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Wednesday, May 23, 2007

Digital ISO Speeds

Just Like Film Speeds, But Different

Digital ISO Speeds

Films are processed in developers and produce transparent negative or positive images that can be read with a densitometer. Digital images are processed by the camera (or by the user after shooting via special software, in the case of RAW images), and don't exist in solid form like film images. Viewed digital images vary with the setup of the monitor on which they're viewed or with the prints made from the images. And the fact that digital SLRs (and some consumer digital cameras) allow you to set different ISO speeds further complicates things. So digital ISO speeds must be based on different criteria than ISO film speeds.

There are actually two digital ISO speeds: the ISO speed and the ISO speed latitude. The former is analogous to an ISO film speed; the ISO speed latitude covers the range of settable ISO speeds that will produce acceptable (per the ISO standard's criteria) images with a particular digital camera.

Digital ISO speeds are based on the amount of exposure required to produce an image of specified brightness while restricting image noise to a specified acceptable level. For the main digital ISO speed, that noise level is quite low. For the ISO speed latitude range, the upper speed limit is determined by a higher but still acceptable specified noise level (“noise limited”), while the lower limit is determined by highlight clipping (“saturation limited”).

Because exposure duration, temperature and humidity can affect digital image quality, the ISO standard cites specifics for each of these. In real life, we shoot at a wide range of shutter speeds, temperatures and humidities so, as is the case with film speeds, the controlled laboratory criteria don't necessarily apply to the wide range of real-world photographic situations. But like ISO film speeds, digital ISO speeds provide us with a standard, a starting point.

A Big Difference

When you change the ISO setting on a film camera, the camera's meter reduces exposure (if you changed to a higher speed) or increases exposure (if you set a lower speed)—but the sensitivity of the film in the camera remains the same. If you have ISO 200 film in the camera and set the meter to ISO 400, the images will be underexposed (unless you push-process the film, which presents its own problems). If you have ISO 200 film in the camera and set the meter to ISO 100, the images will be overexposed (unless you pull-process the film). With a film camera, if you want to shoot at a different ISO, you must set the meter to that ISO and then put a roll of film with that speed in the camera.

When you change the ISO setting on a digital camera, the camera adjusts the exposure accordingly, like a film camera. But it also adjusts the in-camera image processing to match—you don't have to “change film” to shoot at a different ISO. In fact, that's one of the big advantages of shooting with a digital SLR instead of a film camera: you can shoot every shot the ideal ISO speed.

Image sensors have an innate “native” sensitivity, generally in the ISO 100 to 200 range. When you set a higher ISO speed, amplifiers in the image sensor's circuitry increase the gain before sending the image data to the A/D converter to be digitized. The sensor's sensitivity doesn't actually increase; the camera is just amplifying the data it produces. In the process, image noise is also increased, making the image “grainier”—sort of like what happens when you “push” film speed. But generally, digital SLRs produce better image quality at higher ISOs than film, especially pushed films.

If you set a lower ISO speed than the sensor's native sensitivity, the camera's image processor adjusts the image data after the A/D converter converts it to digital form. In the process, the dynamic range is reduced. So it's best to shoot at the sensor's native ISO whenever possible.

All Digital ISOs Are Not Equal

Just as some ISO 400 color slide films produce better image quality than others, some digital SLRs produce better image quality at a given ISO setting than others, particularly at the higher ISO settings. An even bigger difference exists between the digital SLRs, with their relatively large image sensors, and the compact consumer digital cameras, with their fingernail-sized sensors. Bigger sensors contain bigger pixels for a given megapixel count, and bigger pixels mean less image noise and a better dynamic range, all other things being equal. So if you buy a consumer digital camera as a take-anywhere tool or backup to your digital SLR, don't be surprised when you find its image quality, especially at ISO 400 and higher, noticeably worse than that of your D-SLR.


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