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 ring.

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 is changed.

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