Tuesday, February 3, 2009
D-SLR State Of The Art, Part II
What to know about the latest digital sensors, ISO and image quality
How practical are such stratospheric ISO settings? That depends, to a degree, on your personal standards and the difficulty of the light you’re working with. Those super-high ISO figures are “expanded” settings, noticeably noisier than the camera’s “normal” ISO range (that’s why they’re not included in the normal range). But the ISO settings in each camera’s normal range are generally quite usable when images are properly exposed. Even ISO 3200 is pretty fast: no current color 35mm film carries that rating.
While noise is a concern at higher ISO settings, it’s usually preferable to crank up the ISO in order to use a fast enough shutter speed for a tack-sharp shot than to have a noiseless image that’s blurry from camera shake.
Besides going to much higher ISOs than their predecessors, today’s digital SLRs also produce much better image quality at any given ISO. This is due, in part, to improvements in sensor and image-processing technology and, in part, to higher pixel counts. More megapixels means it’s possible to record greater detail. A higher pixel count also means you can make a bigger print, or a better-looking print than a lower-resolution image could produce at a given size.
Listed among the specs of both film and digital cameras, you might think ISO is a technical abbreviation, but the International Organization for Stan-dardization (“ISO” is its international acronym) is a global group that establishes worldwide standards for everything from space-vehicle engineering and textile technology to business-to-business dealings. The ISO includes standards bodies from more than 150 countries, including the U.S. representative, the American National Standards Institute (ANSI, formerly the American Standards Association, or ASA).
ISO film speeds are based on different criteria than those used for digital ISO speeds. To further complicate things, there are two variations on ISO ratings for digital cameras: SOS, or Standard Output Sensitivity; and REI, or Recommended Exposure Index. Since 2006, all Japanese camera makers have been required to state in their published product specifications which rating method they use.
The Digital ISO Difference
Since films produce negatives and slides that can be read with densitometers, while digital images exist only as digital data until displayed on a screen or printed, the ISO speed-determining systems have to be different for the two imaging media. Essentially, the high-end sensor ISO limit is based on image noise, while the low-end limit is based on highlight clipping. (You can order the full ISO standards from the ISO website, www.iso.org; search for 12232.) But the end result is the desired standardization. In theory, if you put a roll of ISO 200 film in a 35mm SLR and expose it in noon daylight for, say, 1⁄200 sec. at ƒ/16, and set a D-SLR to ISO 200 and make an exposure of 1⁄200 sec. at ƒ/16 in the same light, the images should be of equal brightness.
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