Tip of the week: Remember that ISO effects not only your exposure, but also the quality of the photograph. The ISO number gives a numerical value to the current sensitivity of the film or digital sensor. For film photography, the ISO is determined by the type of film used (Kodachrome 64, Velvia 50, T-Max 400, and Tri-X 125). For film photography, photographers are stuck using the same ISO sensitivity until they change their roll of film (typically every 36 exposures). Digital photographers have the opportunity to change the ISO in camera to determine how sensitive the digital sensor is to light. Since the ISO is manipulated in camera much like shutter speed or aperture, digital cameras can alter the ISO in between each and every photograph. This gives a tremendous advantage for digital shooters when working in unpredictable lighting situations.
Lower ISOs deliver superior image quality, however are less sensitive to light. Since lower ISOs are less sensitive, the camera will have to let in additional light either through a larger aperture or by increasing the length of the shutter speed. For digital cameras, lower ISO settings will create cleaner images with less grain (digital noise). Lower digital ISOs also allow for a greater exposure latitude and capture additional detail in the brightest highlights and darkest shadows. For the ultimate image quality, most photographers try to use the lowest ISO possible on their camera. Higher ISOs are more sensitive to light, however they have degraded image quality. If you are OK with additional grain and less exposure latitude, higher ISOs will allow for a brighter exposure without having to increase the amount of light coming through the lens or shutter. For many photographers, low ISOs are used in bright lighting conditions and high ISOs are used in low light conditions.