"Jim" <> wrote:
>In addition there are extension tubes which enable focusing closer than the
>mount allows.
The problem with extension tubes is that manufacturers limit the
close focus mechanical range *because* of the generally
worsening quality of the optics as the focus distance is
reduced. Hence, a nice tack sharp lense that can focus to 1
foot may not be anything like tack sharp with enough extension
to focus it at 6 inches...
Generally though, for lenses specifically optimized for close
focus, extension tubes or bellows are nice. Macro lenses,
enlarging lenses, and regular (not zoom) lenses that are
reversed will all generally provide good results.
The above is particularly significant if close focusing also
requires a "flat field" lense. One of the big tradeoffs between
optimizing for close focus or not is a balance between
astigmatism and flatness of field. If a flat field is not
required (e.g., insect macro images) then a lense that is not
particularly noted for a flat field might well exhibit less
astigmatism, and produce a sharper image. But if the object to
be photographed is a postage stamp, a flat field lense will make
a significant difference.
>Which one you use depends on the nature of the use. For the one use that
>you specified, a couple of close up adapters should work fine.
>Just keep the aperature toward the small end of the range to reduce the
>inevitable distortions.
Using smaller apertures will extend the depth of field, and
might reduce spherical aberrations, but not the inevitable
pin cushion or barrel distortions...
>The various diopter strengths are needed to allow for differing distances
>from the lens.
>For example, the focal length of one of these adapters is 1000/diopter.
>Consequently, with the lens set at infinity, the furtherest distance from
>the lens would be 1000/diopter.
To put that into perspective, a +2 diopter will result in the
"infinity" focus position on a lense actually focusing at
1000/2, or 500mm. Which perhaps isn't really very interesting
because it doesn't indicate how *close* it will be when the
focusing mechanism is set to its closest focus setting.
>The two element adapters are intended for use with telephoto lenses because
>the simple single element lenses cause way too much degradation of the
>image.
Not true. Multi element close up lenses are "achromatic",
meaning the different elements are designed to cancel the
chromatic aberrations of each other. That works exactly the
same regardless of the focal length of the lense they are
mounted on.
However, diopters provide more dramatic magnification when
attached to longer focal length lenses, hence it is typically
more useful to put them on short or medium focal length
telephoto lenses than anything shorter. A close up lense in
front of a wide angle lense has very little effect.
--
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska)