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Wednesday, February 9, 2011

The Truth About Digital ISO

Digital Photo Pro sorts through the confusion and misinformation to get to the heart of digital ISO

This Article Features Photo Zoom


In the early days of digital imaging, CCDs produced better image quality than CMOS sensors, largely due to the higher noise and smaller “fill factors” of CMOS sensors. (Fill factor is the percentage of the pixel area that’s devoted to gathering light—sensors contain circuitry, as well as the photodiodes.) Today, most DSLRs (including the top-performing ones at higher ISOs) utilize CMOS sensors. On the other hand, all medium-format digital cameras and digital backs use CCDs. Today, both sensor types can turn out amazingly good image quality.

The lesson here is that you won’t necessarily get the highest image quality by shooting at the camera’s lowest ISO setting. You should test with your gear to see which ISO setting(s) produce the cleanest images.
The original ISO standard 12232:1998, “Photography—Electronic still-picture cameras—Determination of ISO speed,” provided three “official” ways to determine digital-camera ISO speeds based upon saturation (highlight clipping) or noise limits. The standard was revised and renamed in 2006: ISO 12232:2006, “Photography—Digital still cameras—Determination of exposure index, ISO speed ratings, standard output sensitivity, and recommended exposure index.” The 2006 standard added two methods developed by the Japan-based CIPA (Camera & Imaging Products Association). These more recent methods—REI and SOS—are the ones primarily used today for digital still cameras.

Essentially, the SOS (Standard Output Specification) method is based on physically measuring the light responsiveness of the imaging system (sensor, processor, etc.), akin to the standard for ISO film speeds. The REI (Recommended Exposure Index) is based on producing what the camera manufacturer considers a good image. The SOS method isn’t applicable to RAW output, and both methods are applicable only to sRGB output, so if you shoot, say, RAW files in Adobe RGB 1998 color space, the ISO speeds are just guidelines. (Recent cameras from Canon, Nikon and Sony use the REI method while Olympus, Pentax and Samsung use the SOS method, and Sigma, with the unique Foveon image sensor, doesn’t specify which method it uses.)

Changing ISO
When you want to change ISO with film, you either switch to a film of the desired ISO rating, or push- or pull-process a slower or faster film to approximate that speed. When you want to change ISO with a digital camera, you merely set the camera’s ISO to the desired rating—no need to switch film. In fact, you can shoot each image at a different ISO if you wish, one of digital imaging’s many advantages over roll film.

If you switch from a film of one ISO to a film of another, you’re actually getting the new ISO—an ISO 400 film is four times as sensitive to light as an ISO 100 film, producing the same image brightness two shutter speeds faster or two ƒ-stops smaller than the ISO 100 film. If you push-process an ISO 100 film to ISO 400, you’re not actually increasing its sensitivity; you’re just chemically amplifying the underexposed latent image.

When you change a digital camera from one ISO setting to the next, it’s actually akin to pushing or pulling film in processing. That’s to say it’s not the same as changing film emulsions. As mentioned earlier, an image sensor has an innate sensitivity to light (called its native sensitivity); higher ISOs are produced by increasing the gain or other manipulations to amplify the underexposed image, and these manipulations generally reduce image quality—they increase image noise, decrease dynamic range and adversely affect color performance. The results depend on what the camera manufacturer does to produce the various speeds. Bear in mind that even images shot at the sensor’s native ISO are processed; the sensor data is just data until the in-camera processor or a RAW converter turns it into an image.


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