by James Richardson

Created on: July 11, 2008

ISO is a general term used originally in film photography to refer to the speed of film. A higher ISO film has a larger grain and generally absorbs light more readily than its lower ISO counterparts. This makes high-ISO film great for fast action, low light, and other challenging photographic scenarios. One of the big downsides to high-ISO film is that the larger grain directly translates into grainer prints.

Serious film photographers often have to remove a roll of film and insert another one with an ISO speed that better matches a current shooting situation. For example: say a nature photographer is shooting wildlife in the early dawn hours. He very well may use a high-ISO film to accommodate for the low light-especially if he chooses not to use his flash to avoid startling his subjects. But then say the sun is now peaking in the eastern sky and he wants to shoot the landscape as it is bathed in the sunrise. The brighter conditions may merit a lower-ISO film. So what is the film purist to do? He probably will need to rewind the high-ISO film, whether or not the roll is finished, and drop in a fresh low-ISO roll. This gets expensive in a hurry!

While there are still many respectable photographers that adhere to traditional film, digital is making huge inroads into the photographic scene. Digital cameras offer the photographer the ability to change ISO speed with a simple menu setting. That means no more partially completed rolls of film. It also allows the photographer to adjust for constantly changing conditions with the touch of a button.

Additionally, most digital cameras have an "intelligent" or "auto" ISO mode where the camera calculates the best ISO speed based on the current conditions. This is very useful for the amateur photographer as it enables him to get more out of what digital shooting has to offer. However, to get the most out of a digital camera, it pays dividends to understand how to manually adjust and work those ISO settings.

Basically, it breaks down to this: low light and/or fast action are probably the two situations most hobbyists will need to use high-ISO numbers, this because a quicker shutter speed is required to freeze movement. An ISO of 400, 800, or possibly higher usually allows for faster shutter speeds despite the lack of ambient light. Lower ISOs of 100, 200, or less work well in most well-lit and flash-lit situations. Just as in film, the downside to high ISO is graininess. Digital cameras don't produce quite the same type of grain, but rather they produce spotty flecks of color, especially in shadowed areas. This digital grain is actually called color noise, and it can be quite annoying. Fortunately, most cameras have some type of noise suppression system on board. While noise suppression can hurt fine details, most of the time it is virtually unnoticeable. If additional noise clean up is needed, many digital photo-editing programs allow some degree of noise reduction as well.

The key with ISO is to never use a higher ISO than is necessary. Try putting your camera in auto mode and shooting in a variety of different lighting situations. See what ISO the camera selects and observe how your photos are turning out. Too blurry? A quicker shutter speed and higher ISO may be in order. Since each camera model is a little different, the best way to learn what ISOs work best in what situations is simply to experiment. Ideally, what you want to discover the lowest ISO setting that will yield the sharpest pictures with as little noise as possible.

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