In general, lenses should have a wide maximum aperture because this allows a lot of light to be transmitted to the photographer's eye and the recording medium. The surface of the lens elements should also follow precise curves and must not exhibit even the smallest inconsistency in form as such errors cause distortion of images. Lenses should refract different wavelengths of light (colours) to equal extents to maintain true contrast in the image. The images formed should exhibit uniform illumination in all areas so that the film or sensor is correctly exposed throughout the frame. Finally, a lens should be practical for the purpose for which it is designed. It should be reasonably transportable and convenient to use under the relevant circumstances. Weight and bulk are consequently significant purchasing considerations.
It is of course not possible to create a perfect lens suitable for all circumstances. Lens design inevitably involves compromise, and designers are obliged to give priority to certain features, or sacrifice performance in one area to enhance it in another. Price is also always a consideration for manufacturers. Lenses of very high quality simply cannot be manufactured at low cost.
More or less any lens can be used for photographing everyday subjects such as landscapes and people, but the most useful ones are in the standard, wide-angle and short telephoto ranges. The focal length of lenses is measured in relation to film format. A standard lens for the 35mm format, designed to produce images that match human perception, has a focal length of about 50mm. In the case of medium format, a comparable lens would have a focal length of about 80mm.
Wide-angle lenses having fields of view as broad as 100 degrees, and extensive depth of field, are useful for photographing landscapes and people in their surroundings. Wide-angle zoom lenses, which for 35mm cameras typically have a range of about 20 - 35mm, are useful because the zoom gives a little flexibility when framing shots. Large maximum apertures are important if work is to be undertaken in poor light
Telephoto lenses let a photographer close in and isolate a subject from the environment. For 35mm cameras, focal lengths of 85mm, 105mm, 135mm, 200mm and 300mm are commonly used. The shorter ones in this range are commonly used for photographing people at distances that imply some involvement with the subject, and are therefore also known as portraiture lenses. However, they are also useful in many other circumstances. The longer ones are perhaps more useful for candid and wildlife work, but can once again be used in a wide variety of situations. Lenses having a focal length of 400mm or more are bulky and difficult to handle without a tripod, but indispensable when photographing camera-shy wildlife
It is generally good practice to fit lenses with a high-quality skylight or ultraviolet filter to protect the front element from damage. The quality of the filter is important because a poor-product is likely to ruin degrade the performance of an expensive lens. Lens hoods also offer some protection but their principal role is to prevent light from outside the subject area reaching the film. They reduce flare, maximize contrast and colour saturation, and are particularly important when directing a lens close to a light source. Flexible rubber hoods are convenient because they fold flat and are consequently easy to store.