At least a couple of things at work here.
A 75mm lens for 6x9 has to create an image which is in focus, evenly
illuminated, etc. over a much larger area of film than a 75mm lens for
35mm. Than means many of the elements behind the front, esp the rear
elements, can be smaller and lighter and the overall support structure
can be smaller and lighter. Assuming from the subject line that you are
talking about a Mamiya 6x9, it has a much larger image circle than a
Rollei 6x6.
Once you get into wide angle lenses, SLR lenses have to have their
optical center behind the physical lens so they don't interfere with the
mirror, which requires more complex design with more elements than a
lens for a rangefinder, which can protrude deeply into the camera.
There is also simple quality of engineering. One of the photo mags years
ago showed pictures of two torn down lenses, a Leica and a cheap third
party lens. The cheap lens had 3-4 times as many parts to accomplish the
same things . More parts means bigger and heavier.
andrew fildes wrote:
However, you're definitely more limited in choice of lenses especially on
the long end. Not to mention issues with size and weight. First off, there's
the issue that every lens you have is a longer focal length than the 35mm
equivalent, they're also usually bigger. My Mamiya's 80mm 2.8 is huge
compared to my Zuiko 85mm 2.0 for instance.
--
Andrew "Frugal" Dacey
Always fascinated by this problem of size. Look at the 75mm f2.8 Planar on
a Rolleiflex compared to that Mamiya. Small, even with the shutter built in
although there is no focussing rack of course. I suspect that there are
other things at work here. SLR lens, 35mm or MF, have to be larger I
suspect to cope with the extended lens to film distance required, although
the Rollei has a fair distance in the box.
I just borrowed an old Olympus Wide-S rangefinder (late 50's?) which boasts
a Zuiko 35/2! In this incarnation, that beloved Zuik has a 46mm filter ring
(rather than 55) and is a lot smaller than the OM version, despite having a
Seikosha MXL shutter built into it!
AndrewF
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