To capture clear images, high-quality cameras generally use lenses that require manual or automatic focusing. Unless the lens is focused correctly, the subject will appear soft and blurry.
In contrast, the majority of simple cameras, including cell-phones, disposable and inexpensive snap-shot cameras use a fixed-focus lens that does not need to be focused. Fixed-focus lenses, also known as focus-free lenses, are designed to render everything in a scene in focus.
Fixed-focus lenses are not the same thing as autofocus lenses. The automatic mechanism of an autofocus lens still must adjust the lens to achieve maximum sharpness. A fixed-focus lens, on the other hand, never needs to be adjusted to deliver sharp images.
At first, this seems odd. Users of low-cost cameras have no need to be concerned with focusing, while those who have invested substantial amounts of cash in a high-quality camera system have to take focusing in consideration on every shot.
There are several reasons why you will not find fixed-focus lenses on top quality cameras. It is not an issue of lens quality, as many fixed-focus lenses have very good optics.
Striving for depth-of-field
Focus-free lenses accomplish their magic by relying on depth-of-field and wide-angle optics. Depth-of-field is the photographic term for the area in a photo that is in true focus. Wide angle lenses generally produce a wide area of sharpness, while telephotos almost always reduce depth-of-field to a minimum. Because fixed-focus lenses require great amounts of depth of field, there are few, if any, focus free telephotos.
Lens aperture also contributes to the depth of field in a photo. The term aperture refers to the opening in the lens that allows light to pass through to the sensor. Obviously, large apertures pass more light, while small apertures reduce the light reaching the sensor.
Fixed-focus lenses require small apertures
In addition to regulating the amount of light passing through the lens, the aperture opening also controls depth of field. Smaller openings extend depth of field, while larger apertures reduce it. To ensure the fixed-focus lens maintains proper focus, camera makers have to use small apertures to create a wide depth-of-field area. This, in turn reduces the light falling on the sensor. In bright sunlight, this is not an issue, but when the light becomes dim, a focus free lens cannot record well-exposed images without boosting the ISO into an unacceptable range.
No close focusing
Finally, most fixed-focus lenses are calibrated to what is known as the hyperfocal distance. This is a photographic concept that establishes the closest distance to the lens that objects can remain in focus, while still maintaining sharpness at infinity. When a focus-free lens is set up in this way, everything from half the hyperfocal distance to infinity will appear in focus.
While the hyperfocal distance creates a deep area of focus within a photograph, it also prevents the camera from focusing at close distances. With the exception of some telephoto lenses, most dSLRs and high-quality range-finders can capture images at distances of two feet or less. The typical focus-free lens cannot accurately capture subjects nearer than six to eight feet from the camera.
The combination of wide- ngle optics and and the lack of close focusing capabilities means that the photographer often cannot fill the frame with the subject. Portraits can be problematic, because the fixed-focus lens cannot get get close enough to capture a nice head shot. These optics can do a credible job on landscapes and other far away subjects, but they are useless at close range.
Despite the drawbacks, a focus-free lens is well suited to unsophisticated, no-frills photography. Fixed-focus optics can be used by anyone, especially casual and easy-going snap-shooters. They are excellent for training children the basics of photography. At the same time, a creative photo craftsman can produce wonderful images with a focus-free lens. There is an oft-repeated mantra that great photos reflect the skill of the photographer, not the price of the camera. The same thing can be said about lenses.
About the Author
Would you like to read more photography articles like this one? Visit http://alphatracks.com and click on the Photography Basics link. While you are there you can subscribe to the RSS feed or choose to receive posts by email.
Tom Bonner provides photography and web design services to clients in the Gastonia, North Carolina area. You can find out more about Tom at http://adventuresindesign.com.
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