It used to be that you had to change your film roll to adjust your camera's sensitivity to light. Now it's just a setting that you can adjust from the menu called ISO. It's a numeric scale, so if you set your camera to ISO 400 it is twice as sensitive to light as when it's set to 200. Depending on the type of photograph you're trying to take, the ISO you'll want to use can vary greatly.
The first thing you have to understand is that ISO is only one third of the equation when it comes to taking a properly exposed photograph. The other two variables – aperture and shutter speed – work in conjunction with your camera's light sensitivity in order to capture a photo that is the proper brightness.
Let's just say, to keep things simple, that the aperture – how much light the lens captures – is fixed. Boosting the ISO lets you shorten the amount time it takes the shutter to open and close, making it possible to freeze motion. Conversely, there are some instances when you may want to lower the ISO and use a longer shutter speed – landscape photographers sometimes use this technique to smooth the appearance of a river or waterfall.
Even though the sensitivity exhibited at a certain ISO is standardised – ISO 100 is the same no matter what camera you are using – the performance is not. Generally speaking, image sensors with lower pixel density are capable of producing photos with less image noise at a wider range of ISO settings than those that are crammed with pixels.
Pixel density is pretty easy to figure out. A typical point-and-shoot camera has an image sensor that is 4.6 x 6.2mm in size – that's really, really small. A typical APS-C D-SLR has a sensor that is 18 x 24mm. If both cameras have the same resolution, say 16-megapixels, the pixel density of the camera with the physically larger image sensor is much lower, giving it an innate edge at higher ISOs.
The higher you boost a camera's ISO, the more noise you introduce into your images. Digital noise is akin to film grain, so lots of noise is great for artsy shots or those times when you want a retro Instagram look, but it's something that you'll want to avoid in traditional portraiture and family snapshots.
For us, when testing using the Imatest software suite, our cut-off point for acceptable image noise is 1.5 per cent, and we also like to check to see if a camera is keeping noise low at the expense of fine image detail. Cameras can apply digital noise reduction to photos, and if it's too aggressive, fine detail is wiped away along with noise. Some more advanced cameras let you customise the level of noise reduction that is applied to JPG photos to suit your liking. If you have a camera that supports this and prefer to shoot in Raw format, you can always customise the level of noise reduction that is applied during your post-processing workflow.
When you're shopping for a new camera, it's important to not only look at the highest ISO that is possible, but to also read reviews to see how the camera performs at that setting. It may well be the case that two cameras can shoot at ISO 3200, but one has a larger sensor and can capture clean images through ISO 1600, whereas the other can only manage the same feat through ISO 200. Do your research carefully.
Also, to get more out of your digital camera, check out our 10 top tips to help novices take better photos.