Originally posted by SEVASTIANOS Can someone please explain to me how this is even possible? This goes against my understanding of everything I have read on this forum as well as every website I have researched. Why do websites like B&H say for example that a Pentax 645 120mm lens is equivalent to a 96 focal length on a 35mm? If that is the case then why does my 45mm 6x7 lens look like a 45mm on my Pentax 645z as
Sorry to hear about your troubles.
Photoptimist already explained why your 45mm lens (made for 6x7) gives you the same field of view like your 45-85mm lens (made for 645 & used at 45mm) when used on the same (645Z) camera.
The reason, why those equivalent calculations and indications exist, is, to give someone who already has experience with a certain camera (format) some ideas which lenses he/she will need when switching to another sensor size.
When you use a lens with a focal lenght of 45mm on a APS-C camera it will be a short telephoto lens, on a FF camera it will be a normal lens, on 645digital it will have the field of view on the border between wide angle and normal lens, on a 645 camera (film) it will be a wide angle lens and on a 6x7 camera it will be an ultra wide angle lens
(lenses made for different cameras (sensor sizes) do, as photopimist already said, only differ in the image circle size they cover and the flange focal distance (and lens mount) -> lenses intended for larger formats are larger in size; you can usually use lenses, which are built for a larger format, on smaller formats with the help of adapters but not the other way around -> with the exception of some APS-C lenses that also cover the FF image circle)
The equivalent example from B&H you mentioned means, that when you use a 96mm lens (zoom lens @96mm) on your K-1, you will get the same field of view as when you use a 120mm lens on the 645D/645Z (and not that this 120mm lens will work like a 96mm lens when you use it on a K-1)