Labs

Digital Cameras

How do they work?

We're here to tell you which cameras are the best buys. But if you want to understand why, it helps to know the basics of how they work.

Perhaps surprisingly, digital cameras work very much like their film ancestors. Light bounces off grandma and enters the camera via a lens, which focuses the image through a hole called the aperture and onto a surface that's capable of recording it. In a conventional camera this surface is photographic film; in a digital camera it's an electronic light sensor. Decent cameras use only one type of sensor: the charge coupled device or CCD. Avoid CMOS sensors - these were used by some old and cheap cameras and webcams, but they're not half as good as their CCD cousins.

The CCD is really an array of many sensors. A camera with more sensors will produce an image with more detail. In today's market, A 2-megapixel camera (which has 2 million sensors) is run-of-the-mill, with the best having over double this. However, simply having more pixels does not mean that the image will always be better, as we'll see from the cameras on test this month, which have between 2 and 4 million sensors.

Once the image has been recorded by the sensor, it's compressed by the camera's electronics - nearly always to the standard JPEG image format - and stored in memory. Some cameras come with built-in memory, but most have a plug-in memory card, of which there are four main types: SmartMedia, MultiMedia Card (MMC), Secure Digital (SD) and CompactFlash.

What makes a good digital camera?

So far so simple, but taking photos in tricky conditions makes things a lot more complex. So a good digital camera incorporates features to deal with this.

First of all, a certain amount of light needs to fall on the CCD to be registered as a bright, accurate image. To cope with low light conditions, most digital cameras can just wait for long enough for sufficient light to arrive - it may take a whole second, say, for enough light to enter the camera. Better cameras also let you open up the aperture of the camera to let in sufficient light in a shorter time. But there are advantages and disadvantages to both approaches.

Increasing the size of the aperture certainly lets more light through, but also means it hits the lens at a greater range of angles. And this in turn shortens the 'depth of field', which means that while the object you're aiming at may be in focus, the areas behind and in front of it will be blurry. You may want to use this as a special effect, in which case having a variable aperture is very handy. But if you need to get the whole frame in focus, you'll have to decrease the aperture again and allow more light in another way.

Keeping the shutter open for longer will allow you to take pictures with a good depth of field in low light. But it will also cause large amounts of motion blur if anything moves in the picture or if the camera itself moves. Using a shutter speed of less than 1/60 of a second really requires a tripod. You might want to introduce blur into a picture for a sense of dynamic movement, in which case you should probably still use a tripod and rely on the movement of your subject. That is, unless the effect you're after involves waving the camera around in front of a static object (which can create interesting patterns).

There is a third option on many digital cameras, and that is to amplify the signal that comes out of the CCD. This is equivalent to using a 'faster' conventional film with a higher ISO rating (usually printed on the box: ISO 100 is slow, while ISO 400 is fast), so is typically called the ISO setting even on digital cameras. The trouble is, amplifying the picture also amplifies the digital noise, and the penalty is a grainy image. An ISO setting is a definite bonus, but unpredictable - so it's typically only accessible in manual rather than automatic mode.

Digital cameras also need to cope with different types of light. Film camera enthusiasts will know that taking photos in daylight using normal film works fine. Indoor shots lit by lightbulbs, on the other hand, are excessively warm and orange, while fluorescent lights create an overly cold effect. So film buffs use different types of film for differently-lit environments. Digital users are luckier because most digital cameras can adjust for different lighting conditions at the flick of a switch.

What makes a really good digital camera?

The ideal digital camera does more than just cope with tricky lighting. It should be easy to use, ready to operate as soon as it's turned on and produce clean and detailed pictures that reproduce colours accurately. It should be able to display the results very quickly and offer the user an option to delete it. Its automatic mode should be able to handle various lighting conditions and balance shutter speed with aperture size in an intelligent way.

There should be an optical zoom, and there needs to be plenty of memory to store the final images. With today's high-resolution cameras, anything less than 16Mb is just plain mean. In fact, unless you fancy lugging a laptop around on holiday with you, it's a good idea to run down to your local camera shop and get yourself an extra 64Mb card.

Retrieving the images from the camera using a PC should require almost no effort. Cameras with removable memory are ideal because you can not only switch cards while on the move, but you'll also have the option of attaching a card reader to your PC. You won't have to mess around with installing camera drivers in Windows, or weird pieces of proprietary software for downloading the images.

The camera's lens should be of good quality, with no obvious distortion, and there should be plenty of sensors on the CCD - these days choose nothing less than 2 million. The flash should be bright and come with an anti-red eye mode that flashes briefly before the main flash. This makes the subject's pupils shrink so that when the main belt of light comes it doesn't reflect off the back of the eye, which is what causes that spooky devil-person effect.

The camera's manual mode should provide lots of shooting options, including the ability to change the ISO rating, the lighting type (such as daylight, flourescent and tungsten), and exposure compensation. This last setting is helpful in situations where the background to your subject is very bright, such as a sun-filled window, or when you want to pick out dark areas behind a well-lit subject.

A self-timer for taking group shots including the photographer is useful, and if the camera is capable of shooting movies it'll be a lot more useful record audio too. You'll need a decent LCD screen to preview your video or still images, otherwise you won't know which ones to keep and which ones to ditch.

Battery life is critically important, as is the ability for the camera to power down when left switched on for more than a couple of minutes. Lithium ion rechargeables are best for both performance and your pocket.

Finally, fiddly controls or important controls that can only be accessed via torturous menus are not helpful. A good camera will make the most important settings instantly available, either through the use of a physical switch/button or a menu no more than one click away.