ISO is sometimes referred to as the third element of the 'exposure triangle' as it can be used alongside aperture and shutter speed to control how light or dark your picture will be. Changing the ISO setting alters the sensitivity of the sensor inside the camera. Making the sensor more sensitive to light allows the use of faster shutter speeds or enables pictures to be taken in low-light situations without having to resort to using flash.
The advantages of being able to use a faster shutter speed include reducing the possibility of blurred photos caused by tiny movements of the camera while the shutter is open and being able to freeze motion when photographing moving subjects.
Cheaper models only allow ISO settings of up to around 1600 or 3200, while more expensive cameras have higher settings, although the highest settings are sometimes hidden away and need to be enabled in the menus.
Some models have such high ISO settings that they almost allow pictures to be taken in the dark, but you do have to be aware that higher settings will produce 'noise' - a speckled, or grainy, effect on your pictures - so make sure you also get a camera with a noise reduction facility built in. Obviously there is only so much you can expect a camera to do. If lighting levels are very poor you are going to struggle to get a high quality photo. In some circumstances there is very little you can do unless you are able to use flash.
Digital SLR Basics
Digital SLRs and Digital Cameras Key Differences
Digital SLR or Compact System Cameras
Digital SLR Handling
Help for Beginners
Lenses and Accessories
Buying a Camera Lens
Digital SLR Accessories
Learn More About Features
Resolution and Sensor Size
Manual Exposure Modes
Help With Tricky Lighting
Live View and Articulated Screens
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Current and Recommended Models
Summary of Current Models
Recommended Digital SLRs
Digital SLR Guide Author
This guide was written by Ian Younger
The amount of noise produced when using higher ISO settings can also depend on the overall quality of your Digital SLR. As a rule of thumb you can expect a more expensive Digital SLRs to produce lower noise levels. This is an area where there has been considerable improvement over time. As technology moves forward further enhancements can be expected, especially as this is one area that can make a difference between someone buying one camera instead of another.
If you find yourself taking lowlight pictures on a regular basis and you are unable to get the picture quality you desire, there is something you can do. If you use lenses with larger apertures (f/1.2, f/1.4, f/1.8), this can overcome some of the common issues of lowlight photography. Lenses with these large apertures tend to have shorter focal lengths, so you will need to be able to get in relatively close to your subject for this to work.