FILM SPEED (ISO)
ISO is actually an acronym, which stands for International Standards Organization, and the ISO rating along with the shutter speed and aperture setting are the three elements that determine the final exposure of the photographic image. The ISO rating, which ranges in value from 25 to 6400 (or beyond), indicates the specific light sensitivity. The lower the number, the less sensitive to light the film stock or image sensor is. Conversely, a higher number indicates a higher sensitivity to light, thereby allowing that film or image sensor to work better in low light conditions.
ISO SENSITIVITY & IMAGE NOISE
With film stocks, the lower ISO rating also meant that the photosensitive grains of salt on the film acetate were very fine, thus producing a smoother, cleaner image. A higher ISO had larger, jagged grains of salt, thus producing “rougher” or grainier images. In digital photography, the same logic applies… the lower the ISO rating, the less sensitive the image sensor is and therefore the smoother the image, because there is less digital noise in the image. The higher the ISO rating (more sensitive) the stronger the image sensor has to work to establish an effective image, which thereby produces more digital noise (those multi-colored speckles in the shadows and in the midtones). So what is digital noise? It is any light signal that does not originate from the subject, and therefore creates random color in an image. The digital camera engineers have designed the image sensor to perform best at the lowest ISO (just like with film). On most digital cameras this is ISO 100, although some high end DSLRs have a mode that brings the ISO down to 50 or even 25.
ISO SENSITIVITY & IMAGE QUALITY
It is important to remember that the lower the ISO rating the better the image quality is going to be. While most digital cameras have a default “Auto ISO” setting, this reduces your control because it can automatically set a higher ISO, which will result in a grainier (noisier) image when other settings could have created an acceptable exposure with less noise. Increasing ISO affects image quality in two major ways: 1) The distinction between fine details is reduced. 2) When you enlarge the image and make digital prints, or when you save the image as a jpeg (which has a high level of compression), the inherent high level of digital noise will result in a “muddier” image after conversion.