Introduction
Until a couple of years ago compact digital camera manufacturers were happy to admit that small, high resolution sensors lacked the sensitivity to offer high ISO modes - anything over ISO 400 was very rare indeed. This is unfortunate because there are many times when the ability to shoot in low light without flash is either essential or at the very least highly desirable. From low-light landscapes to atmospheric portraits to high speed sports there are endless situations where a higher sensitivity setting offers huge benefits (something SLR users take for granted).
The issue of high ISO performance on compacts first raised its head with the launch in 2005 of the Fujifilm FinePix F10, which was the first camera to offer anything close to usable ISO 800 and ISO 1600 performance (later refined in the FinePix F30 and FinePix F31fd) - a unique selling point in a market full of barely discernible models from a wide range of manufacturers. Within 18 months every compact camera worth its salt was offering ISO 800, ISO 1000, ISO 1600 or even higher - and special 'high sensitivity' or 'low light' subject modes. Perhaps unsurprisingly there's a lot more to high ISO performance than mere numbers, and - particularly with the latest 6-10MP compacts - we believe consumers are being misled by the manufacturers when it comes to the actual performance of most cameras at higher ISO settings and in low light. So we decided to put together a short article on the truth behind those headline high ISO modes.
We've started with some technical background information - if you just want to see how the various high ISO options actually compare then feel free to skip to page 3.
Some background information
Sensor sizes
The CCD sensor used in most compact digital cameras is small; very small indeed. Typically they measure from around 9mm to 14mm diagonally - compared to 30mm on most digital SLRs (and 43mm on 'full frame' SLR models). And yet - with only one or two exceptions - there is little difference between the pixel counts offered by compact cameras and SLRs - at the time of writing both types of camera max out at about 10 million pixels (although some 12 million pixel compact cameras are starting to appear).
It doesn't take a rocket scientist to work out that if you've got two sensors with 10 million pixels on them and one is over 10x smaller than the other, the pixels themselves will also have to be a lot, lot smaller to squeeze them all in. And so they are. The trouble with very small pixels is that they are a lot less effective at gathering light. This low sensitivity to light means the camera's processor has to work with a very weak signal and that brings a whole new set of problems.
The rectangles above show the relative sizes of three typical sensors). In order to fit the same number of pixels onto each sensor the pixels themselves obviously have to be made much smaller. The first two are used in digital SLR cameras, the smaller two are only used in compact models
The problem of low sensitivity
Low sensitivity means you need a lot of light to produce a signal from the sensor strong enough to record an image. In bright sunlight this isn't really an issue, but we don't always shoot in bright light. We also want to be able to take pictures in low light, or to be able to use very short exposures for freezing motion.
ISO settings
In the days of film we would buy a higher sensitivity film (with a higher ISO or ASA rating) for use in low light or for shooting fast action. In the digital era you can change the ISO setting in-camera - from around ISO 50 (low) to ISO 1600 or even higher.
It's important to remember, however, that what you are really doing is amplifying the signal from the CCD - you can't actually increase the sensitivity of the sensor itself. Of course you rarely get something for nothing in this world, and there are some serious trade-offs associated with increasing the ISO setting too far.
Noise
Like all electrical circuits, CCD sensors are not perfect, and produce some noise - unwanted random errors in the signal (think of it as being analogous to the interference you get with poor TV or radio reception). Noise in digital images appears as 'graininess' or specks of false color.
Although it increases with longer exposures and at higher temperatures, broadly speaking for normal photography the amount of noise produced in a sensor is fairly consistent. At low ISO settings the amount of noise relative to the signal produced by the CCD is very low, so it doesn't have a huge impact on the picture quality. At higher ISO settings the camera's processor is having to amplify a much weaker signal, and as it doesn't know what's noise and what isn't, the noise gets amplified too and you end up with an image where the noise is much more visible. This relationship - between the relative strength of the signal (containing real image information) and the noise (containing unwanted false information) - is known as the Signal to Noise Ratio; the higher the S/N Ratio the less noisy your pictures will look.
Most digital SLRs produce essentially noise-free images at their lowest ISO settings (usually ISO 100-200), and - thanks to their relatively sensitive CCD or CMOS sensors - don't suffer too badly from noise at higher (ISO 400-800) settings either. A few models can produce perfectly usable results at ISO 1600 or even ISO 3200.
Small sensor compacts, by comparison, tend only to be able to produce low noise images at their very lowest ISO setting (usually ISO 50-100). At any higher setting noise becomes very intrusive and once you get to ISO 400 or higher the results are so noisy that they are only suitable for very small prints.
Viewing the output from a typical small sensor 8MP compact at 100% / actual pixels (in this case the Ricoh GR-D) reveals how much noise has an impact on the overall quality of the photograph. It is always best to use the lowest ISO setting possible.
http://www.dpreview.com/articles/compactcamerahighiso/
