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Wednesday, May 23, 2007

Digital ISO Speeds

Just Like Film Speeds, But Different

Digital ISO SpeedsEvery photographer is familiar with ISO speeds, those numbers we dial into our exposure meters and cameras so they can lead us—albeit, sometimes somewhat circuitously—to properly exposed images. Most films have ISO speeds, and digital cameras have ISO equivalents. Actually, the digital figures are ISO speeds, too: Like ISO film speeds, they're assigned based on standards issued by the International Organization for Standardization. ISO isn't a photo term; it's the global acronym for the body that publishes worldwide standards for everything from space-vehicle engineering and textile technology to business-to-business dealings. As you'll see, ISO covers much more than just film speeds.

But there's a fundamental difference between ISO film speed and digital ISO speed because there's a fundamental difference between films and digital image sensors. When you put a faster film into a camera, the new film is more sensitive to light than the one you replaced. When you switch a digital SLR from one ISO setting to a higher one, the sensitivity of the digital image sensor doesn't change; you're just amplifying the data that it produces. More details on this in a moment, but first, what exactly is an ISO speed?

An ISO speed is a mathematical expression of a photosensitive material's sensitivity to light. Originally, ISO speeds included both the arithmetic ASA value (used in the U.S.) and the logarithmic DIN value used in Europe (ISO 400/33º for Kodak Tri-X, for example), but now they just use the arithmetic value (ISO 400). In the arithmetic system, each doubling of the ISO number indicates a doubling of film sensitivity: An ISO 400 film is twice as “fast” as an ISO 200 film and half as “fast” as an ISO 800 film.

ISO film speeds and digital ISO speeds are assigned by the film and digital camera manufacturers based upon criteria set forth in the appropriate ISO standards. The purpose of ISO speeds is to provide a consistent standard for exposure: Theoretically, if you use any manufacturer's ISO 100 film and set your camera's meter to ISO 100, or set any digital camera's meter to ISO 100, the resulting images should look about the same, exposure-wise. (This isn't always the case in practice due to differences in exposure meters and their use among other things, but it's the goal.)

ISO Film Speeds

ISO standards are copyrighted, and the ISO speed standards run four to 14 pages in length, so we can't reproduce them in this magazine. If you want to read the actual standards in their entirety, you can purchase them on the ISO website, www.ISO.org. But basically, ISO film speeds are established by exposing the film in question to a range of exposure values, then giving it a specified degree of development in a specified developer. The point where a specified minimum density occurs on the resulting film curve determines the film speed.

Of course, laboratory-based film speeds don't always apply perfectly in real-world photography. In fact, several alternative methods of determining working film speeds for black-and-white pictorial photography have been devised, most notably the Zone System made famous by Ansel Adams. And many color film shooters rate their emulsions at something other than their ISO speeds because they prefer the results they get that way.

Your ideal speed for a given film depends on such things as the subject matter you shoot, the meter you use (and how you use it) and your personal taste. But you have to start somewhere, and ISO speeds provide a good standardized starting point.


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