Close-up lenses don't attach to your camera's lens mount, but instead screw into the filter thread at the front of a camera lens. Because of this, they are sometimes called close-up filters. However, since they do not filter light, this is not strictly correct. You will also see them referred to as supplementary lenses because they are used in addition to another lens.

Whatever you call them, a close-up lens takes up very little room, yet can transform the capabilities of your other lenses, enabling them to focus closer than normal, much as if you attached a magnifying glass. It is an ideal accessory to carry when you want to travel light.

Over the years, Canon has made a range of different close-up lenses with two designs ? single element and double element. The double element kind are identified by the letter D. Both kinds have been available in a range of focal lengths including 240mm, 250mm, 450mm and 500mm ? this determines the magnification. They also come in different filter thread sizes such as 52mm, 58mm, 72mm and 77mm, but not every permutation of focal length and thread size is available.

The most important factor when choosing a close-up lens is performance, and this depends on the construction. Single element close-up lenses are relatively inexpensive and may be adequate for occasional use, but they are not as good as their double element equivalents. In simple terms, all single element lenses show aberrations, usually chromatic. By adding a second element, the aberrations from the two elements can be made to cancel each other out. The resulting double-element or "doublet" lens may not be completely free of aberrations, but it will usually be a lot better than a single element lens.

The improved performance is particularly noticeable at the edges of the image. This means that if you are photographing a flower, where the subject is mostly in the centre of the frame, a single element close-up lens might be adequate. However, if you are shooting a flat subject that extends to the edges of the frame, such as a postcard or stamps, a doublet lens will give much better sharpness at the edges, especially at wider lens apertures.

When you attach a close-up lens to your camera lens it acts a little like reading glasses for a far-sighted person. The camera is no longer able to focus on infinity, but it has a clear vision of close subjects that were previously outside its focusing range. For general guidance, the Type 250D close-up lens is designed for camera lenses with focal lengths from 35mm to 135mm. However, good results are possible using the close-up lenses with focal lengths outside of this range.

The power of close-up lenses is sometimes expressed in dioptres. The dioptre power is calculated by dividing 1,000 by the focal length of the close-up lens. This means that the Type 250 lens has a power of +4 dioptres.

Working distance is the distance between the front surface of the close-up lens and the point of the subject on which the lens is focused. Do not confuse this with the focusing distance, which is the distance between the focal plane and the subject. Most cameras indicate this film or sensor position with a symbol – a circle with a line through it – on the top of the body. If you are shooting with a close-up lens on the front of a telephoto lens, the difference between the two distances can be considerable.

The focusing distance is needed in some close-up calculations, while the working distance lets you know how much space there is between the front of the camera and the subject, so you can position any lighting.

It's not generally recommended to use a Speedlite on-camera for close-up work. First, the minimum working distance for most Speedlites is around 0.7 metre. Second, at close distances the difference in position of the Speedlite and the lens means that most of the light from the flash is likely to miss the main area of the subject. Even if you use a Speedlite with a flash head that can be angled down by a small amount, you run the risk of the subject being in the shadow thrown by the lens.

One solution is to use the Speedlite off-camera, either connected wirelessly to the camera or via the Off Camera Shoe Cord. With the Speedlite away from the hot-shoe, you can tilt it so that the light is aimed directly at the subject. You can also move the Speedlite back (up to 60cm from the camera with the Off Camera Shoe Cord) to bring it within its normal working range. Using the flash away from the camera has the benefit of producing better texture lighting on the subject.

The camera's built-in flash is also unsuitable for many close-up shots, because of the risk of the illumination being partially obstructed by the lens barrel.

For simple photographs with a close-up lens, the best illumination is daylight. The camera's normal exposure metering is not affected by the addition of the close-up lens and no increase in the amount of exposure is required. It's a good idea to use a tripod, though, to avoid the effects of camera shake at the increased magnifications.

So how can you determine the magnification that a close-up lens gives when attached to a lens at a particular focusing distance? With the close-up lens in place, set the focus on the camera lens to the value you want (maximum or minimum are the best values).

Find a ruler that has a clear scale marked in millimetres. Position the ruler at right angles to the lens axis, and move the camera back and forth in front of it until the scale on the ruler comes into sharp focus. Move the centre of the viewfinder over the middle of the scale, so that the millimetre markings run at least the full width of the frame. (Don't try aligning the zero point of the ruler with the edge of the viewfinder – because of parallax error and the difference between the image frame and the viewfinder field of view, you risk ending up with a gap between the frame edge and the start of the scale.) Now take a picture.

Take a look at the processed result and count the number of millimetres visible. Let's say there are 72. The width of a full-frame sensor is 35.9mm, so the magnification is 35.9 divided by 72, which is 0.499. This means that the magnification for that setup is 0.5x.

If you're travelling and space in your kitbag is critical, then you might prefer a close-up lens, which is much more compact than an extension tube. Otherwise, the choice depends largely on what lenses you have and what magnification you want to achieve with them. Close-up lenses have more effect when they are used on a camera lens with a long focal length than on shorter lenses. This is exactly opposite to the effect of extension tubes, where the amount of magnification decreases as the camera lens focal length increases. In practice, this means that extension tubes are more useful with short lenses (up to 100mm, for example) whereas close-up lenses are more useful with longer lenses.

When you require greater working distance, the combination of a telephoto lens with a close-up lens will usually be the best choice. If you use an extension tube on a wide-angle lens, you may find you're having to work extremely close to the subject to get it in focus. In addition, close-up lenses do not reduce the amount of light reaching the sensor, whereas extension tubes do. This can sometimes be critical when you need a reasonably fast shutter speed or a small aperture (or both).

If you have a long telephoto zoom, you can make good use of a close-up lens – and in practical terms it is a better option than using extension tubes with a telephoto lens. If you use extension tubes with zooms, you will find that the focus changes as you zoom in and out, which means you continually have to re-focus. This doesn't happen with close-up lenses. Once you have focused, you can change your composition by zooming in and out, just as you can with any zoom used normally. Always use the camera on a tripod – the increased magnification will also increase the effects of any camera shake.

If you have a macro lens, you can make good use of a close-up lens to bring about even larger magnifications, although a close-up lens typically loses effectiveness at shorter focal lengths (less than about 35mm). If you're aiming for really high magnifications, try using a macro lens + extension tubes + a close-up lens.