Zoom Lens Considerations
The two touch system is slower, but it does
have some advantages of its own. Because zooming and focusing are done
by separate rings, altering one setting will not disturb the other. In
addition, because of a less complex internal structure, a two touch lens
is generally less bulky than a lens with combined controls.
There seems to be little doubt that anyone
would prefer the parfocal, or true, zoom, because of its greater
convenience and faster handling. With it very accurate focus can be
achieved simply by focusing at the longest zoom setting (where shallow
depth of field is a great help) and then zooming back to the required
magnification. Then, too, the ability to maintain the point of focus
allows a parfocal zoom to create slow shutter speed zooming effects by
changing focal lengths smoothly while the shutter is open.
However, all the advantages are not with the
parfocal zoom. Because it is simpler mechanically and optically, the
varifocal is usually lighter and more compact, with greater lens speed
and the ability to focus closer without special levers and buttons. In
addition, it will probably deliver better image quality and cost less.
Focal Length Range of Zoom Lenses
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