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Old May 31st 09, 05:22 AM posted to rec.photo.digital,uk.rec.photo.misc
freddie
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Default Another question - How to convert medium format lens to equivalency of a 50mm normal lens (35mm camera) in APS-C digital cameras

On Sat, 30 May 2009 20:37:59 -0700 (PDT), Nicko
wrote:

Using the lens adapter almost certainly throws everything off


It depends if that lens adapter has an optical element in it or not.
Sometimes, not always, they contain a low-power negative achromat to focus
the image further back to compensate for the depth of the adapter itself,
increasing the camera lens' apparent focal-length. Not unlike a
between-body-and-lens telextender but of lower power, or a how a Barlow
lens is used for astronomical purposes. It all depends on what two
manufacturer's designs have to be mated. I have an old SLR Pentax
screw-mount to Olympus bayonet adapter that works in this manner. Quite
well too. The depth of the old Pentax screw-mount design requires a deeper
adapter with a required negative lens. Other adapters for other lens makers
do not because the Olympus design is already shallow-bodied, there's plenty
of working room to mate them without much change. If the adapter doesn't
contain an optical element in it then there'll be no change in the
calculations to find the 35mm equivalent focal-lengths for an APS-C frame
size.