In article ,
Siddhartha Jain writes
>Why is that macro capability exists only in prime (as in fixed focal
>length!!!) lenses? Other than primes, only a few cheap telephoto zoom
>lenses seem to have macro.
>
>In contrast, all P&S digicams have a macro feature and some like the
>Oly C-750 that I owned had a super macro too.
>
>- Siddhartha
>
First, some terminology. Whatever the opinions of the "words mean
whatever I want them to mean" school of thought, this is a highly
technical area, and if you don't use the right terminology you will not
find the answers in reputable sources.
General photography uses magnifications of < 0.1.
"Close up photography" is the term applied to the range of
magnifications 0.1 to 1.0.
"Photomacrography" is the term applied to the range of magnification
from 1.0 to a rather ill-defined upper limit, usually about 50.0 (i.e.
where the image is 50 times the size of the object). If you try to look
up under the name "macrophotography" you will not find what you want in
rigorous text books, as that means something else.
There are mostly good reasons for this division; the equipment is
generally different, and the simplifications of optics equations that
work in one area break down (become inaccurate) in others.
If a lens designed for normal photography is set in a mount which allows
it to extend further from the position for normal photography - i.e. to
focus a lot closer - then unpleasant things happen to the image.
Spherical aberration (SA) rises sharply, contrast decreases, and
astigmatism, coma and field curvature become obtrusive.
To design lenses which work well in the m = 1.0 region and beyond,
manufacturers have to use different design criteria. Different basic
designs are often used, and may be used "back to front", i.e. the
reverse way to that used in normal photography. Floating elements are
often used to keep SA under control as the focus point moves, and to
control field flatness. Steps also may need to be taken to control
geometric distortion. Also, even after all these measures, it is
necessary to limit apertures to f/2.8 or similar to keep SA within
reasonable limits.
With all this, you can see that it would be quite difficult to do this
with a zoom lens, which already starts out with an often compromised
design, lots of elements, and several complex cams to move the various
bits around. So far as I am aware, there is no conventional zoom lens
which is a true macro.
You will see from the above that even most of the dedicated macro lenses
for 35mm and related format DSLRs, used alone, barely qualify for the
term macro. There is a whole world of specialised macro lenses for use
in the range m = 1.0 to m = 50.0 which is rarely mentioned in these
groups. These rarely have their own focussing movements, but are
designed for use on bellows, adapted microscopes or dedicated stands. In
truth it is not what the focus mount does which defines the lens, it is
how well it works optically at the required magnification.
(In case anyone raises it, the floating elements in many macro lenses
with focussing mounts for 35mm cameras will often alter the focal length
appreciably, so they might be thought of as a kind of zoom, or at least
vari-focal lens. This however seems pure sophistry. Also, there exists
among the true macro lenses for the region well above m = 1.0 at least
one variable magnification lens, the Zeiss Luminar 2.5 - 5.0x. A zoom it
is not, in any accepted sense at least.)
Many manufacturers take close focus in their zoom designs to the point
where quality starts to be compromised, and limit the action to this
point. Others, rather dubiously, extend them to the point where they
should know better, where quality is materially compromised, and then
call them "macro". Frankly, they should be sued under trade description
legislation.
If you want to learn more about photomacrography, there are several good
books and at least one discussion site on the subject.
David
--
David Littlewood
>> Stay informed about: Why do only primes have macro 
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