A close-up lens is designed for "flat-fieldness" at it's "macro" position
whereas a normal lens won't focus to a flat plane when highly extended.
Close-up lenses can be used as a normal lens.
A close-up lens is also generally designed to go to 1:1 (lifesize) or 1:2
(half-lifesize) without any additional extension. This is very handy as it
doesn't require any additional equipment or componants. Disadvantage? You
must turn, and turn, and turn, and turn, and turn....
Extension tubes (what I have used for years) on normal lenses can be used
and for outdoor photography works very well. For example, I use extension
on my 100/2.8 to achieve the nearly perfect "flower portrait." With the
100/2.8 I can get down to 1:2 without any nasty things happening to the
focus plane. In fact, the edge-to-center focus plane variation with this
lens is about 3mm. Stopping down to F5.6 gives me sufficient DOF to cover
for this variation. I personally find F5.6-F8 works very well with this
lens/extension combo.
Extension tubes give you the advantage of using high-quality, lower-cost
lenses for close-ups. Inexpensive extension tubes add a whole new
dimension to your photography. For copy work, though, I'd still stick with
a true macro lens.
Diopters add close-up capability without loss of light that a macro lens or
extended lens will give you. When lightloss is a critical issue, there are
few options and the diopters are terrific ways of quickly getting close
without changing lenses. In fact, a single zoom lens and a diopter is all
you need on a "weightfree" fieldtrip.
Diopters aren't perfect though. You will lose flat-fieldness and
resolution at the edges of the image and they are highly suseptible to
flare. Be very carefull about shielding the lens from the flashes and
other bright lights and keep them very clean. I personally use one that
was for the IS-1 on my 100/2.8 and it really works out well. In fact, I've
got one magazine cover taken with this combo.
There is a time and place for all three close-up options. When travelling
light I take the Diopter. When on an "art" trip, I take the extension
tubes. When I am rich, I'll get the 90/2.
Ken Norton
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