ISO… what exactly is that obtuse and confusing number on a roll of film? What does it mean in regards to digital photography? And how does it affect your photography?
Simply put the ISO rating is an algorithmic value that indicates the image sensor’s (or film stock’s) sensitivity to light. Since digital cameras use a CCD or CMOS image sensor instead, an ISO equivalent is providing when rating a given digital camera’s ISO. This is so all those decades of photographic knowledge can uniformly and easily transfers over to the digital realm.
ISO, which is an acronym for International Organization of Standards, is a numeric value that helps determine — along with the shutter speed and aperture — the necessary exposure value required to register an effective image and accurately reproduce the subject that’s filling up your viewfinder. The ISO sensitivity value functions in such a way that the higher the ISO number, the shorter the exposure length required and the converse is also true.
Just like with film, the higher or lower ISO also indicates how much “grain” is in the final image. When you select a higher ISO, you’re cranking up the image sensor’s sensitivity to light and an unintended result is the addition of more electronic noise into your photo (“noise” is any light signal that doesn’t originate from your subject). The camera’s engineers have designed the image sensor to perform best at the lowest ISO rating a given camera will allow. On most digital cameras this is ISO 100, and some have ISO 50 others ISO 200. If you look around you can still find film stocks with ISO 25, the finest grain available.
A side note about the “grain” in a digital image; with film the graininess is sometimes used for artistic effect and can contribute to the overall mood of the final photograph. Whereas, digital noise is quite undesirable as it appears as clumps of distracting multi-colored dots or freckles on your image (although you might find that this has some artistic merit!).
If you’re looking for smooth, appealing and color-accurate images, then a lower ISO is required. But if you want to shoot in extremely low light conditions or stop fast-moving action (like rain or LeBron James dunking the basketball), then you “dial up” the ISO rating. Since a lower ISO dictates a longer exposure time for a good image, you’ll have to adjust your shutter speed and/or aperture to compensate. Be careful though, unless your subject remains relatively still and/or if you set your shutter speed below 1/30 you’ll get a motion blur (which is also sometimes desirable).
In addition, the actual size of the image sensor determines what ISO rating the camera can use without being afflicted by unwanted noise. Image sensor size is not the pixel value; it’s the actual physical size of the image sensor. Consumer cameras (or point-n-shoots) usually have a small image sensor, where ISO ratings of 400 or above produce high (almost unacceptable) amounts of noise. However, with dSLRs the image sensor is much bigger, usually equivalent to the APS film size (23x15mm) and therefore the noise level is less. Some manufacturers (like Canon and Nikon) are currently producing cameras where the image sensor is the same size as a 35mm film frame (known as full-frame). On a larger sensor the pixels are also larger, so they can receive more light and thus require a lower ISO (i.e. less digital noise) to capture the specific image you want under potentially adverse lighting conditions.
So, if you are shooting on a sunny day or with more than adequate indoor lighting or maybe just a more static shot, by selecting a lower ISO you will end up with smoother, more appealing, reproducible and color-accurate images.
You might say that a little bit of noise isn’t aesthetically unappealing or if you’re in a situation where you have to decide between getting the shot you want with a bit of noise or no image, then by all means take the picture and rely on the camera’s on-board noise reduction technology to reduce the grain.
Lastly, what is deemed an acceptable level of noise ultimately matters in the size of your prints or the final presentation display size. There are several 3rd party noise reduction programs that can reduce the noise. This usually requires an extensive amount of processing and you have to weigh the pros and cons of doing this, versus the size of the final print. And like I said before, you can creatively balance the f-stop and shutter speed so that you can use a lower ISO thus controlling the “grain.”