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Don't Forget the ISO

The common formula for exposure involves two variables: aperture and shutter speed. But the sensitivity of the recording medium, commonly referred to as ISO speed has as much of a bearing on the outcome as either of these.

The concept of ISO is simple enough. The higher the number, the less light you needed to take a correctly exposed photograph. The numbers themselves form a standardized scale specified by the International Standards Organization. A few readers may have been at this photography thing long enough to remember that before there was an ISO standard there were American (ASA) and European (DIN) standards for film speed but the gist was the same.

Back in the days of film photography, you had few chances to choose an ISO speed. You purchased a roll rated at a particular film speed and set your camera accordingly. Yes, you could push process a roll, a technique I won't get into here, but for the most part you shot the film the way the film manufacturer told you to. Film photographers basically took ISO for granted because there wasn't much you could do about it.

Those film photographers that wanted the highest quality results possible tended towards slower ISO speed film (lower numbers). Those that needed to capture fast movement under low light shot with faster film knowing that by doing so they were accepting more grain in order to get the shot at all. The rules of picking a film speed were thus pretty straightforward, and once chosen were set for the entire roll.

Not much changed in the early days of digital photography. While it was then possible change ISO between every shot, few photographers did so. The costs were still too high. If you didn't need the faster speed to get the shot, your best bet was to keep the ISO as low as possible to get the best quality. The rules didn't really change much from film other than that film grain was replaced by digital noise.

As the years went by the technology behind digital photography improved. With each new generation of digital sensor it became possible to have more leeway in setting ISO and still get acceptably good results. The latest crop of cameras includes many that do perfectly well at ISO 800, 1600 or even higher. But even as it has become possible to leverage ISO as a third, independent variable in the exposure equation few photographers do so. The tendency of many is to pick a compromise ISO setting and leave it there.

The tradeoff between depth of field and preventing motion blur is a common one. Close down the aperture enough to get the entire scene in focus and you end up with an exposure time long enough to show motion. Flowers blow in the wind. If you want them to capture them without movement you generally need to accept a wider open aperture to still get a good exposure. Trying to achieve both at the same time leaves you with an underexposed shot. But by raising the ISO to compensate for the underexposure you can have your cake and eat it too. Or your aperture and shutter speed. You know what I mean.

Modern digital cameras have changed the rules. No longer is it necessary to pick an ISO based solely on minimizing noise, or accept lower quality with a higher ISO in order to get the shot at all. Within a range of ISO settings you're now free to use ISO as part of the broader goal of creativity in your photography without risk of compromise. Not all cameras equally capable of course and I urge you to determine the range of usable ISO speeds on your camera. Once you do, you'll have a new tool available to you.

When you're shooting, don't forget the ISO.

Date posted: November 4, 2012


Copyright © 2012 Bob Johnson, Earthbound Light - all rights reserved.
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The Exposure "Stops" Here
Some Thoughts on Exposure in the Era of Digital Photography
Diplomacy in Action: In Search Of ISO

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