A good camera is worthless without a quality lens. Compact and bridge digital cameras have built-in lenses, but the advent of affordable reflex or hybrid models with lenses that can be changed now allows budding photographers to make the most of top-quality optical systems designed for all types of use.
Choosing your lens
Focal length range
The most important factor when choosing a lens is its focal length range, which defines the angle of the image that can be taken. Zoom lenses have long focal lengths of 50mm or more, while a wide-angle photography can be taken with short focal lengths. Basic 18-55mm type lenses cover all the focal lengths from 18 to 55mm, and can be used to zoom slightly and to take wide-angle pictures.
Since the "standard" focal length depends on the type of sensor in your camera, it is sometimes difficult to directly compare focal lengths between different types of cameras. This is why manufacturers often define focal lengths in terms of the 35-mm equivalent focal length, in order to provide a reliable basis for comparison.
Fixed focal length lenses are also available for most focal lengths. While you will not be able to directly change the framing of your photo with this type of lens, they do offer higher quality, because they are optimized for a particular focal length.
The second most important factor when choosing a lens is its aperture, which corresponds to the quantity of light that the optical system can capture. The diaphragm used to adjust this quantity of light works very much like the pupil in our eyes. The aperture is expressed by the ratio between the focal length and the diameter of the entrance pupil. By way of example, for a lens with a focal length of 50mm, f/2.0 indicates that the maximum aperture is 25mm. The lower this divisor, the greater the quantity of incoming light.
The aperture also significantly impacts the depth of field. The greater the aperture (i.e., the lower the f/ value), the shorter the depth in which the image is sharp. This effect is often used by photographers to create blurred backgrounds and other artistic effects.
Every manufacturer uses one or more types of mounts for their various camera formats. Some models can use different mounts or mounts that hark back to conventional film cameras, but they are sometimes subject to a number of restrictions (e.g., the autofocus does not work). Some mounts can also be used with different sensor formats. However, depending on the type of mount (with or without an adapter), the distance between the lens and the sensor may vary, thereby impacting the actual focal length when the mount is used.
Which lens for which use?
The lens you should choose depends on the way you use your camera.
For highly specific usages, there can be no doubt that lenses with a fixed focal length offer unmatched photographic quality at a reasonable price. Their maximum aperture is also greater than with lenses with a variable focal length. But they are not very versatile, and require the photographer to have several lenses at all times.
For beginners looking for both good performance and versatility, the best compromise is a standard zoom lens capable of taking wide-angle pictures, plus a telephoto lens with a variable focal length of up to about 200mm. With these two lenses, you will be able to take perfectly good photographs under most circumstances.
The main advantage of a hybrid or reflex model is that it can use several different lenses. But the so-called superzoom lenses offer an even greater amplitude than standard zooms, with focal lengths that are as long as those found in dedicated telephoto lenses. If you can only take one lens on holiday with you, then superzooms are unbeatable in terms of versatility. But their optical qualities and maximum aperture are both inferior to those of standard zooms and telephoto lenses.
Just like with compact models, if your camera does not have a sensor with built-in stabilization, then you need a lens with image stabilization, especially when photographing distant subjects or for hand-held photography. While these lenses may be more expensive, they will significantly increase the number of good photos taken under difficult conditions.