Home Cameras SLRs D-SLR's: State Of The Art, Part II
Tuesday, February 3, 2009

D-SLR State Of The Art, Part II

What to know about the latest digital sensors, ISO and image quality

This Article Features Photo Zoom

dslrsNikon shook up the D-SLR industry late in 2007 with the announcement that its new D3 model provided ISO settings as high as 25,600. Now, Nikon’s D700 and Canon’s EOS 5D Mark II also go up there, Canon’s EOS 50D has a top ISO setting of 12,800, and a dozen current D-SLRs have settings of at least 6400.

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.

dslrs dslrs Canon’s top-of-the-line EOS-1Ds Mark III and new EOS 5D Mark II use essentially the same full-frame, 21.1-megapixel sensor, but the newer camera provides a much higher ISO range: a normal range of 100-6400 and a top speed of 25,600 versus a normal range of 100-1600 and a top setting of 3200 for the EOS-1Ds Mark III. What’s the difference? Newer technology in the EOS 5D Mark II, including a far more powerful DIGIC 4 image processor, a new RGB filter for the sensor and proprietary improvements throughout to improve sensitivity and decrease noise.

What’s ISO?

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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