Zoom range

A lens's zoom range is the ratio between its minimum and maximum focal lengths. So a 28-84mm lens, for example, has a zoom range of 3x, because its magnification at 84mm is 3x its magnification at 28mm.

Zoom range is an important selling point, especially for compact cameras, where the average buyer may have difficulty working out the lens's range from its focal lengths alone.

Superzoom cameras like the Canon PowerShot SX30 IS have the largest zoom range of all. This camera has a 30x optical zoom, and the diagram below shows the huge difference in scale between the wideangle end of the zoom range (the tiny figure on the left) and telephoto end (the large figure on the right).

It can be misleading, though, if you start to assume that the bigger the number  the better the lens. One problem is that the zoom range doesn't tell you the lens's widest angle of view, which is very important. Being able to magnify distant subjects is all very well, but in everyday photography a wideangle lens is more useful for 'getting everything in'.

It is important, then, to check the focal lengths anyway. A 5x zoom with a focal range of 24-100mm equivalent is likely to prove a lot more useful in everyday use than a 10x zoom with a focal range of 35-350mm, because a lot of the time the 35mm minimum focal length just won't prove 'wide' enough.

The other problem is that zooms with a very long range don't always perform that well, particularly at their maximum zoom, where pictures can often come out quite soft-looking and low in contrast. This can happen even if the camera uses a very high shutter speed, demonstrating that it's the lens that's the problem, not camera shake or user error.

The performance of superzoom cameras can often be quite disappointing for this reason, and this applies to pretty well all lenses with an extra-long zoom range. They're more versatile than normal lenses, but the quality is usually compromised to some degree.

Maximum aperture

The maximum aperture of a lens represents its light-gathering ability, and it's an important selling point. The smaller the number, the wider (or 'faster') the lens aperture.

This is the scale of aperture values. These are whole 'f-stops', but there are apertures in between ('half stops' or 'one-third stops'):

• f1 (very rare)
• f1.4 (some fixed focal length lenses)
• f2 (some zooms)
• f2.8
• f4 (most standard zooms start at f3.5, which is mid-way between f2.8 and f4
• f5.6 (the maximum aperture of most standard zooms at their maximum focal length*)
• f8
• f11
• f16
• f22

* Zoom lenses rarely offer the same maximum aperture throughout their zoom range, which is why the lens specs will quote something like 'Nikkor 18-55mm f3.5-5.6'. That means the maximum aperture at 18mm is f3.5, but at 55mm it drops to f5.6.

Each aperture value in the list above is one stop (or 1EV) 'faster' than the one below it. F1.4 is two stops 'faster' than f2.8, which means you can use an ISO setting two stops lower or a shutter speed two stops faster.

There are zoom lenses with constant maximum apertures, but these are much bigger, heavier and more expensive.

Fixed focal length lenses like this Nikon 50mm f1.4 have wider maximum apertures because their optical design is simpler and the designers can push the envelope in different directions.

Focal factor

The focal length of a lens doesn't really tell you its actual angle of view because this depends on the sensor size too. But you can use a 'focal factor' to work out what the equivalent focal length will be.

For example, most digital SLRs have an APS-C sized sensor measuring around 24 x 16mm. This is smaller than a full-frame sensor, or a 35mm negative size, which is what most people go by. The difference is 1.6x (a full-frame sensor is 1.6x wider).

So if you use a 50mm lens on an APS-C camera, you multiply this focal factor by 1.6x, which gives you an equivalent focal length of 80mm.

In other words, because the sensor is smaller, a 50mm lens fitted to an APS-C camera actually looks like an 80mm lens.

Vignetting | Both an aberration and a creative effect

Vignetting (also called corner shading) is where the lens doesn't produce the same brightness at the edges of the frame as it does in the middle. It can also happen when the lens’s image circle is just a little too small for the sensor/film area and the areas furthest from the centre (the corners) appear darker. It’s common with cheaper lenses or zooms at the limits of their performance – wide open and minimum focal length, for example.

You can see it in this shot, where the top corners are clearly darker. It's possible to fix this using software (Photoshop's Lens Correction filter, or DxO Optics Pro, for example), but it's better if the lens doesn't produce vignetting in the first place.

Vignetting can also be applied deliberately using software to produce an old-fashioned effect, though photographers prefer to add it by choice and under careful control as and when they want it.

Wideangle lenses | 28mm equivalent or less

This is a lens which captures a wider angle of view that normal or, in the case of a zoom, one which offers a 'wideangle' view at its shortest focal length. 28mm equivalent is considered a 'wideangle', while shorter focal lengths are widere still. Most compact digital cameras only zoom out to the equivalent of a 35mm, which doesn't in fact give a particularly 'wide' view. With compact cameras, then, a 'wideangle' lens is a selling point.

Digital SLRs come with kit lenses which offer a minimum focal length of 28mm, so they're all wideangles. You can get super-wideangle lenses which go wider still, though.

This picture was taken with a super-wideangle 14mm equivalent lens, which is about as wide as conventional lenses go without resorting to fisheyes and obvious distortion.

