Zoom Lens Considerations
One touch or two. Another decision that must be made before purchasing a
zoom lens is whether to select a one touch or two touch control system.
In a one touch system a single, large ring controls both the zooming and
the focusing. Turned to the right or left, the ring adjust the focus.
Pulled back or pushed forward, it shortens or extends the focal length.
The two operations may be performed separately or together. In a two
touch system focusing and zooming are each controlled by a separate
The one touch system is faster to use, especially when following action.
However, combining the controls increases mechanical complexity, weight
and bulk. A further drawback is that either the zoom or the focus
position may be disturbed as the other is being adjusted.
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.
Parfocal or Varifocal. A further choice open to zoom lens purchasers is
between parfocal and varifocal types. Most zooms are parfocal, meaning
that they maintain their focus regardless of zooming action. Varifocal
zooms, on the other hand, must be refocused each time the focal length
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.
The choice is not an easy one. If you prefer to work slowly and
carefully and your budget is modest, a varifocal could be the answer.
However, if you need the convenience of constant focus and donít mind a
smaller maximum aperture and higher price, choose the parfocal. Handle
and look closely at each type before you make a final decision. You may
want one of each.
More about zoom lenses
Focal Length Range of Zoom Lenses
Tips about Zoom Ratio
Standard or Macro Focusing
Zoom Lens Consideration
Tips on Handling Zoom Lens
Advantage of Zoom Lens vs Unifocal Lens
Disadvantage of Zoom Lens vs Unifocal Lens