Intermediate lens mounts are a subclass of lens mounts. One could say that they are not ‘proper’ lens mounts because there has never been a camera that would have natively accepted intermediate mount lenses. Instead, the point of intermediate mounts is that they:
• Have a long flange focal distance to make room for adapters, and
• They offer a wide range of adapters that allow the use of that lens on a broad range of lens mounts.
A brief history of intermediate mounts
For a time period (ca 1960-1985) intermediate mounts were relatively popular, as they did offer some clear advantages.
For lens manufacturers the advantages were:
Design and manufacture a lens (to have an intermediate mount) and offer adapters to allow its use on a wide range of systems.
For users/buyers the advantages were:
Buy a lens with an adapter for your current system, and should you later change system, you simply needed to buy a (relatively inexpensive) adapter to keep using your lens.
Using intermediate mounts
Importantly, many intermediate mounts are somewhat finicky and were never intended for regular adapter changes. Some intermediate mounts actually needed tools for replacing one adapter with another.
This is because the intention was never that you’d have (e.g) a Canon FD camera, one Adaptall->FD adapter and a bunch of Adaptall lenses. Instead you were supposed to buy all your lenses with a pre-installed adapter, and – should your next body use another mount – you were recommended to take all your lenses to a camera shop where your adapters would be changed (typically you would be offered some trade-in value for the adapters you were no longer using).
One old-timer camera salesperson I talked to said that a lot of the negative experiences photographers had with intermediate mounts were due to improper installation of adapters. Having dabbled a bit with interchangeable mounts (with little issues), I do not want to insinuate that you’d need to be a lens technician to change adapters, but it’s definitely not something i would recommend you do in the field, in the midst of a shoot or in inclement weather…
The contemporary relevance of intermediate mounts
Today, with a resurgence of film photography, some of these intermediate mount lenses are again finding use, and photography retailers are congratulating themselves for not having designated remaining adapter stocks to landfills (or kicking themselves if they had).
However, for those intending to adapt legacy lenses to modern mirrorless cameras, intermediate mounts have a bit less appeal as such photographers are less bound to using a single mount. However, if you get your hands on an intermediate mount lens using a mount that you do not have an adapter for (e.g. Miranda), it is quite likely both easier and cheaper for you to buy an intermediate adapter to a mount you already have an adapter for than getting a new adapter (e.g. Miranda->Nikon Z)
Finally, there is currently some discussion (here’s JAPB’s contribution to that discussion) that would point to that the concept of having an intermediate mount still has some appeal.