"Oh, that 35/shift..."
Save your money for the 24 shift. The price might be falling -- one just
went for around $750 on eBay.
The 24 shift is an exquisite chunk o' glass. It's worth owning just to drool
in the general vicinity of.
Plastic-barreled lenses have several theoretical advantages. The right
choice of plastics could eliminate the need for lubrication. And the cam
system in a plastic zoom lens can have "any" shape, something not practical
with a metal-barreled lens.
Alas, all the plastic-barreled lenses I've seen have sent me looking for a
trashcan.
Here's an easy way to remember how to compute the bellows factor. Forget the
formula; if you understand the following principles, you'll get it right.
>> The magnification is the lens extension (from infinity) divided by the
lens's focal length.
>> By definition, a lens's focal length is the distance from the lens's
optical center to the film plane. As you focus closer, the lens's focal
length increases. The f-number -- which is the focal length divided by the
aperture -- also increases, which requires an exposure increase.
>> The effective aperture is the nominal aperture multiplied by (1 plus the
magnification). The exposure varies as the square of the f-stop, so it will
be the square of (1 plus the magnification).
If this doesn't make immediate sense, think of it this way: If you stand at
one point and focus closer, you'll see the image in the viewfinder get
larger. That means the light from the lens is spread over a larger area at
the film plane.
QED
Exceptions! These rules don't apply to telephoto and inverted-telephoto
(retrofocus) lenses, whose exit pupils are (respectively) closer to and
farther from the film plane than they would be in a "normal" lens. But such
lenses are not commonly used for close-up work, and a TTL meter provides the
correct reading, anyway.
< This message was delivered via the Olympus Mailing List >
< For questions, mailto:owner-olympus@xxxxxxxxxxxxxxx >
< Web Page: http://Zuiko.sls.bc.ca/swright/olympuslist.html >
|