One of the most interesting accessories for the Pentax 67 system is a lens adapter for Pentax 645. You can now buy for cheap the body of its ordinary non-AF models, and it can become a good additional or backup camera for the primary Pentax 67.
For one thing, it is possible to keep a different type of film there to be able to shoot simultaneously on the black-and-white or color film. There is also no need to take with you lenses for Pentax 645. Take, for instance, the 90mm F2.8, which may work similar to the normal lens, while the 105mm F2.4 will become a great portrait lens almost equivalent to the 125mm focal length in the 6×4.5 format.
This adapter allows the use of any 67 lens on 645 bodies in full manual and aperture priority (Av) exposure modes. It has aperture coupling and automatic diaphragm release levers which allow to use Pentax 67 lenses almost like 645 lenses. It permits open-aperture metering while the aperture value is not displayed in the Pentax 645 viewfinder. It is like setting aperture ring of the 645 lens to the non “A” value.
Also, it is possible to use Pentax 67 macro extension tubes for shooting macro and closeups with automatic aperture release continuing to work well. In the original manual Asahi says that with some lenses corners get slightly darkened when shooting with open aperture or light falls off at corners when aperture is closed by some f-stops. I’ve used the adapter with my 90mm F2.8, 105mm F2.4, 135mm F4 Macro and 165mm F2.8 lenses and I’ve never noticed any light falling off.
I have for some time enjoyed the original non-AF body, having bought it for just 100 USD. And I can say that its exposure meter works fine, and AA batteries are available everywhere. Therefore, you won’t find yourself in a situation that once happened to me.
It is also worth mentioning that inter-frame distances, which can at times pose a problem in the Pentax 67, are simply non-existent with 645 cameras due to the completely different film transport mechanism in these cameras.
As we know, a lens of X mm focal length is a lens of X mm focal length, and no matter what camera you put it on. So, the difference is that the Pentax 645 is a smaller format camera, which “crops” the inside of the image circle more than the 67 format does. In a nutshell, it is like using only 41.5mm of film length instead of exposing 70mm. Take a look at first and second examples to understand how cropping is actually applied.
This always creates the effect of a “longer” focal length, which can also be called an equivalent focal length. To calculate it and compare with native 645 lenses a focal length of the 67 lens should be multiplied by a “crop” factor. The Pentax 645 has 56×41.5mm frame size, so its diagonal is 70mm. The Pentax 67 has a 55×70mm frame size and a diagonal of 89mm. The “crop” factor is 89/70=1.27 and, as a result, a 90mm lens “turns into” 114mm, 105mm, into 133mm, 135mm, into 172mm, and 165mm, into 210mm.
Unfortunately, the 6×4.5 format proved to be not very convenient for me, if not entirely uncomfortable. And eventually I replaced the camera and adapter with another Pentax 6X7 body with MLU which costs a bit more. I found it easier to use two almost identical cameras on a photo shoot. But Pentax 645 allows to take 15 frames on a single type 120 film roll (16 frames for Pentax Pentax 645N and 645NII), which may be important in case of a limited film stock. Do not forget that this adapter also allows you to mount lenses on digital Pentax 645d and 645z which only operate in stop-down metering mode .
There is another third party adapter with the same full-automatic functions: Fotodiox PRO PT67-PT645 (II). It is a bit cheaper than a gennuine Pentax adapter but still has all the same features, including 67 outer bayonet mount.