Digital camera sensors work by converting light energy (photons) into electrical energy (electrons). This energy forms a signal that’s transferred to the camera’s processor.

The sensor is made up of millions of individual photosites, each one corresponding to a pixel in the final image. A six-megapixel digital SLR has six million pixels on its sensor.

Actually, there are a few more than this, but many pixels around the edges are used for calibration and other purposes. This is why you’ll usually see two figures quoted for sensor resolution: total pixels and effective pixels. The effective pixels are those used to form the image and are the most frequently used.

But there’s a complication. Each photosite, or pixel, is sensitive only to the quantity of light, not its colour. Film manufacturers got round this by incorporating several layers of film, each sensitive to a different colour. Most digital sensors, with the exception of the Foveon X3 sensor (see the boxout), have only one layer. What the makers do, then, is cover each pixel with a red, green or blue ‘micro-filter’.

In fact, it’s been found that the human eye is most sensitive to the green part of the spectrum and manufacturers arrange pixels in groups of four: two green (two pixels with green filters over them, in other words), one red and one blue. There are twice as may green pixels as blue or red pixels. This arrangement is known as the ‘Bayer pattern’.

The image data now has to be processed to produce full-colour data for each pixel. The camera looks at the surrounding pixels to estimate or interpolate this colour information. For example, a green pixel would have data from the red and blue pixels around it added, in order to produce a full-colour RGB (red, green and blue) value. The same applies to red and blue pixels. This colour interpolation is sometimes called ‘demosaicing’, since in it’s non-interpolated form, the arrangement of red, green and blue pixels, resembles a mosaic.

Like other interpolation processes, demosaicing is a technical bodge. Colours (or the outlines of coloured objects) are softened slightly and you may sometimes see slight colour bleed from one area to another when images are magnified on the screen.

There’s another source of blurring with digital images. DSLRs use anti-aliasing or low-pass filters in front of the sensor. These blur the image slightly before it’s recorded by the sensor. This is to prevent regular rectangular patterns in fabrics, distant fences or the corrugated cladding on industrial buildings interfering with the rectangular array of pixels on the sensor and producing moiré as a result. Some DSLRs can appear sharper than others even if the sensor resolution is the same. This may be due to differences in the strengths of the anti-aliasing filters.

This filter, incidentally, covers the sensor completely. If you ever need to clean dust off the sensor, it’s the anti-aliasing filter you’re cleaning, not the sensor surface itself. The filter surface is usually quite hard and resilient and will only be damaged with careless or vigorous cleaning. Nevertheless, you undertake any sensor cleaning at your own risk!

The combination of colour interpolation (demosaicing) and the anti-aliasing filter takes the edge off a digital SLR’s resolution. This is counteracted by the sharpening applied by most cameras as the image is processed.

The number of megapixels on the sensor is no longer a reliable guide as to how good  a camera’s pictures will be. In the early days of digital photography, sensor resolution was the limiting factor, but now we’ve gone past that point and the physical size of the sensor is paramount.

There are two main reasons for this: the first is electronic and the second is optical. The pixels on a sensor turn photos into electrons. The smaller the pixel, the smaller its capacity for holding electrons. This has two bad effects. First, it restricts the maximum brightness level the pixel can record without overflowing and this restricts the camera’s overall contrast or 'dynamic range'. Second, random background noise is unavoidable, and the smaller the pixel the greater the noise compared to the brightness value the pixel records. That’s why you’ll often hear the small, high-resolution sensors used in digital compacts referred to as being noisy. Noise is worse at high ISOs because cameras increase sensitivity by amplifying the pixel values – including the random noise. This makes the signal-to-noise ratio much worse, and that’s why high-ISO shots show pronounced ‘speckling’.

The latest ten-megapixel compacts use extremely small, extremely high-resolution sensors where noise has become a serious issue and is visible even at the lowest ISOs.

In-camera noise reduction is used as the image is processed and saved in order to disguise this noise. A secondary effect of this is that while coarse outlines appear crisp, fine textural details are ‘smudged’.

The optical issues are just as serious. There are extremely complex mathematical and optical calculations involved in predicting visual sharpness and lens performance with different sensor sizes and resolutions. If they were condensed to fit the space we have here, they’d make even less sense, so we’ll opt instead for a much simpler comparison.

The boxout (over the page) on sensor sizes shows the differences between digital SLRs and compacts graphically. The key point here is what we’ll call the 'enlargement factor'. In order to get an A4 print from a DSLR you need to enlarge the original image, as captured by the sensor, by a factor of 13. To do the same with an image captured by the tiny sensor in a digital compact requires an enlargement of 50.

This means the lens in the compact camera would have to be four times sharper than the lens on the SLR to give an equivalent result. And that’s never going to happen, partly because there are optical laws (diffraction) limiting resolution even with perfect lenses at these small sizes, and partly because if makers knew how to make compact camera lenses four times sharper they’d be using the same techniques to make SLR lenses sharper too.

In fact, there’s an old adage from the days of film that makes just as much sense when applied to digital sensors. It goes: “A good big ‘un will always beat a good little ‘un.” That was used to justify upgrading from 35mm to medium-format, for example, or from large-format to 5 x 4”. Today, you’ll see a considerable jump in image quality from a compact camera to a digital SLR with the usual APS-C sized sensor, and another jump if you move to a full-frame SLR. There’s another jump again when you enter the world of tethered digital backs, which have larger sensors still.

