Gordon Laing

As the figure which defines the amount of detail captured, the number of megapixels has become the primary measure of quality for digital cameras. But, like so many things, it's not that simple, as having more megapixels can sometimes reduce your overall picture quality.

This sounds hard to believe. After all, more megapixels means more detail and being able to make bigger enlargements or severe crops without losing quality.

But take a close look at digital camera images and you'll notice numerous randomly coloured specks, especially in areas of flat colour. This is mostly due to electronic noise on the sensor, and could get worse with higher resolutions.

Here's why. Each sensor has a grid of light-sensitive elements which act like buckets for light. After the exposure, the amount of light captured in each element is proportionately converted into electrical charge, then amplified and digitised.

Noise becomes a problem when there's insufficient light captured for a decent signal. This gets worse with smaller elements which are less sensitive.

There are two ways you can increase the resolution of a sensor: keep the elements the same size and increase the sensor area; or keep the sensor the same size and shrink the elements to squeeze more in.

It's clear that the latter will be susceptible to greater noise yet, due to manufacturing costs and optical designs, it's a common approach for new sensors.

The first 3megapixel cameras employed a 1/1.8in sensor which measured 8.93mm across its diagonal and kept noise reasonably under control. When 4megapixel cameras arrived, though, their sensors were almost all the same size as previous 3megapixel models, thereby employing smaller elements with higher noise levels.

Fortunately when the first 5megapixel cameras were launched, the sensors were increased to the 2/3in size with an 11mm diagonal and roughly the same size elements (and therefore noise levels) of original 3megapixel sensors.

Sadly the subsequent generations of 5megapixel cameras switched to a 1/1.8in sensor identical in size to the original 3megapixel sensors. New 8megapixel all-in-ones also use the same 2/3in sensor size as the first 5megapixel models.

Both cases demand smaller elements to squeeze these higher resolutions into the same space, and therefore suffer from greater potential noise. While the industry often claims that superior manufacturing techniques and post-processing eliminate the problem, I'd argue that new sensitivity settings reveal it as a genuine concern.

All digital cameras can increase their sensitivity, but at the cost of greater noise. Decreasing the sensitivity has the opposite effect and lowers noise levels. And what's this? Many new cameras with high resolutions and relatively small sensors now offer low sensitivity options such as 50 or 64 ISO; indeed these are present on all the new 8megapixel cameras.

Admittedly these low sensitivities seem to do the trick. I've found shooting at 50 or 64 ISO on the new 8megapixel cameras produces relatively low-noise images. Increase to 100 ISO, though, and noise begins to creep in. By the time you're at 200 ISO it's seriously affecting the picture.

The trouble is that it's rarely practical to shoot at 50 or 64 ISO, especially in the overcast UK, where 200 ISO is usually more appropriate.

You are, of course, welcome to shoot at lower sensitivities, but you'll need more light: four times more if it's the difference between 50 and 200 ISO. And if your optics can't gather it, you'll need a longer exposure which increases the risk of camera shake.

And therein lies the rub. Unless you have bright conditions, a steady hand or don't mind carrying a tripod, these low sensitivities are impractical, yet necessary to keep noise levels under check.

With noise becoming a personal issue I upgraded to a digital SLR, which may only have 6megapixels but features a sensor measuring 30mm across its diagonal.

Compare that to the 11mm diagonal of the sensor in 8megapixel all-in-ones and difference in pixel size is monumental. At 400 ISO, this bigger sensor produces roughly the same noise levels as 8megapixel models do at 64 ISO. At lower sensitivities, the bigger sensor produces incomparably smooth pictures. This is why 6megapixels worked out better than eight.

I may capture less detail, but the improvement on noise levels, not to mention flexibility of shooting at higher sensitivities, is far preferable.

Of course digital SLRs are only an option at the high end of the market. Compact consumer digital cameras will always have smaller sensors and therefore noise as a greater concern, especially as resolutions increase.

While many users won't notice noise on their images, I fear it's being compromised in favour of higher resolutions. And does the average photographer really need five or more megapixels?

Sadly, higher resolutions have become a marketing numbers game and are coming at the cost of greater noise. If you're concerned, look carefully at the sensor size of your next digital camera, as megapixels aren't the be-all and end-all of image quality. Either that or join me in a prayer for a long, bright summer.

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