controls are well laid out. But there is a line across the prism where it has desilvered. This appears to be a common problem as pretty much everyone I've seen is like this. I got a good price on an unmetered prism with the same issue.

On closer inspection, this doesn't seem to be a prism at all. It just looks like a mirror. Is it really a mirror? If so, how does one replace it? I'm assuming the screws are hidden under the leatherette, as there don't seem to be any visible.

shutterfinger · Nov 30, 2020

There are several versions of the 645 so check out the service manuals at https://learncamerarepair.com/productlist.php?category=2&secondary=22 and pick the one that is correct for the camera you have.

Marcelo Paniagua · Nov 30, 2020

Best way to know if prism/mirror is check if image is correctly aligned horizontally . If image left side is displayed left side on the prism viewfinder, then there is a prism.

Marcelo

reddesert · Nov 30, 2020

All of the Mamiya 645 prisms are pentaprisms, like a 35mm SLR, not mirrors. (Edit: this is wrong as MattKing pointed out below, there are a few pentamirror viewfinders in the later 645 system like the 645E.) The only Mamiya mirrored viewfinders that I know [knew] about are the older Porroflex 3-mirror system for the Mamiya TLR and possibly a periscope like mirror finder for the TLR.

The desilvering in the 645 prisms appears to be due to foam that contacts the upper surface of the prism. Some types of prism seem to get it worse than others. I don't know of any way you could fix it other than disassembling it and having a new aluminum reflective coating deposited by a coating shop.

MattKing · Nov 30, 2020

reddesert said:
All of the Mamiya 645 prisms are pentaprisms, like a 35mm SLR, not mirrors. The only Mamiya mirrored viewfinders that I know about are the older Porroflex 3-mirror system for the Mamiya TLR and possibly a periscope like mirror finder for the TLR.

Mamiya also used penta-mirror finders in the 645e and some of the accessory finders available for the 645 Pro and Pro-Tl - the AE Reflex finder 402 being an example.
They function similarly to the penta-prisms - images are right side up and they are laterally correct.
They are lighter, simpler and (likely) less expensive to manufacture. They also incorporate a variable diopter adjustment.
They provide a less bright image, and their view of the corners of the image isn't as good as with penta-prisms.
With respect to the 645 Pro and Pro-Tl, the penta-mirror finders often came as part of a kit.
You can recognize them as having a tube between the eyepiece and the rest of the finder.

reddesert · Dec 1, 2020

MattKing said:
Mamiya also used penta-mirror finders in the 645e and some of the accessory finders available for the 645 Pro and Pro-Tl - the AE Reflex finder 402 being an example.
They function similarly to the penta-prisms - images are right side up and they are laterally correct.
They are lighter, simpler and (likely) less expensive to manufacture. They also incorporate a variable diopter adjustment.
They provide a less bright image, and their view of the corners of the image isn't as good as with penta-prisms.
With respect to the 645 Pro and Pro-Tl, the penta-mirror finders often came as part of a kit.
You can recognize them as having a tube between the eyepiece and the rest of the finder.

Thank you, I was so fixated on the Porroflex (having just taken one apart) that I forgot about the existence of pentamirrors. There are also some inexpensive 35mm SLRs that used pentamirrors, with the same disadvantage of a dimmer view.

The original 645 system (not the 645E / Pro / Super), which is what I should have restricted my comment to, all used pentaprisms. Some of those have the desilvering problem. I'm less familiar with the Pro / Super era and its viewfinders.

Deleted member 88956 · Dec 1, 2020

Which 645 did you get? The line you speak of suggests the original, in which case as already stated there are no mirror equipped options.

Mamiya Prism

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