However, zoom lenses do have a couple of intrinsic disadvantages. One is that their maximum apertures are lower than those of prime lenses. Whereas a 50mm prime lens might have a maximum aperture of f/1.8, a typical ‘standard zoom’ might have a maximum aperture off/4 at this focal length.
Lens mounts
Each digital SLR brand uses a different lens mount. A Nikon lens, for example, will not fit a Canon camera. However, you do not have to buy lenses made by your camera’s maker. Independent companies such as Sigma, for example, make lenses which can be supplied in different mounts according to the brand of camera you are using. These lenses are just as serviceable as those supplied by leading camera firms.
EQUIVALENT FOCAL LENGTH
Photographers using 35mm cameras are used to judging the angle of view of a lens by its focal length. However, with a couple of exceptions, digital SLRs have physically smaller sensors, so that the angle of view of the lens is reduced and it appears to have a longer focal length. You need to multiply the actual focal length by a factor of 1.5 or 1.6 to get its ‘effective’ focal length. For example, on a digital SLR a 50mm lens effectively becomes an 80mm lens.
Independent lenses versus marque lenses
Lenses made by independent companies are generally much cheaper than those offered by the camera maker. The optical performance is often very good and it may be difficult to see the difference in image quality between photographs taken using a good-quality independent lens and those taken on a more expensive ‘marque’ lens.
Having said that, when you buy a lens you are not just paying for image quality. Marque lenses may be better made than those from independent companies and are consequently more likely to withstand years of hard use. Their design and finish will be consistent with other lenses in the same range, and with the camera bodies which they are designed to accompany, and the lens range may include more sophisticated and specialized lenses that you cannot get elsewhere.
Standard Lenses
Digital cameras may be sold in ‘body-only’ form, but this option will normally only appeal to buyers who already have compatible lenses. Most buyers will choose a camera kit that includes both the body and a standard lens.
Although most new cameras come with zoom lenses fitted these days, until fairly recently film cameras were supplied with only a fixed focal length lens. Atypical standard lens has a focal length of 50mm or thereabouts, and is designed for general-purpose photography. Its angle of view closely matches that of the human eye, although some photographers feel that a 45mm lens is closer to the ideal. With a lens like this, the framing and perspective of shots looks natural, and this type has a large maximum aperture of f/1.8 or f/1.4. This makes it especially useful for shooting in low light or where you want shallow depth-of-field.
50mm standard lenses are light and compact, but some makers have produced shorter ‘pancake’ lenses for photographers who want their camera and lens to be slimmer still. These have a focal length of around 40mm and a smaller maximum aperture off/2.8, but their optical quality is usually excellent. They are more expensive, but for many photographers they are also more useful as standard lenses.
Standard zoom lenses
The zoom lenses that are a standard feature of most cameras sold today offer greater flexibility than the fixed focal length lenses of the past, covering a range of focal lengths from wide-angle through to telephoto.
On a film camera, the usual standard zoom has a focal range of 28-90mm. On a digital SLR it is usually 18-55mm, which gives a comparable angle of view. You can use a 28-90mm film lens on a digital SLR, but the focal range is not ideal, because it does not provide proper wide-angle coverage on the smaller sensor.
Zoom ranges are sometimes quoted as a factor – for example, ‘3x’, ‘4x’ or ‘5x’. This figure indicates the change in magnification across the zoom range. A 28-gomm zoom offers a 3.2x zoom range, for example, while a 28-135mm is a 4.8x zoom.
Zoom lenses with a longer focal range are more desirable, but they are also more expensive and much heavier. A lighter, inexpensive lens may be more useful on a camera that is going to be carried around all day.
USES FOR STANDARD LENSES
Standard prime lenses are good for full-length portraits, because you stand far enough away to prevent distortion but not so far away that you run out of space when working in small rooms.
The large maximum aperture of a standard prime lens will make it ideal for taking pictures at parties and celebrations where you don’t want to use flash, which could disturb the subjects.
A standard 28-90mm zoom lens (or its digital equivalent) is perfect for everyday photography, because it is compact and light and you will seldom need focal lengths outside this range.
Standard zooms are perfect for travel photography as well, in situations where there is often little time to change lenses or viewpoints. For this kind of work, a slightly longer zoom range (28-135mm, for example) can be an advantage.
Wide-Angle Lenses
A wide-angle lens enables you to get more into the frame of your image. At the same time it makes objects seem smaller and further away. A wide-angle lens has an obvious, practical value if you are trying to photograph a sweeping landscape or if you are taking pictures in confined spaces.
The term ‘wide-angle’ covers a range of focal lengths. On a 35mm film camera, a 35mm lens would be considered a modest wide-angle and a 28mm lens a ‘proper’ wide-angle. 20mm lenses are ‘super-wides’, and considerably more expensive.
The standard zooms supplied with modern cameras usually have a minimum focal length of 28mm, which is probably as wide as you will usually need to go.
Just as there are super-wide fixed focal length lenses, so there are wide-angle zooms, which cover similar focal lengths, such as 16-40mm on a 35mm camera, or a 10-20mm lens on a digital SLR. These lenses are bulkier and more expensive than standard zooms.
WIDE-ANGLE PERSPECTIVE
Wide-angle lenses have a profound effect on perspective. This is not because of their inherent optical properties – the explanation is, in fact, much simpler. Wide-angle lenses make objects appear smaller, with the result that you move closer to fill the frame. The fact of doing this exaggerates the difference in size between nearby objects and those further away. In turn, this produces the ‘keystoning’ effect which is often seen when tall buildings have been photographed from their base. However, this exaggeration of perspective can have creative uses, as well. The dramatic size differences between close and distant objects produce a strong feeling of three-dimensional depth and compositional ‘movement’ in images taken in this way.
In common with most specialist equipment, wide-angle lenses have both advantages and disadvantages. Their most obvious advantage is that they enable you to get more into your picture. They also offer more depth-of-field than lenses of longer focal lengths at a given aperture, so that it is easier to get both close-up and distant subjects sharp at the same time.
Image distortion
One downside of wide-angle lenses is that they can introduce distortion. This distortion manifests itself in two different ways. The most obvious is perspective distortion, in which the walls of tall buildings, for example, appear to converge, or objects at the edge of the frame appear to be toppling inwards. This undesirable effect is called ‘converging verticals’, or ‘keystoning’. It is not a fault in the lens, but simply a reflection of the fact that you tend to stand closer to your subject with a wide-angle lens, and this means