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Guide to Sensors

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.

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