Last month, we discussed how to choose the perfect first lens for your DSLR, and looked over the two most important aspects of any lens: focal length and f-stop. However, once you get outside of the lens and into the camera body itself, you need to look into a few other settings. One of these is ISO sensitivity, which determines how sensitive the camera's sensor is.
Digital camera ISO sensitivity is similar to the speed of film, right down to the numbers of the settings. By setting your digital camera to ISO 100, it becomes roughly as sensitive as ISO 100 film. Similarly, by setting it to ISO 1600, it becomes approximately equivalent to high-speed ISO 1600 film. The "faster" (more sensitive) the setting, the less light it needs to form a picture. A camera set at ISO 100 might require a very long exposure at the risk of an unsteady (and blurred) shot, while one set at ISO 1600 might only need a quick snap.
better it looks. However, high ISO sensitivity has a very big drawback, and it's the same drawback found in high-speed films: the noise. The more sensitive the sensor or film is, the more variations in light it can pick up, which manifest as a fuzzy graininess.
ISO 100 on the left, ISO 6400 on the right.
Whenever possible, you should use the lowest ISO settings that still let you get the exposure at the right shutter speed and f-stop you desire. It will always produce a better picture than higher ISO settings on the same camera. If you can't do that, use a flash to better illuminate the subject to let. If you can't do that, only then consider cranking up the ISO sensitivity.
ISO 100 on the left, ISO 1600 on the right.
reciprocal rule of shooting is a good general measure of how fast you want to shoot and when you need to push the ISO sensitivity up to meet that speed. The rule is that the slowest shutter speed you should use is equal to the focal length of your lens (multiplied by your camera's crop factor, which on a non-full frame SLR is about 1.5 to 1.7). If you're shooting with a 50mm lens on a Canon Rebel T1i, then you don't want to shoot slower than around 1/75ths of a second. If you can't get a good exposure at the widest f-stop setting at that shutter speed, then consider increasing the ISO. Of course, you can only follow this rule to the letter when you use an SLR or Micro Four-Thirds camera; a snapshot camera's focal length and crop factor are unpredictable, so you have to wing it.