The digital photography terrain is paved with acronyms, and one of the most important ones to understand is known as ISO. What’s interesting is that ISO isn’t an acronym, really--it was created by the International Organization for Standardization to refer to sensitivity of film to light. The term (or acronym) ISO replaced the film equivalent term ASA (American Standards Association). In the days of shooting film, you would purchase film according to its ASA, and this would indicate how sensitive the film would be to light. “Faster film” meant it was more sensitive to light; “slow film” meant less sensitivity to light.
So, how does this translate today? If we aren’t shooting film, why do we need to worry about ISO?
Well, ISO is one of the key components in creating a properly exposed image. ISO in today’s technology refers to the camera sensor’s sensitivity to light. Photographers will often bump up the ISO in situations such as low light conditions or in circumstances where shutter speed has already been decreased as much as possible. By increasing our ISO setting, we are increasing our camera sensor’s sensitivity to light. As you increase the ISO, less light is needed for the shot. Each time you double the ISO, it equates to needing only half the amount of light to create the same exposure.
One challenge of shooting at higher ISO settings is that of ‘noise.’ The term ‘noise’ refers to the stray speckles than can be created in an image. In the film equivalent, noise was referred to as film grain. You may not always notice image noise and it may not become apparent until you enlarge an image considerably, but image noise can detract from a quality image in that the photo will look more grainy and speckled. Smaller compact cameras are often more prone to image noise; some cameras can start showing evidence of noise at an ISO of 400 and above. This is due to the fact that a compact model camera’s sensor is much smaller than one used in a DSLR, and a smaller sensor means increased sensitivity at lower ISO numbers.
Some of today’s high end DSLR cameras offer ISO ranges from 100-6,400, which can then be further expanded higher due to mathematical algorithms. And some of these cameras can shoot at these high ISOs with minimal noise. If such a camera isn’t in your budget, simply practicing varying the ISO settings in different conditions is the best method for helping you understand the relationship between ISO, shutter speed and aperture.
Learning to adjust ISO in various lighting conditions is an important cornerstone in advancing your shooting skills. It’s also a great opportunity to get more comfortable with your camera’s more advanced settings.