That is an excellent question, John.
You are right that a close-up filter like the 500D is like "reading
glasses"
for your lens, but as you have recognized, you're putting another
piece of
glass in front of your lens, which "could" degrade the quality of your
image, and the benefit is not that great. The 500D, though, is
very good
glass...I have one myself, but I rarely use it.
What it sounds like you want is "working distance". To answer your
dilemma,
you'll need a longer macro lens, such as the 180mm offerings from
Canon,
Sigma, and Tamron. I've used the Sigma, and its optics are
excellent and it
handles very well. Your working distance with the 180mm lens will be
roughly 9 inches when you're at full 1:1 magnification. This is,
as they
say, as good as it gets. You could add extension tubes to the 180,
but that
would shorten your working distance. You could also add a 500D,
but it
would again shorten the working distance. That's where
teleconverters come
in. If you pair the Sigma 2x TC with the Sigma 180mm macro, for
example,
you can either double the working distance and still get 1:1, or
you could
double the magnification to 2:1 (2x) at the minimum focusing
distance (about
9 inches), or something in between.
By the way, extension tubes have less and less effect with longer
lenses,
because the magnification you get from tubes is directly related to
the
"shortness" of the lens you pair them with. For example, if you
put 50mm
worth of tubes onto a 50mm lens, you'll be adding 1x to the
magnification
(because you have 50mm worth of extension on a 50mm lens). If you
put those
same 50mm worth of tubes on a 100mm lens, you'll add only one-half
"x" of
magnification because the tubes make up only one-half of the length
of the
lens.
I hope this makes sense. I'm glad you asked this question, because
I am in
the process of wrapping up a long article on insect macro
photography, and I
completely neglected to mention all of this...I'll have to go and
add it
now.
--
My Extreme Macro Bug Gallery:
http://www.beautifulbugs.com
Ever wondered about the MP-E lens? Read my review here:
http://www.vividlight.com/articles/2914.htm
.
