Wednesday, May 23, 2007
Digital ISO Speeds
Just Like Film Speeds, But Different
From a practical standpoint, it's a good idea to test your digital camera to see how it performs at different ISO settings in a variety of conditions. Shoot the same scene at different ISO settings and see which results you prefer. Some D-SLRs have an ISO-bracketing feature, which makes this process much easier.
Shooting at higher ISO settings increases image noise, but so does shooting at longer exposure times. So is it better to shoot a low-light scene at a higher ISO with a faster shutter speed, or at a lower ISO with a longer shutter speed (assuming that action-freezing or -blurring shutter speeds aren't a consideration)? Again, the best way to answer that question is to try it both ways with your camera and see which results you prefer. Because I like to work handheld, I tend to go with higher ISO settings and shorter shutter speeds, but if you use a tripod, you might prefer to do the opposite. Many digital SLRs have long-exposure and high-ISO noise-reduction features; see how well they work at different ISO settings and exposure times with your camera.
If you shoot RAW images, you might also test to see whether your camera gives better results exposing correctly at a higher ISO or using the same shutter speed/ƒ-stop combination at a lower ISO (underexposing) and then “pushing” the image using the RAW software. Note that you don't want to underexpose at higher ISO settings with a digital camera. A number of current digital SLRs have given me superb results at ISO 1600 when I nailed the exposure (no manipulation in Photoshop required), but no digital camera will produce good image quality when high-ISO images are underexposed.
When you get a new digital camera, try it at all its ISO settings and a wide range of shutter speeds to see what it can do. And bracket exposures whenever possible while you learn the intricacies of the new camera—it's not like you're wasting film. Once you're comfortable with the camera, use the ISO setting you need to get the shot, expose correctly and enjoy the results.
A digital image sensor has a native ISO. When you adjust the ISO setting, the sensor itself isn't changing; rather, the amplification is changing. This is fundamentally different from film.
The International Organization for Standardization (ISO) includes national standards bodies from 156 countries. The American National Standards Institute (ANSI) is the U.S. representative. ANSI replaced the American Standards Association (ASA) in the 1970s, for those of you who remember the old ASA speeds.
Like the CMOS sensor shown on page 88, this CCD sensor has a native ISO that can't be changed. The amplifiers further along in the camera's circuitry boost the signal to change the “speed.” Boosting the signal also boosts noise, however. Everything is a trade-off.
ISO Film & Digital Speed Standards
• ISO 2240: 2003,“Photography— Colour reversal camera films— Determination of ISO speed”
• ISO 5800: 1987, “Photography— Colour negative films for
still photography—Determination of ISO speed”
• ISO 6: 1993, “Photography— Black-and-white pictorial still camera negative film/process systems—Determination of ISO speed”
• ISO 12232: 1998, “Photography— Electronic still-picture cameras— Determination of ISO speed”
Film ISO is adjusted through the chemistry of a particular emulsion. Silver-halide crystals and other chemicals are reformulated to give the film a faster or slower ISO. Elements such as crystal size and inclusion of additives alter the film's native ISO.
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