Lenses

Q. What do zoom lens range numbers mean?
A.. In 35mm photography, think of 50mm as a dividing point. A 50mm lens has about the same angle of view (46 degrees) as the human eye and it neither reduces nor enlarges the subject. The farther a lens is below 50, the wider angle it will cover. The farther above 50 a lens goes, the stronger its telephoto effect. A camera with a 28-70mm lens will have more wide-angle capability than a 35-90mm lens, but not as much telephoto power.

Q. I know a 300mm lens is more powerful than a 200mm lens, but by how much?
A. When comparing telephoto lenses for 35mm cameras, divide the focal length by 50 to find the magnification factor. A 200mm lens brings the subject in 4x closer than a 50mm lens (which views the subject like the human eye). A 300mm lens will bring the subject in 6x closer.

Q. Is there an adapter that will let me use a lens from one brand of camera on a second camera which has a different lens mount?
A.Generally speaking, no. Adapters exist to convert screw-mount lenses to various bayonet-mount cameras, but that is about all. Virtually no bayonet-to-bayonet lens adapters exist. Such adapters would add distance between the film and lens, preventing the lens from focusing at infinity unless the adapter contained optical elements to maintain infinity focus. These lens elements in turn would increase the lens focal length and add to the cost of the adapter.

Q. What are teleconverters for 35mm SLR cameras and how are they used?
A. Teleconverters are optical devises that mount between the lens and camera body and contain several lens elements to multiply the magnifying power of the lens. They are available in strengths ranging from 1.4x to 2x. Teleconverters reduce the amount of light reaching the film by up to two f-stops. A 70-210mm zoom lens with a 2x teleconverter in effect becomes a 140-420mm lens.

Q. What does the optical term aspherical mean?
A. Light rays entering the edges of a lens must be refracted (bent) more severely than rays entering the center of the lens and as a result, do not focus precisely at the same plane as the center rays. This creates an optical fault called spherical aberration and results in poor edge sharpness, lens flare and reduced contrast. Adding a specially shaped lens element, called an aspherical element, to the lens design focuses the edge rays at the same plane as the center rays.

Q. What does apochromatic mean?
A. Apochromatic lenses (also called APO) are highly corrected and include special elements to focus all colors of light at precisely the same plane.