Originally Posted by Manolo Gozales
I think you're answering the wrong question. We're not talking about putting the Mamiya 150mm onto a 35mm cammera, but equating the field of view one gets using a 150mm lens to some unknown 35mm system lens.
Now, say that the standard lens on the Mamiya is a 90mm. This would give (approximately) the same field of view as a 50mm lens on a 35mm camera. A 150mm is 150/90= 1.67x longer than the 90, so this should equate (again approximately) to a 35mm camera lens length of 50x1.67=83.5mm. So a 150mm 6x7 lens, and therefore any associated aux vf, gives approximately the same FOV of an 85mm lens.
I thought the question was "What lens on an R-D 1 would match this finder?", and if so, I'm pretty sure the geometry works the same way regardless.
You can't necessarily go by 35mm equivalents because medium-format "normal" lenses tend to run wider in terms of angle of view than 35mm lenses -- 50mm is actually longish for the 35mm film format, but we got stuck with it because when Oskar Barnack was experimenting with his prototype for the Leica, the handiest way for him to get a lens for it was to swipe the 50mm tele lens off a movie camera!
Really, it all comes down to angles of view, and the geometry says that a 150mm lens that covers a format height of 57mm will have the same angle of view as a porportionally shorter lens covering a proportionally smaller format. In other words, if lens X on camera Y has a focal length of 2.63x its format height, it will have the same angle of view as lens X' on camera Y' that also has a focal length of 2.63x its format height. That's because the triangle formed by lens to film base, and the triangle formed by lens to subject area, are conjugates
-- they'll always have the same proportions even though their sizes are different.
So, a 2.63x lens on a Mamiya (the 150mm on a 57mm frame height) will have the same angle of view as a 2.63x lens on a camera with a smaller format (such as the R-D 1 with its 15.6mm-high APS sensor. 15.6 x 2.63 is 41.03, so the finder that covers this angle of view on the Mamiya will cover the same angle of view on the R-D 1, and should work with this focal length of lens.
Better? Or is this still the wrong question?
I'm not trying to be argumentative, so please let me know if I've made any obvious logical errors or if my answer doesn't make sense. I figure that this is going to be a common problem for R-D 1 users -- there are so many lenses you can put on the thing, and then you've got to figure out what finder to use for each -- that it will help everyone if we can come up with a simple, clear way to work this out.