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SLR lenses at a glance
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Photo by Dennis Bodzash. Zoom lenses are
versatile (top) while prime lenses often have
large apertures (bottom).
Despite the fact that there are hundreds of different photographic lenses for digital SLRs on the market, at the most basic level, they can be broken down into two types: zooms and primes. There are far more ways to categorize lenses, but the two previously stated make the best place to start.
Zoom lenses
The zoom lens is characterized by a changing (ex. 70-200mm) focal length. The obvious advantage here is the fact that the magnifying power changes, allowing the photographer to stay in one spot to frame images properly. Some zoom lenses have a short zoom range (ex.10-20mm) while others have a very long one (ex. 18-270mm). Obviously, the the larger range in between the focal length extremes, the greater the versatility and zooming power. At one time, zoom lenses were considered cheap and for amateurs who demanded convenience over optical quality. Today, this statement could not be farther from the truth. Optical engineers have greatly raised the standards for zoom lenses, equaling, and sometimes surpassing the image quality of fixed focal length prime lenses. Today, when anyone buys a camera that comes with a bundled lens, chances are that it will be a zoom. For all their convenience, zoom lenses have some disadvantages. First is the aperture. With the exception of Olympus (due to its small Four-Thirds sensor), no camera maker produces a zoom lenses with an aperture faster than f2.8. In comparison, Canon once produced a 50mm fixed focal length lens with the astounding aperture of f1, which lets in eight times as much light as a f2.8 optic. A second downfall is that zoom lenses are optical compromises because they must provide adequate image quality at all focal lengths. This fact can result in jack of all trades lens in that it does everything, but nothing particularly well. This problem is especially true for long zoom range lenses. However, recent “hyper-zoom” lenses are starting to dispel this fact. Most professional grade zooms have a zoom ratio of 4x or less (ex. 24-105) and retain good to excellent image quality at all focal lengths. The very best zooms can even equal of beat fixed focal length lenses in image quality.
Prime (fixed focal length) lenses
The second type of lens is called a prime lens and this type is characterized by having a fixed focal length. In the days before quality zooms, no self respecting, serious photographer would be without a collection of prime lenses in his or her camera bag. Even today with the rise of the zoom lens, prime lenses still have a very important place in a photographer's optical arsenal. First, by having a fixed focal length, a prime lens must be optimized for performance at only a single focal length, which often results in exceptional image quality. In the center of the frame at least, a cheap prime will often outperform a more expensive zoom with its inherently slower aperture. A second advantage of a prime lens is that, by having a single focal length, the lens can be made to have a much faster aperture than a zoom. Many wide angle prime lenses have an aperture of f1.4, which lets in four times as much light as a f2.8 zoom. By letting in more light, prime lenses are invaluable for low light, indoor shooting when a photographer does not want to use a flash. Obviously, the downside to a prime lens is the fixed focal length. There are times when a photographer can only move around so much, which can limit the usefulness of a prime lens in some situations. If going out to shoot with a prime, it is vital to anticipate what lens will be the most useful for most of the time. Generally, a lens in the range of 30mm for a crop camera and a 50mm for a full frame camera will make an ideal general purpose lens if only one lens can be used, the difference is because of the crop factor. The second downfall of primes arises because many photographers will not be content to carry just one. Changing lenses opens the sensor to the air, and dust. While the dust will not hurt anything, the dark spots in the pictures are highly unaesthetic. This problem can be minimized by pointing the camera down when changing the lens.
Tomorrow's article will return to the theme of different size sensors, focusing on how this fact impacts lenses.
For more info:
Focal length simulation tool, this is very cool, check it out!
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