Compact Camera High ISO modes contd.

As mentioned on the previous page digital SLRs can usually produce perfectly acceptable results at ISO 800 or higher, with the best models allowing you to shoot right up to ISO 3200 - a high enough setting to allow hand-held photography by candlelight.

Increasingly manufacturers of compact cameras are also using 'high sensitivity' as a selling point; either by including ISO 800, 1000 or 1600 presets or by adding special 'low light' modes that turn the sensitivity up as high as ISO 3200 (or even 6400 in one case). It's important to note that these settings rarely produce acceptable results - the pictures will be either noisy or blurry or (often) both.

Manufacturers can't work miracles

The 'base' or 'optimum' sensitivity of most compact camera sensors is roughly ISO 100. Each step on the ISO scale represents a doubling of the 'equivalent' sensitivity.

Of course what actually happens is that for every step up the ISO scale the camera halves the exposure and amplifies the signal to produce a correctly-exposed photograph. With tiny pixels (which are literally 'counting photons' anyway) this inevitably leads to a significant increase in noise. By the time you hit ISO 1600 the camera's processor is working with a signal from the CCD that is somewhere in the region of 5-10% that is is at base ISO. Since the amount of noise in the circuit remains roughly the same it's no surprise that it becomes more and more visible.

As you increase the ISO setting the amount of noise as a proportion of the entire signal rises dramatically (S/N ratio is decreasing). Note that this simple diagram is not to scale and is merely designed to represent visually the effect of amplifying an increasingly weak signal.

Noise Reduction

All cameras use some form of noise reduction to attempt to reduce the visual impact of noise at higher ISO settings, but few are very effective. The very nature of noise makes it very hard to remove without also removing the fine detail from the image.

The very high megapixel counts used in modern small-sensor compacts mean noise reduction is often used even at the lowest ISO settings, and this has an impact on low contrast detail such as hair or foliage. The destructive effect of noise reduction gets worse as you increase the ISO setting so that by the time you hit ISO 400 or higher the result is unacceptable for anything but the smallest print.

Low noise reduction (left) produces a 'grainy' looking ISO 400 image, but one that still has some detail. Using high noise reduction (right) produces a much smoother image, but most of the fine detail has been sacrificed.

Pixel Binning

As well as noise reduction many manufacturers employ a technique known as 'pixel binning' for high ISO modes. Pixel binning - put simply - involves combining the signal from four or more adjacent pixels into a single 'super pixel'. The idea is that you trade off resolution (lower total pixel count) for sensitivity (since each 'super pixel' contains the output of several pixels added together).

Our experience is that the reality is usually a lot less satisfying. This is partly due to the inherently low sensitivity of small, high pixel count sensors (four noisy pixels does not make one noise-free 'super pixel') and partly because many manufacturers 'upsize' the result so as to allow a 'full resolution' high ISO mode, which simply reveals the loss of detail in all its glory.

The examples above show pixel binning high sensitivity modes from two different cameras (click on the crops above to see the full studio test shot). The image on the left is from a 6MP camera (Panasonic DMC-FZ7), which resizes back up to full resolution after pixel binning. The image on the right is from an 8MP camera (Olympus Stylus 800) that leaves the pixel-binned result at its lower resolution (in this case 2MP).