Digital Photography is one of the fastest-growing consumer product areas, and the pace of change in technology can be bewildering!

In the last few years we have seen even quite modestly priced digital cameras able to rival film quality, at least for prints up to 9"x6.

However, with so many cameras on the market, which one should you choose? What features are worthwhile and what are not? Increasingly it becomes difficult to distinguish between the numerous brands, resolutions and features on offer.

So lets start with that resolution issue, the most often quoted (and misquoted) digital camera attributes.

At the time of writing, consumer digital cameras offer up to 8 Megapixels (Mp), with professional models offering up to 22 Mp. It is obvious that a Megapixel is a milllion pixels, but what exactly is a pixel? The word actually comes from the words picture element. Simply put, if you were to view a digital image closely, you would see that the image is actually made up of numerous tiny dots. Each dot in the recorded image has a certain brightness and colour, and corresponds to a tiny individual light receptor (pixel) in the camera. These receptors are grouped together in a rectangular array and form the cameras sensor, the equivalent of conventional film.

If we arrange these receptors in several hundred rows, with each row containing several hundred receptors, for example 1700 rows each containing 2500 receptors, we now have a sensor with just over 4 million receptors, or a resolution of 4Mp (1700 x 2500 = ~4 million). Similarly 2000 rows with 3000 receptors gives us 6Mp (2000 x 3000 = ~6 million), and so on.

More Megapixels gives us more dots, which practically enables us to display our image at greater sizes before each individual dot become apparent and the appearance of image quality deteriorates as a result. Most books and magazines commercially printed use a printed resolution of 300 dots per inch (dpi), so our 4Mp camera is good for printed images up to nearly 8.5 inches (2500/300 = 8.3) by about 5.5 inches (1700/300 = 5.6) at this print resolution. Using the same logic, our 6Mp camera is good for prints 10 inches by about 6.5 inches at 300dpi.

The more Megapixels, the more storage space you will need, both on your PC but also in camera. Consider what use you are going to put your camera to: if you are going to be away from base for weeks on end, you will need more storage; if you only intend to go on day trips before downloading your shots from the camera to PC, then you may not need as much storage.

In very general terms, I would budget around 25% on top of the camera price for memory, and plan to buy twice as much memory as you think you will need. As a very rough guide, files sizes will be about 3 times the number of Megapixels, so a 3Mp camera will produce 9 Megabyte(MB) files at the highest quality setting. In this example a 256MB card will hold about 28 images (3 images = 27MB).

The size of the captured image file also as an impact on the cameras speed and responsiveness, because bigger files take longer to write to the memory card. Most cameras do not write directly to the memory card (this would be far to slow), instead the image is copied to much faster memory integral to the camera, called buffer memory. The buffer memory is then copied to the removeable storage media.

The number of images that can be captured in a given period is measured in frames per second (fps). With digital cameras however, once the buffer memory is full, the camera cannot capture another image until enough space has been freed up by copying data to the removeable storage media.

So, there is another aspect of digital camera speed and responsiveness to consider: the number of images that can be continuously captured at the specified fps for a given image size/quality setting, before the buffer memory fills up. This is often referred to as burst-mode. For example, a professional Digital SLR may have a frame rate of 8 fps, and a burst mode of, say, 40 frames when using jpeg-fine. Practically speaking, that means hold down the shutter and it will take 40 frames in 5 seconds, and then stop until enough space is free in the buffer memory to re-commence shooting - even though you are still holding the shutter down.

The final aspect to be considered is shutter-lag (sometimes referred to as latency). Previously, film cameras captured an image almost instantaneously with the the press of the shutter button. Digital cameras do not always display this sort of response, although the latest models are generally much better than early ones.