Pancake lenses | Magically miniaturise your camera

A 'pancake' lens is an ultra-slim fixed focal length lens for digital SLRs or hybrid cameras like the Micro Four Thirds models from Olympus and Panasonic. Pancake lenses often have a wide maximum aperture too, making them good for low light and shallow depth of field effects.

This is Panasonic's 20mm f1.7, designed for its G1, GH1 and GF1 cameras. It's equivalent to a 40mm lens in 35mm/full-frame camera terms. Olympus has produced a 17mm (34mm equivalent) f2.8 pancake lens for its Pen E-P1 model.

Pancake lenses appeal to photographers looking for smaller camera/lens combinations, the simplicity of a fixed focal length and the faster maximum apertures and superior optical quality. The resolution may be no higher than a good-quality zoom's, but you can expect far less distortion and chromatic aberration.

Lens mounts | Every maker's is different

The lens mount is the mechanism that attaches a lens to an SLR body. It consists of a twist-lock 'bayonet' mechanism to allow the lens to be attached quickly and a series of electrical contacts so that the lens and the body can exchange the information needed for today's high-tech auto-exposure and autofocus systems.

The bayonet mechanism and the electrical contacts are visible in this shot of the Canon EOS 1000D with its lens removed.

Each maker produces its own proprietary lens mount which is incompatible with the rest. You can't fit Canon lenses to Nikons or Olympus lenses to Pentaxes. There may even be some incompatibilities within the maker's own range. Canon's EF full-frame lenses can be used on its full-frame cameras and it's smaller APS-C models. However, the EF-S lenses designed for the APS-C models can't be used on the full frame EOS SLRs.

Manufacturers are constantly developing their camera and lens ranges, and it's always wise to check compatibility with your existing equipment before you buy, or that your existing lenses can take full advantage of the features in the latest bodies, say.

Camera makers only make lenses for their own cameras, but independent lens makers like Tamron and Sigma may offer their lenses in several different mounts. When you buy an independent lens, you must specify the lens mount you want.

Superzooms | Longer range, lower standards

A 'superzoom' is generally reckoned to be a camera or a lens with a focal range of 10x or more. In other words, its maximum focal length is 10x its minimum focal length. The advantage is that you can tackle a wide range of subjects with a single camera or lens, though there are issues with image quality (see below).

This is the Olympus SP-590UZ. It's a typical 'superzoom' camera. It's a compact with a small sensor and a non-interchangeable lens which has a 26x zoom range, covering a focal range equivalent to 26-676mm; in other words, from extra-wideangle to super-telephoto. The quality from the small sensor is OK at low ISOs but deteriorates quickly at higher settings. Nevertheless, the low cost compared to a digital SLR, the range of photographic controls and the huge versatility makes this a popular kind of camera.

But although superzooms like the SP-590UZ are powerful cameras, they're not especially compact. You certainly wouldn't be able to put one in your pocket. For this, you need a 'superzoom compact' like the Panasonic TZ10.

The TZ10 has a 12x zoom equivalent to 25-300mm. You sacrifice the very longest focal lengths, but these aren't used that much anyway. And in return you get a camera which is not much larger than a regular pocket-sized compact. In many ways, then, a compact superzoom is more practical.

You can also get 'superzoom' lenses for digital SLRs. This is the Tamron 18-270mm VC, which boasts a 15x zoom range that's currently the longest for a digital SLR lens, and it covers a focal range equivalent to 28-420mm. The 'VC' stands for Tamron's Vibration Control image stabilisation system. Some kind of image stabilisation system is essential for any superzoom lens or camera.

Superzooms are amazingly versatile, but there are some significant issues:

• Loss of sharpness at longer focal lengths. People often buy a superzoom for its extra focal range, only to be disappointed by the sharpness. This is quite normal, unfortunately, though the best examples can still yield acceptable results at full zoom. The sharpness at short-medium focal lengths is fine.

• Distortion. With a standard zoom you expect a degree of barrel distortion at the wideangle end of the zoom range and pincushion distortion at the telephoto end, but superzoom lenses are usually a lot worse.

• Chromatic aberration: Generally, a superzoom will show higher levels of chromatic aberration (colour fringing) than a normal superzoom at most focal lengths, but at full zoom it can be particularly bad.

• Size and weight: This doesn't affect compact superzoom cameras particularly, but it is an issue with superzoom lenses for SLRs. The Tamron 18-270mm VC is compact for its type, but it's still a monster, especially when extended to its full zoom.

Lensbaby lenses | Lo-fi photography meets high art

Where most lens makers are using ever more complex optical designs to produce the sharpest possible images, Lensbaby is doing exactly the opposite, producing a range of lenses which celebrates optical aberrations rather than trying to eliminate them.

It's a modular system based around three lens holders with movable lens panels. These are adjusted to change the region of sharp focus produced by the lens. It's like an extremely crude version of the tilt movements provided by large-format studio cameras. A range of simple lenses can be dropped into these holders, and you choose the lens which creates the effect you're trying to achieve.

Lensbabys are far to simplistic to accomodate the auto-exposure and aperture control systems used on modern D-SLR bodies, so it means a return to manual focus and manual exposure. It takes a good deal of practice and experimentation to work out how the lens movements affect the image, and the results aren't to everyone's taste, but Lensbabys do have something of a cult following.