To see what happens and the effects of puting a mf lens on a 35mm camera find soeone with a Bronica ETRS series camera who has a 35mm back. You will see there is neither magnification or reduction in image size and the image size stays the same but the area covered is reduced. When you read that a 75mm mf lens is an equiv to about a 50mm 35mm lens, it refers to the area of coverage and not the magnification. If your friend has both the 50mm and 75mm lens switching to the 35m back and from the 75mm lens to the 50mm lens and you will see the area of coverage with the 35mm back and 50mm lens is very close to the 120 back with the 75mm lens.
This seems to be very confusing to the average person on the street now because of the digital advertising of focal lens lengths being published and then noting their equiv 35mm lengths.
- Lens focal length doesn't change between formats. Focal length is the property of the lens, NOT the format!
OTOH, as a side note, image circle / coverage requirements change between formats and this fact has to be taken into account when using lenses at different film formats, specifically in the case you plan to use a lens on a larger format. (Since you're currently reducing the format, image circle won't be a problem in your case; because the lens covers / was designed for a larger area than what is needed for the 35mm format...)
- A 150mm MF lens mounted on a 6x4.5 camera will give the equivalent (diagonal) field of view of a 92mm lens mounted on a 35mm camera. (I think this is where people got confused; what changes between formats is the field of view, again, not the focal length!)
- A 150mm MF lens will give the equivalent field of view of a ...SURPRISE!!!... 150mm lens when mounted on a 35mm camera!
Hope this helps & thanks for the funny thread! :D