Labs

Digital Picks

How do they work?

There's surprisingly little difference between a digital camera and a traditional film camera. Both use a similar type of lens, and offer variable shutter speeds for controlling the duration of an exposure. Many can also adjust the size of the aperture through which light can enter. These controls are usually adjusted by the camera's automatic exposure system, ensuring that your snaps are well exposed.

But where a film camera uses light-sensitive film to record the image that comes through the lens, digital cameras use a chip containing an array of tiny light sensors. This records the image as an electrical signal, which is then turned into digital information by the camera's ADC (analogue-to-digital converter).

Digital camera image sensor chips come in two varieties. CMOS chips are cheaper to make, but produce lower quality. As a result, they're usually only seen in low-cost Webcams. (Canon uses CMOS chips in its high-end digital SLRs, but a different, highly-developed sort.) Better digital cameras use CCDs, and all but two of the cameras on test here are CCD-based.

Like film, CCDs have a sensitivity rating, quoted in terms of its equivalent ISO film speed. Many cameras allow you to change the CCD's sensitivity manually, so that it behaves more like film with a higher ISO rating. This enable you to use faster shutter speeds even in lower lighting, reducing the amount of visible camera shake. Trouble is, it does this simply by amplifying the signal output from the CCD - and this has the unfortunate side-effect of increasing the amount of digital 'noise' (random-coloured pixels) scattered throughout the image.

So can digital cameras match film for detail? It all depends on the number of 'pixels', or dots of colour, recorded by the CCD. This figure is the digital camera's resolution, and is quoted in millions of pixels, or 'megapixels'. The 2-megapixel cameras reviewed here produce files measuring 1,600x1,200 pixels. At the 300dpi setting recommended for high-quality printing, this means you can produce images measuring around 6in x 4in. If you want to produce bigger enlargements, a 3-megapixel model or even a 4-megapixel camera will produce progressively sharper results. For ordinary prints, e-mails or Web pages, though, 2 megapixels is fine.

In most cameras, the image data produced by the CCD is saved in the JPEG graphics format, which uses a technique called compression to squeeze down the size of image files without compromising too much image quality. These files are then stored either in internal memory or on a removable card. When you adjust the 'quality' setting on a digital camera, you're changing the amount of JPEG compression that's applied to an image. Using a lower quality setting means the images will suffer slightly, but they'll also take up less space, meaning you can take more photos before the memory fills up.

These JPEG files can then be transferred from camera to PC by hooking up the camera using the USB cable supplied. Once the files are on your PC, they can be opened in an image editing program. Many cameras come bundled with software. If your doesn't, we'd recommend our Top 50 favourite, Adobe Photoshop Elements 2.

While the cheapest digital cameras here offer internal memory storage, most use removable memory cards. This means you can buy larger-capacity cards for storing more images, or to plug them into a card reader or some specially equipped printers.

Memory cards come in several types. Compact Flash cards are physically the largest, though still only 4cm square. They're also the cheapest type available, with 128MB cards selling for less than £30. SmartMedia cards are smaller and slimmer, but less common these days. Cheaper cameras tend to use the newer SD (Secure Digital) card format, while FujiFilm and Olympus are pioneering the introduction of even tinier xD Picture cards.

What makes a good digital camera?

We've already mentioned the importance of resolution (megapixels) as an indication of picture quality. But outright resolution isn't the only factor that affects image quality.

The quality of the lens used will have an effect, as will the type of imaging chip. The accuracy of the exposure and focusing systems are important, and the colour rendition will depend on the camera's 'white balance' circuitry. Like video cameras, digital cameras can take into account the colour of the prevailing light, so that even shots taken under artificial lighting will look realistic. The only way to assess these factors is to actually get out and take pictures. Specs alone will only tell you half the story, as at least two cameras in this roundup demonstrate. That's why it's vital to read a group test like this before buying!

Some features are more important than others, too. Cheaper cameras come with a fixed focal length (non-zoom) lens. This means you can't zoom in close for action shots, sports or tightly-framed portraits. Non-zoom cameras usually offer a 'digital zoom', but this isn't the same thing. Digital zooms simply blow up the central part of the image, and you end up with a substantial loss of detail. 2-megapixel cameras don't have a big reserve of image detail anyway, so digital zooming is seldom worth using. If you want a zoom lens, you'll simply have to save the extra cash to buy a camera with a proper 'optical' zoom.

For beginners, or those who simply don't want to get involved with the technicalities, ease of use is vital. You'll never get the best from your camera if you can't work out how to use it. Can you just pick it up, click the button and get great results every time? And if you want to explore some of the more advanced options, how easy is it to get at them?

Design, handling and build quality are important too - and these, too, are factors you can't evaluate from an advert or spec sheet. We've said how inexpensive digital cameras have become, but £100-200 is still a fair outlay, and you don't want to end up with something that feels like it fell out of a Christmas cracker. Does the camera feel robust? Are the controls easy to use, and do the button have a positive, 'clicky' action? What about the LCD panel on the back - is it crisp and clear, or murky and blurred? This is where some of the biggest differences lie, yet it's the kind of thing - again - that you can't evaluate without actually handling and using the camera.