Focal Length - Informational Resource
The focal length of a lens will tell its angle of view and how much the object will be magnified for a given position. Wider angle lens has small focal length while the telephoto lens has larger focal lengths. Focal length will also give the perspective of an image. But this is disputable among photographers. Perspective control is important in photography composition and is determined by a person’s choice of focal length. Lens focal length can be influenced by many factors. For example, if the lens focal length is less than 21mm or extreme wide angle, it is best for architecture. Lens focal lengths of 21 to 35 mm or wide angle are best for landscapes while focal lengths of 135 to 300mm or telephoto are best for sports and wildlife.
Focal lengths may also have an important impact on how easy it is to get sharp handheld photo. Longer focal lengths need shorter exposure times to lessen blurring caused by shaky hand. Imagine you are trying to focus light at something then your hand will shake, the effect is that the object you are trying to give light to will not be seen fully. That is how the photo will come out, blurred. One common rule for measuring how fast the exposure for a given focal length, is the “one over focal length rule”. This rule states that for a 35 mm camera, the exposure time needs to be at least as fast as one over the focal length in seconds. It means, if you are using a 200 mm focal length on a 35 mm camera, the exposure time needs must be at least 1/200 seconds.
When a photographer can vary the focal lengths within a pre-identified range what he is using is a zoom lens. The advantage of a zoom lens is that it is easier to attain a variety of perspective. This advantage is usually best for dynamic subject matter such as children’s photography and photojournalism. Prime lenses have been in existence way before the zoom lenses were made available. Both prime lenses and zoom lenses have its advantages and disadvantages. The advantages of prime lenses are in speed, cost, and weight. A less expensive prime lens can actually provide as good image quality as a high-end zoom lens. Moreover, a small part of the focal length range is needed for a zoom lens, while a prime lens with the same focal length will be also smaller and lighter.
If you are considering buying a lens, specifications usually list the maximum and minimum available apertures. Lenses with a wider range of aperture settings provide greater artistic flexibility in terms of exposure options and depth of field. Portrait and indoor sports photography usually requires lenses with very big maximum apertures. The narrow depth of field in a portrait helps isolate the subjects from their background. Other considerations in buying lenses include cost, weight and size. If you are not sure what you need, it is best that you consult a professional photographer or the seller themselves. You would not want to waste money and be disappointed that what you bought is not what you need.
Lens Focal Length Explained