150mm Finder Question.

[mtokue]

Hi Guys, I'm usually over in the RD-1 Threads, but I have a question
that I hope someone overhere can Help with.
I have a friend who used to have a bunch of Mamiyas unfortunately
The bulk of which went walkabout(Stolen) on his last assignment in Cambodia.
He has decided to go with Pentax 6x7 when he gets his insurance.
Anyway my question is this. One of the few things he has left is a
Finder for the 150mm and as he doesn't need it anymore he has
offered it to me at a very good price (about 45US).
Now. I do not own a Mamiya M6 or M7, but I was wondering if
someone could tell me what the rough FOV would be equal to
on a 35mm system.
Thanks
Mike.

[mtokue]

Hey ManGo, Thanks for that, I remember looking through it a while back,
And all I remember is that it was nice & bright. So I'm wondering whether I
coud adapt it to use on the RD-1
Thanks
Mike.

[jlw]

This is really rough, but you can make a super-crude approximation of this for any lens by using the old geometric principle of similar triangles: the rays from a lens form a triangle whose base is the frame size (vertical or horizontal) and whose height is the focal length of the lens. Triangles that are geometrically similar always have the same base:height ratio, so you can use this to see what focal lengths will be equivalent on different format sizes.

On a 6x7 cm camera (actual frame height about 57mm) a 150mm lens would make a triangle whose height (150mm) is 2.63x its base.

On a 35mm camera (24mm frame height) that would correspond to a lens with a focal length of 63mm.

On an R-D 1, with its 15.6mm-high APS-size sensor, it would correspond to a 41mm lens.

Of course the widths will differ, because (as Manolo already has pointed out) the proportions of 6x6cm, 6x7cm, 35mm, and APS-size formats are all different -- although 35mm and APS are pretty close. The 35mm/APS format is proportionally wider than 6x7, so if your finder is for a 6x7cm camera, your actual field of view will be wider than what the finder shows.

However, if you like to crop your digital images to fit on 8x10 paper, you're already losing image off the sides -- so that 150mm finder might be a pretty good match for a nice new C-V 40mm f/1.4 on your R-D 1!

[hoot]

I'm confused; there are no external finders for Mamiya 6, and as far as I know, the only external finders for Mamiya 7 are those for the two wide angle lenses.

[mtokue]

Sorry ManGo wasn't my intentionto cause a headache!
Hoot, I'm 99.9% sure that he got it with the lens but I'm not sure
what camera it was for the 6,7 or 7II

Mike

PS I have decided to get it so I'll let you know how I find it with regards to use on the RD-1,And Jlw thanks for your explanation too!

[jlw]

Could that finder have been for the Mamiya Universal, rather than a Mamiya 6 or 7? The Universal was an older, very versatile RF camera that could accept either roll-film or sheetfilm backs and a range of interchangeable leaf-shutter lenses.

[jlw]

Originally Posted by Manolo Gozales

Hey jlw

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.

Regards

ManGo

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.