At 16:40 3/23/02, Mark Lloyd wrote [in part]:
Diopter lenses are another option, I personally
haven't used diopter since I've always had access to a
dedicated macro, but I hear they can be quite useful.
Be wary though cause diopters are like filters and can
be very bad like filters, they also put more glass in
the picture and can create flare like filters. A good
set of dioppters from makers like Hoya can run $40 or
more, and better brands naturally cost more. Be wary
of noname brands.
I looked into these once again some time ago. The "no-name" single
element, uncoated generic ones are generally atrocious optically. There
are some excellent multi-element ones considering it's adding more glass to
the original compound lens. However, the price of them is about the same
as the Olympus auto-tubes. The pro for [excellent] close-up aux lenses is
zero light falloff, which occurs with lens extension into macro ranges (the
"falloff" is not edge/corner, it's over the entire image).
An Aside for new and future extension tube users:
Experience in use develops a "feel" for which tube length(s) to use on
which lens. Likewise, estimating standoff distance also improves. I got
better at it after starting to think about how big the subject is compared
to 35mm film frame size and doing a rough estimate of the desired
magnification before reaching for a lens and tube(s).
A 35mm SLR lens is extended one focal length from the film plane when
focused at infinity without any tubes. Zero additional extension with lens
at infinity focus is technically "zero" magnification (of objects at the
critical focus distance: infinity). Magnification at the critical focus
distance increases from zero to life-size as additional lens extension
(including use of lens focus helical) goes from zero to the lens focal
length. Thus if I want something about 1/4 life size on film using an 85mm
lens (a subject field of 4 x 6 inches), I need to use the 14mm Oly tube
plus much of the lens' focus helical, or the 20mm tube from the AT-21
set. If I select the 135mm lens, it requires the 12+20 tubes from the
AT-21 set (the 36mm tube is just barely too long) or the 7+25 tubes from
the Oly set.
-- John
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