We should note that third party lenses are pretty much limited to 35mm cameras.
Medium format cameras have rarely had third party lenses available.
Manufacturers have a natural desire to keep all the profits from lens sales. As one example, simple lens mount adapters often cost hundreds of dollars, or are no longer made.
It is also true that the market for third party medium format lenses is thought to be much smaller than for most 35mm systems.
Medium format photographers demand the maximum optical quality, so lower cost third party lenses are not as popular as in 35mm.
Medium format zoom lenses are very rare for the same reason, and very costly due to low sales volumes and complexity and size of these lenses.
There have been a few major exceptions, chiefly the Kiev- Pentacon- Exakta series lenses (by Schneider, Zeiss, Arsenal..) and the Bronica S2/EC series.
The Kiev-88 and Kiev-60 cameras had a variety of lenses by Schneider, Zeiss, Pentacon, and Arsenal/Kiev. These multiple lens lines are partly the result of the breakup of Germany into East and West Germany during the Cold War, each with their own camera production and lens lineups. The latest lenses are made by Arsenal in the Ukraine and sold by Kalimex s.r.o. in the Czech Republic.
The Bronica S2/EC
series had OEM lenses by both Nikon and Zenza, and third party lenses by
Komura among half a dozen other sources. Many of these lens makers would
be considered third party lens makers. Examples include:
One interesting point is that many 35mm long-focus lenses (non-compact design telephotos) could cover medium format 6x6cm as well as 35mm film.
Lens heads are special lenses which do not include the camera mounting hardware. They are usually designed for bellows mounting, with the bellows mounting on the camera instead of the lens own mounting. You only have the optical head of the lens.
See 135mm F/4 Preset Nikkor Bellows Lens
The above dual format 35mm and 6x6cm lenses are all 35mm telephoto lens heads. Telephoto lenses have lots of coverage, especially when stopped down. By contrast, wide angle lenses have a difficult time covering the full 6x6cm frame. Moreover, telephoto lenses can easily be mounted at longer distances from the film plane by their very nature, unlike wide angle lenses. So it is relatively easy to find 35mm long-focus design telephoto lenses which can be used to cover 6x6cm.
How easy this is can be illustrated by Cambridge Camera Exchange's use of some Samyang/Phoenix 500mm f/8 lenses from 35mm in Dr. Optik adapters for mounting on 6x4.5cm and 6x6cm focal plane shutter cameras. Models include the Pentax 645 and Mamiya 645 systems. Herbert Keppler of Popular Photography reviewed these lenses in November 1997, concluding they showed surprising sharpness even into the corners of the medium format frame.
Yet these remounted 500mm f/8 lenses cost only $395 US, instead of the thousands of dollars usually asked for medium format lenses (albeit auto-diaphragm rather than preset lenses).
Why should the medium format camera have a focal plane shutter? None of these 35mm lenses has an integral leaf shutter built into the lens. So if you hope to use 35mm lenses on leaf shutter cameras (e.g., Hasselblad 500c/m cameras), you have to supply a leaf shutter with the lens mount that matches the Hasselblad mount.
There is a very rare and pricey microscope adapter with a lens mount and leaf shutter built into the adapter, but the cost is quite high. Similarly, you could use a Hasselblad lens with bad glass as the source for such a leaf shutter mount. But otherwise, there is no way to mount and use the 35mm lenses without supplying a leaf shutter for the lens.
Focal plane shutter cameras such as the Mamiya 645, the Bronica S2a/EC, the Kiev-88, the Rolleiflex SL-66, or the Pentax 67 are all much easier to adapt to use 35mm telephoto lens heads. The shutter is there in the camera body. So you just have to supply the lens mount of the right type and distance, and a way to stop down the lens, and you are in business!
Most 35mm macro lenses and many regular lenses can be used for macrophotography on medium format focal plane cameras. The extra length of the longer medium format camera body acts as an extension tube with the 35mm macrolens or regular lens. The usual macrophotography tools such as bellows, extension tubes, and reversing rings can also be used with good effect.
Finally, there exist a wide variety of 35mm front-of-the-lens adapters
which can be used on medium format cameras:
In general, medium format lens options are restricted to the OEM, unless you decide to homebrew your own lenses.