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Old 27th January 2017, 07:15 AM
Martin Rick Martin Rick is offline
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Default A couple of questions

Why is my 50mm f2.8 enlarger lans so much smaller than most on camera 50 mm f2.8 lenses?
What governs the coverage of a lens, i.e, what's the difference between a 135mm tele for 35mm and one of the same focal length for half plate?

Last edited by Martin Rick; 27th January 2017 at 07:23 AM.
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Old 27th January 2017, 09:40 AM
Lostlabours Lostlabours is offline
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Because it doesn't have a focussing helical and all the mechanism for auto diaphragm.

Coverage is largely design both optical and physical (casusing vignetting). A 135mm lens for a 35mm camera is a telephoto design these days. a 135mm for a Half plate camera would be a wide angle lens,

Just to confuse you the Schneider 360mm f5.5 Tele Xenar was sold in a barrel in a shutter for up to 7x5 and in a focussing mount for Exacta and M42 35mm cameras, same lens.

The head off a Leitz 135mm Hektor could be used as a 5x4 lens as it wasn't a telephoto design.

Ian
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Old 27th January 2017, 04:54 PM
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Martin Aislabie Martin Aislabie is offline
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An enlarging lens is purely an optical device - it just needs to cover the angle of vision of a 35mm negative.

The enlarger bellows does all the focusing activities and as Ian says, there is a mechanical aperture.

And, at f/2.8 its not exactly fast - so only small pieces of glass are required - even though most top end enlarging lenses still pack in 6 elements.

On a 35mm SLR, the lens and its optics need to be much more complicated.

The rear element of the lens needs clear the mirror swing of the SLR body - which makes the optical design much more complicated (and compromised)

The lens body needs to contain the helical focusing mechanism as well as the automatic diaphragm of the aperture.

As regards speed - most 50mm SLR lenses are f/1.8 or faster - so bigger glass elements are required.

In addition to the optical and mechanical considerations for an SLR lens, there are ergonomic issues too.

The lens needs to be big enough to be able to comfortably focus it and contain an adequately wide aperture ring with all its associated markings.

The lens design compromises necessary to clear the swinging mirror of an SLR are one of the main reasons why Rangefinder cameras optics were claimed to be superior to those of an SLR - particularly at normal to short focal lengths. In the early days (1950s) of SLRs this was clearly true. As to whether the practical optical advantages are still true in the 21st century is more debateable - in terms of purely optical considerations they must be - however, once you reach a certain level of optical excellence, other issues begin to take precedence.

Martin
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