Megapixels don’t matter anymore. Indeed, in the compact market we’re now seeing cameras with a higher number of pixels but lower dynamic range, crippling noise levels (or harmful noise reduction processes) and sharpness levels that don’t show any significant improvement.

So why do camera makers keep on increasing the resolutions in compacts? You have to remember that the most important thing for them is to sell cameras. The number of people who don’t yet have a digital camera is small and dwindling, so the makers increasingly rely on selling cameras to people who’ve already got them. And the only way to do this is to keep producing cameras that are ‘better’ than last year’s. The megapixel rating has become embedded in the consciousness of the buying public as the measure of a digital camera’s performance, and as long as the public clings to this idea, the makers are going to keep pandering to it.

In fairness, there are commercial and practical advantages to small sensors, leaving aside the megapixel angle. The ‘yield’ during the manufacturing process is much higher. Yield is related to the chip area, not the number of pixels. The bigger the chip, the more likely it is to show a flaw that necessitates its rejection.

It’s also easier and cheaper to make lenses for small sensors and it allows really exotic optics like 10x and 12x zooms to be manufactured at a reasonable cost. The optical light path is smaller, allowing smaller camera designs, and the sensors use less power so that batteries last longer (or, more to the point, the makers can fit smaller batteries).

Will we ever be able to buy compact digital cameras with sensors the same size as those in digital SLRs? That would be the dream of many serious photographers looking for a good compact as a second camera. It should be possible, too.

The difference in sensor sizes means that the quoted focal lengths of lenses don’t necessarily tell you their true angle of view. For example, a 3x zoom lens on a typical compact might be 5.8-17.4mm, which on a 35mm camera would be ultra-fisheye. On a digicam, though, it gives the same angle of view as a 35-105mm zoom.

That’s why digital camera makers talk in terms of focal length ‘equivalents’. It’s becoming increasingly common to offer the actual focal lengths and the equivalent focal lengths on the specs sheets, or simply to drop the actual focal lengths altogether.

Once you know the sensor size, converting actual focal lengths to equivalent focal lengths involves a simple mathematical conversion factor. Digital SLR makers often quote this factor. With most digital SLRs that use the APS-C sensor size (roughly 23 x 15.5mm), the focal factor is 1.6. In other words, you multiply the actual focal length of a lens by 1.6 to get its equivalent focal length in 35mm film camera terms. With full-frame cameras like the Canon EOS 5D and 1Ds Mk II, there’s no focal factor at all.

SENSOR SIZES

This shows the relative sizes of camera sensors. The bigger the sensor, the smaller the enlargement factor for prints

1/2.5-inch
This produces the highest noise and lowest quality, but it’s popular for cheap point-and-shoot compacts, super-slim pocket cameras and affordable super-zoom lenses, like the 12x zoom on the Panasonic Lumix FZ8.

1/1.8-inch
Once the most common sensor size for compacts, it’s now confined to higher-end models like the Casio Exilim EX-Z1200. Resolutions have climbed to 12MP, but quality still has a heavy dependence on the lens.

Four Thirds
Pioneered by Olympus and now taken up by Panasonic, the Four Thirds format is used in DSLR models from both companies. It’s smaller than the sensors found on other DSLRs and the noise levels reflect this, but it's better quality than a compact.

APS-C
This is the sensor size used by almost all DSLRs, including the Nikon D80. It offers very good image quality, but while the latest cameras offer 10MP resolution, compared to older and cheaper 6MP models there is some increase in noise.

Full frame
Currently, Canon’s the only manufacturer to produce full-frame SLRs – in other words, the sensor has the same dimensions as 35mm film. The EOS 5D is excellent value and offers a visible quality advantage over SLRs with APS-C sensors.

SuperCCD SR
A common complaint among film photographers is that digital camera sensors don’t have the dynamic range of film and they ‘blow’ highlights too easily. Much of the dynamic range of film comes from the mixture of different sizes of film grains in the emulsion. The larger grains are more sensitive to light and record shadow detail and midtones, while the smaller grains are less sensitive to light and are able to hold on to highlight detail as a result.

Fujifilm’s SuperCCD SR II sensor replicates this effect by alternating high-sensitivity and low-sensitivity photosites. The larger ‘S’ pixels do most of the work, but the smaller and less sensitive ‘R’ pixels record highlight detail that other sensors would ‘blow’. Fans of the SR sensor may have been dismayed by Fujifilm’s decision to stop making the FinePix S3 Pro earlier this year, but will be delighted by the news that it’s to be replaced by a new S5 Pro model incorporating the same double-barrelled sensor design.

CAPTIONS

The sensor sizes in compact digital cameras are quoted as fractions of an inch. These are measured across the diagonal and include the chip rebate around the imaging area. This rather inflates the true size of the sensitised area

Fujifilm’s SuperCCD SR II has two sizes of photosite to extend its dynamic range in the highlights by 400 per cent

The FinePix S5 Pro uses the same SuperCCD SR II sensor design as its predecessor.