Lab

Digital Cameras

If you're yet to be convinced about the benefits of digital cameras over their film-based counterparts, be prepared to change your mind. High-resolution models with immaculate image quality and equally gorgeous styling are now priced within most people's Christmas present budget.

If you already own a digital camera that's more than a year or two old, you'll find that these models offer a massive improvement in image quality, and they're probably a lot more compact and stylish too. For those yet to buy their first digital camera, now is the perfect time to take the plunge.

What are the benefits? You can see your pictures immediately after you've taken them, using the cameras' built-in display screens. What's more, the money you'll save by only printing the pictures you like will soon cover the cost of the camera itself. When the best on test cost just £185, what is there to wait for?

How do digital cameras work?

Many of the features you need to look for when choosing a digital camera are familiar to anyone who has used an old-fashioned film-based model. Both types of camera use a series of lenses to focus light onto a light-sensitive surface. In the case of conventional cameras, this is a piece of film coated in photo-sensitive emulsion. In the case of a digital camera, this usually is a type of light sensor known as a charge-coupled device or CCD. The camera's lenses move in and out to zoom, or to focus on subjects nearer or further away.

The method by which a digital camera correctly exposes an image is similar to that of a conventional camera. The amount of light that reaches the CCD is dictated by the size of the aperture - the hole that lets in light - and for how long it remains open. Many upmarket cameras enable you to adjust focus, aperture size and shutter speed manually, but most budget models rely on an automatic system. This measures the distance of the subject from the camera and the light conditions before the picture is taken, adjusting the focus and exposure accordingly. This allows users to point the camera at a subject, adjust the zoom, and press the shutter button to take a pleasantly exposed photo.

OK, so all this probably sounds fairly familiar - but once the image hits the camera's CCD, everything becomes very different. Rather than the tiny photo-sensitive silver halide crystals used by conventional film, a CCD contains millions of light-sensitive diodes arranged in a grid. Each of these becomes electrically charged when it's hit by light - the more light, the greater the charge. The charge in each cell is then passed to an analogue-to-digital converter, or ADC, which measures the amount of charge that each has accumulated, and turns it into a digital value that your PC can understand. When the data from all the diodes is pieced together, you have your captured electronic image.

At least, that's the theory. In practice, CCDs are more complex than this. Colour film reacts differently to different coloured light, but each diode in a CCD can't discriminate between different colours. To capture colour, each diode uses a colour filter to measure only red, blue or green light. This means that it takes three diodes to measure the true colour of any given point (in fact, four diodes are used, with two green for each red and blue). The brightness of each diode is measured individually, and the camera combines this information to build up an image made of full-colour pixels.

If you do the maths, you'll find that the quoted CCD size of a digital camera is usually a little higher than the number of pixels recorded. A digital camera that takes 2,800x2,100-pixel images may be described as a six-megapixel camera, even though 2,800 multiplied by 2,100 actually equals 5.88 million. This may seem a bit dishonest, but in fact there probably are six million diodes on the CCD. However, some of the diodes on the edge of the CCD sit above the ADC; these ones are deactivated so their charge doesn't interfere with the measurements from other diodes. The CCD sizes we quote are for the number of effective diodes rather than the megapixel figure quoted by the manufacturer. Our figure tells you how detailed the recorded image is.

Once the CCD has captured, measured and converted the light into electronic data, there's still plenty more to be done. Image processing, performed by the camera's processor chip, turns the raw data into what the camera manufacturer believes is an attractive photo. This takes into account white balance, which defines what shade should be considered white - without it, most indoor photography would have a yellow tint, as tungsten light bulbs give off a yellow rather than a white light.

ISO settings in traditional film cameras refer to the speed of the film - that is, how quickly it responds to light. The diodes in CCDs respond to light at a fixed rate, but high ISO settings can be simulated in digital cameras by boosting the output from the CCD. This gives brighter pictures in dim light - the downside is any small errors generated by the optics or CCD will become bigger errors as the signal is boosted. This means more image 'noise', the random speckling caused by inaccuracies in the light that's recorded by the CCD, particularly in dark areas of shadow.

Other settings, such as specialist scene modes, may also be achieved using image processing, but many, such as night modes or exposure compensation (which brightens or darkens the overall image), will be achieved simply by adjusting the aperture size or shutter speed to allow more or less light to be recorded by the CCD.

From then on, the digital camera enters more familiar territory for PC users. The size of the processed image file is reduced using a process called JPEG compression to enable the camera to fit more shots on to the memory card. JPEG compression involves throwing away some image data, but it's smart enough to dispense with information in ways the human eye won't notice, as long as the compression ratio isn't too high. Make the file too small and there will be visible side effects or artefacts in the recorded image. The compressed image is stored to internal memory or a removable memory card, and is eventually transferred to your PC, either by connecting the two via USB or by putting the memory card into a card reader.

What makes a good digital camera?

When buying a camera, at the top of most people's list of concerns will be image quality. This depends on a great number of things: the lens, the auto-focus and auto-exposure metering (which analyses the image before the photo is taken), the aperture and shutter-speed settings, whether automatically or manually chosen, the CCD resolution and quality, the accuracy of the ADC, the digital image processing (which in itself is a multi-stage process) and the JPEG compression. Although some people say that more pixels means better quality, in fact, higher resolutions don't necessarily mean more detail, as our test results prove. The only way to assess a camera's image quality is to test it in a variety of conditions. Of course, that's impractical for most people - which is why tests like this one are so important.

For the first time since we began reviewing digital cameras, every one of the models on test this month turned in some excellent quality images. However, for those who want the best quality snaps in all conditions, there are differences in how well each camera coped with difficult photographic situations, such as shots taken at close range using flash, or those taken in low light. You'll find the results of our tests in the individual reviews and in the box on page 86.

Besides image quality, you need to consider if the camera is strong enough to withstand the knocks of everyday life. Fortunately, none of the cameras on test gave us any cause for concern. All were housed in metal cases with no obvious weaknesses. However, there were wide variations between cameras in areas such as battery life, compactness and ergonomic comfort.

The two cameras that use AA batteries both ran out of power more quickly than the three that came with rechargeable lithium ion batteries. Both were supplied with non-rechargeable AA batteries, leaving you to buy rechargeable batteries and a charger, which will set you back another tenner or so.

Ease of use is another issue. But do you want ease of use at the expense of control over your photographs? Of the five cameras on test, the ones that were easiest to use lacked the precision offered by more sophisticated models.

Another make-or-break feature is the speed at which the camera operates - most importantly, how long a time lag there is between pressing the shutter button and the photo being taken. Also of concern is the length of time the camera takes to recover between photos.

But there are plenty of other features you may decide you'll need: high quality video recording, a continuous mode for taking quick successions of photos, or a decent range of physical controls, allowing manual settings to be changed with ease.

One thing this test did prove, though, is that you should expect to get everything you want at a great price. Every camera here offers impressive value - it's just that some are more impressive than others.

Buying decisions

With all the cameras on test producing excellent photos, it was tough to pick a winner. Ultimately, though, it came down to two. The Canon Digital Ixus 40 has instant charm, with its stylish design and highly portable size. Using it was a joy thanks to its friendly controls and responsive performance. Its picture quality is impressive, but close scrutiny revealed that other cameras fared even better.

With its relatively bulky design and AA batteries, the Fuji FinePix E500 didn't instantly impress, but with each test our esteem grew. It came top in our tests for image quality, and its manual controls are a step up from the competition. At a price that's approaching entry-level but with quality that's anything but, it fully deserves our Best Buy award.