Lens speed


No matter what the focal length or other special qualities of a lens, its maximum aperture or speed is always of great interest. Lens speed is featured prominently in all advertising, for manufacturers know that many photographers rate a large maximum aperture among the most desirable of lens characteristics, equating “speed” with quality. Although the two do not necessarily go together, it is understandable that photographers with large aperture lenses feel a certain pride of ownership.


Besides such purely emotional feelings of satisfaction, there are some real advantages in owning a fast lens – especially one of normal focal length, for it is in this size that lens speed can be made greatest.


1. A fast lens is easier to focus. The finder image is brighter and depth of field is shallower, so images snap in and out of focus with greater certainty.

2. The limited depth of field at its largest aperture also allows a fast lens to be used for effective portraiture when a distracting background has to be thrown out of focus.

3. A fast lens is an ideal choice when pictures are to be taken in dim natural light without the help of flash. An increasing number of photographers use flash whenever there is a lack of bright day light, but this practice could often be a mistake. Natural light is much more interesting and realistic than flash, revealing a roundness in the subject that flash obliterates.

4. At the point where light is so dim that a tripod or other camera support would ordinarily be used (to avoid blurred pictures due to the necessary slow shutter speeds); a fast lens will enable you to sue shutter speeds high enough to continue hand holding your camera.

5. Higher shutter speeds can be used in any light, allowing you to stop fast sports action in a situation in which a slower lens and slower shutter speeds might not be able to capture the subject effectively.

6. With a fast lens adequate shutter speeds can usually be maintained even when dense filters are placed in the light path

7. If you should wish to use better and lower ISO setting in order to obtain sharper and without noise distortion (noise is amplified higher when ISO setting is higher), a fast lens will make this possible.

There are, however some minor disadvantages in owning and using a fast lens.

1. Large aperture lenses are relatively expensive.

2. Fast lenses, because of their larger glass elements, are bigger and heavier than slower ones.

3. Image quality is likely to be lower when a fast lens is used wide open. However, even at moderate apertures some fast lenses may not perform quite as well as slower ones, because lens speed has been given a higher priority than image quality.

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