What is "Prism Separation?"

I've never heard of the term before. I was looking at a 1970's film camera that listed "prism separation" as a defect. I googled the term, but pretty much came up blank. The only "separation" I could find was balsam separation, but I thought that was only in lenses.

Thanks for any help.

No, it's possible in all optical things that are glued. And prisms often are, consisting of different glass pieces joined by glue. What you will see is an unclear, often brown(ish) irregular spot in the viewfinder, most likely along the edges.

It could also be a rather less correct way of saying that the silvering of a prism has begun to come off. That manifests itself as a dark line (it usually occurs on an edge) or spot in the viewfinder.

Prism separation can be repaired by unglueing and reglueing. Soaking the prism in hot water for a while will (probably - my experience is limited to fixing some microscope stereo heads) soften the glue and make it possible to remove it without doing damage. Reglueing the prism in perfect alignment (and dismantling and reassembling the camera to get at the glass itself) however is another task...

Silvering problems are, i believe, much harder to fix.

And that dark vertical line is not caused by separation, but by the loss of silvering (most often black paint, and not silver).

I believe the casue is often a foam rubber pad that is put between the top of the prism and the prism housing. That stuff decays into something nasty that does evil.

Unless they specify otherwise, sellers typically use the term "prism separation" when referring to the dark vertical line that can develop straight down the center of the viewfinder image as some prisms age. Depending on thickness of the line, it can range from slightly annoying to unusably distracting. This type of "separation" is unfortunately very common in highly sought after items like plain unmetered prisms for the Nikon F, most of which have decayed in some way from minor to hopeless. The two that I bought in 100% flawless condition four years ago have begun to develop a couple brown spots and the early stages of the center line, disappointing but predictable given its a known issue with F prisms.

If the seller is being very precise and correct, "separation" might be referring to the eyepiece optics connected to the prism. These are often cemented doublets similar to what you'd find in a lens, and they can separate in the same manner. It may or may not interfere with using the prism finder, depends on the design of the camera.

The Rolleiflex prism finder seems to consist of two cemented parts, and often suffers from balsam separation. You can see a picture of it here (scroll down):

LINK --- Rolleiflex Prism Finder - Fixing a Loose Prism

Genuine "separation" of a multi-part prism is probably more likely with vintage medium format, or in the adjoining eyepiece optics.

Most modern (post-1960s) prisms for 35mm SLRs were solid glass, as are many older variations like Nikon F, so most viewing defects tagged as "separation" will be traced to desilvering. This is usually caused by decaying foam spacer pads between the prism and its housing, as q.g. has said: the same crap as the back seals that rot away, mirror bumper strips that turn to glue or crumble, inaccessibly buried mirror box noise dampers and shutter roller washers, battery chamber positioners, etc. Everywhere this junk was used in a camera poses nasty decomposition risks: sadly by the time mfrs figured that out millions of classic cameras had been built with the stuff (to our everlasting dismay).

Prism desilvering is significantly more prevalent in some 35mm SLR model series than others, mostly contingent on whether the mfr thought to place a plastic, epoxy or tin shield between the prism glass and the body foam, and whether that shield has any cutouts allowing foam contamination. Age isn't as significant as particular camera or prism model: i.e. your chances of encountering a desilvered 1968 Nikon F plain prism are twice that of a 1968 metered Photomic prism and ten to twenty times more likely than a 1968 Nikkormat, while it almost never occurs in any of the F2 and later Nikons. The Olympus OM-1 and OM-2 are often plagued by foam rot desilvering. Unrestored Leicaflex SL and SL2 tend toward desilvering or discoloration.

Getting a prism re-silvered can range from difficult to impossible: few techs are willing or able to do it properly (preferring to install a new prism, or one harvested from a donor body). Usually too costly to be practical, exceptions being the Leicaflex (collectible enough to justify it) and OM1/OM2 (intact replacement prisms can be harvested easily and fairly cheaply from the far less popular but compatible OM10). Caveat emptor: study the track record of any vintage film camera model that tempts you, ask questions of the seller, get a return guarantee, and don't overpay.

I agree that what's being referred to is more likely to be a loss of silvering on one of the edges of the prism. I have on old Nikon F prism that's in bits at the moment because of this very issue. It's not easily fixable.

Other 'prisms' prone to this are early Mamiya 645 eye-level finders that develop a horizontal dark line. You just learn to ignore it, since it's well out-of-focus when your eye is fixed on the screen image. I believe that the affected Mamiya finders are actually Porro-type mirrored finders, but I've never taken one apart to find out.

There are two ways to do silvering.

The one traditional for many years for telescope mirrors uses chemistry including silver nitrate that plates out silver onto glass.

With a concave mirror, one can put the solution in and let it sit for a while. As this gets close to photographic chemistry,

it might not be too far off for many here.

The other is vacuum evaporation. There should be places around that do this for industrial customers.

It isn't so hard to do, and they might even be able to add a mirror into an existing order.

The usual ones have a large bell jar, and can hold a fair amount of area.

Ones I used to know did aluminum, and I don't know what works best on prisms.

Some years ago I needed a (non-camera) part spot welded, found a place that would do it.

The normal customers were industrial, but they found a way to do what I wanted for $5.

Thin Film Coating - Vacuum Process Engineering

sounds like they might be able to do it. I didn't put in "request quote", though.

Thinking back to my years of repairing cameras, I am remembering some instances of this.

The cause of the desilvering could not always be easily determined. It might have been a bad day at the factory, perhaps an environmental issue (humidity, temperature, etc.) not obviously determined.

Or it might have been something as simple as using cello tape to affix the prism; an issue found on a number of FT series Canon cameras, and always fatal to any silvering in contact.

Of course if the desilvering was the only negative impact on the camera, from whatever source, it was probably still capable of making good photographs, once the user was determined to work around the image in the viewfinder.

And I am not picking on Canon..... I think it was a short run of the FT series which had the problem. I had an FT-Bn in the same time period for several years and never had any problem with it.

For those contemplating having a prism resilvered, I would only say that you might easily buy a parts camera with a good prism and get the prism "grafted" into your target camera for much less cost.....