ISO Explained

The term ISO is often tossed around in the context of digital imaging, but to those not familiar with it, ISO can be both confusing and misleading.  Short for International Standards Organization, and sometimes referred to by its former name "ASA," ISO is a numerical representation of the imaging devices' sensitivity to light.  The higher the ISO number, the more sensitive the camera is to light.  In the days of film, part of selecting the right film to used involved selecting the ideal ISO for what you planned on shooting.  If it was too high or too low, then you had to work around it.  With digital cameras, you have the ability to select ISO on the fly; it can be different for every single photo.  It is important to know that while a lot of settings can be changed when shooting a RAW file, ISO is fixed after the image is taken, so be sure to get it right!  

ISO can be thought of as similar to the volume on a stereo.  If you want the music to get louder, you turn up the volume.  This also makes the volume of the static and the various imperfections in the music louder, so there is a trade off.  The "static" in a digital picture is referred to as "noise" or "grain."  Noise in a digital image manifests as small dots of various colors that make an image grainier and less sharp.  It is especially obvious in shadows or large areas of a single color or tone.  As you raise the ISO setting of your digital camera, the digital noise becomes stronger and more offensive.  There will always be a point at which the image becomes almost useless, and it is important to test various ISO settings on your specific camera so you can find that point.

There are two types of noise, luminance and color.  Luminance noise is the true equivalent to film grain, it is less offensive in pictures that color noise, and a bit harder to remove in software.  Color noise is more obvious, it shows up as small dots of random colors in a picture.  It is easier to remove with software, and is far more noticeable in pictures.  Most noise-removal programs have different settings for removing either luminance noise or color noise, so it is important to find a combination of settings that work.  Too much noise reduction will result in far less detail and sharpness, because essentially noise reduction blurs the image enough to remove the grain.  If an image is really noisy, but you can't bring yourself to throw it away, a quick conversion to black and white in your favorite image processing software can give it an entirely new purpose.  

As a rule, it is always best to use the lowest ISO possible.  Every camera is has different levels of quality at varying ISO settings, but even the most expensive camera performs best at the lowest possible setting.  Point and shoot cameras generally have far worse high-ISO performance that digital SLR cameras, this is directly related to the size of the sensor inside the camera.  The bigger the sensor, in general, the less noise there is as you increase the ISO.  All other factors aside, a point and shoot camera with a sensor the size of a postage stamp (or smaller) will almost always be unusable over ISO 800.  The lowest that most cameras go is ISO 80 or ISO 100.  Depending on the camera, there might be only a few options for ISO settings, or many.  Higher end cameras offer more increments between ISO settings, so instead of having to go from ISO 400 to 800, you might also be able to select ISO 640.  

ISO Image1.jpg
"In this comparison, the shot of color patches on the left is from a high end digital SLR with a large sensor, at ISO 400.  The image on the left is of the same color patches, but from a point and shoot camera with a very small sensor, also at ISO 400.  The difference is clear, the digital noise of the point and shoot sensor is far inferior to that of the digital SLR"

It is important to understand that when shooting jpeg images, the camera will do some level of noise reduction automatically.  Sometimes there is a setting to adjust the strength of this processing, be sure to look in your manual to see if this is available.  If so, try to keep it as low as possible.  When shooting RAW, you have far more control over noise reduction in post processing.  The way I feel is that I would always rather start with more detail, even if it is a bit nosier, and do the noise reduction myself, versus giving up control before I even download the photos to my computer. 

There are times when shooting at a high ISO is necessary.  Shooting in low light is the Achilles heel of any digital camera, since a digital sensor is always starving for more light.  A flash can help sometimes, but it will destroy the natural ambiance of the scene, and is not always effective at longer distances.  As the sensitivity of the camera is increased, you are able to select a faster shutter speed to prevent blur, or a smaller aperture to get more depth of field.  Sports photographers know that when shooting indoor sports, for example, they would much rather use a higher ISO and get some noise in their pictures than have to settle with a slower shutter speed and get blurry images.  Noisy pictures can be frustrating, but can almost always be made usable.  Shooting with too slow of a shutter speed cannot be fixed with any amount of post processing.  

I would recommend getting to know your camera's various ISO settings.  Most digital cameras have an "auto ISO" function, where the camera's built in computer will select the most ideal setting.  This is great in a pinch, but the camera might want to go to a higher ISO than necessary, resulting in noise or it might not go high enough, and you can end up with blurry photos.  Learning to control your camera's ISO setting is an easy and fun way to get more creative control over your images without dealing with complex adjustments or overwhelming options.  As a general rule, keep the ISO at 200 or lower when shooting outdoors, and do your best to keep it under 800 when shooting indoors.  This will depend highly on your camera, and I can't emphasize enough how important it is to experiment with different settings before going out and making pictures.  

jf.gifJoshua Lehrer | Website