Thanks for the reply-
That is what I thought also -- you can't change the focal length of a given lens: a 45 is a 45. I hope Michael doesn't mind, I'm going to paste a small section of his article (its from his review of the Pentax 645NII) here to make it eisier to find.
From the essay:
Greater Magnification: When using long lenses there's the greater magnification. To convert to 35mm terms a 67 lens' focal length needs to be multiplied by .5X. A 645 image needs a factor of .6X. So a 300mm Pentax 67 lens (which is like a 150mm on a 35mm camera) become the equivalent of a 180mm lens on a 645. Not a big deal, but helpful. (Of course cropping a 67 image to 645 dimensions accomplishes the same thing, but you know what I mean.)
Focus Confirmation: When used with non-autofocus lenses the 645NII provides a focus confirmation light ó even with 67 lenses. This can be very helpful in finding critical focus, especially in low light levels.
Using the Sweet Spot: All lenses perform better toward the center than at their edges. By using Pentax 67 lenses on the 645 format one is using the lens' "sweet spot", or central area, which exhibits maximum image quality.
Greater Magnification: While having greater magnification is helpful with long lenses it's a hindrance when using wide lenses. My widest, the Pentax 45mm f/4, becomes equivalent to roughly a 28mm instead of a 24mm equivalent lens with the 67 body.
(end of paste).
I understand that what is changing is the size of the film format. Instead of exposing 7 cm of film, you are using only 4.5 cm, (the 6 cm height of the film is a constant) exposed by the same size image circle -- you get the "sweet spot" of that circle with the smaller film.
At least that is what I thought I understood untill I read this article -- and then maybe its my simple mind that has trouble with anything beyond 2+2!!!
It was the last part of article that threw me the curve (NOT out to shoot down the article or Michael in any way please!) and maybe there is a typo there at the end (should be 645 body, instead of 67).
Anyway- you are probably right about just working towards getting a 35mm for the 645--again thanks for the info.
I think your math is backwards. I haven't read the article but a 300 (or 45) is the same FL on any camera it's mounted.
The difference is the 645 is a smaller format and "crops" the inside of the image circle more than the 67 does. This always gives the "effect" of a longer focal length.
a 45 (6x7 lens) mounted on a 645 would lengthen from a "very wide" angle of view to a "wide" angle of view.
Its the identical discussion as is commonly heard in 35mm form cameras i.e. using a lens on the canon full frame (1Ds) vs using the same lens on a crop camera (20D). A 24 on the larger sensor is wider than the 24 on a APS-c (roughly 35mm equivalent).
If you want a 35mm lens for your 645 you have to by a 35mm lens.
Hope this makes sense!