There are five methods listed for determining digital camera ISO in the standards, three from the previous standards and two newer ones from CIPA (Camera & Imaging Products Association) standard DC-004. Today, the Japanese camera companies use one of these last two: Relative Exposure Index (REI) or Standard Output Sensitivity (SOS). The takeaway is that SOS is based on the reproduction of a standard middle tone in the sRGB image, while REI is based on whatever the camera manufacturer considers good images. Note that digital ISO ratings are based on evaluating images, so they don’t apply to RAW files, which aren’t images until they’re processed. Unlike film ISO standards, digital ISO standards don’t specify processing procedures for the RAW data. Canon, Nikon and Sony use REI, and Fujifilm, Panasonic and Pentax use SOS; in practice, we haven’t noticed a difference. Olympus and Sigma don’t state which method they use.
With film, you set the meter’s ISO index for the ISO speed of the film you’re using (or your own tested exposure index; some film photographers find they get better results rating the film lower or higher than the official ISO speed) and expose according to the meter reading (or adjust accordingly per your experience with your gear and that film). Digital cameras provide a range of ISO settings, which makes things easier when you encounter dimmer or brighter light. With film, if you’re using ISO 100 film and want to shoot in a dim setting, you have to switch to a faster film. With digital, you can merely change the ISO setting on the camera to a higher number. Many photographers work this way quite successfully. But it’s important to realize that changing the camera’s ISO setting doesn’t actually change the sensitivity of the image sensor. It just changes the processing of the resulting image data. Changing the ISO changes the gain (either analog, which electronically amplifies the signal, or digital, which just multiplies the digital values after the signal is converted from analog to digital form), and in auto-exposure modes, the metered exposure. But increasing a digital camera’s ISO is more akin to push-processing a slower film to a higher rating than actually changing to a film with a higher ISO speed: Image quality suffers as you raise the ISO (see the next section for why).
In effect, digital ISO is a brightness control. Depending on how the camera applies ISO gain, you actually may be better off shooting at the desired shutter speed and aperture at base ISO (100 or so with most cameras) and brightening the results yourself when you process the image in your RAW converter. You have more control over the results and your computer is more powerful than the one built into the camera. Your computer can handle more powerful noise-reduction algorithms in the camera’s RAW converter. Also, each time you double the ISO setting in-camera, you lop a stop off the highlight end of the dynamic range.
Why High-ISO Digital Images Are Noisy
There are several sources of noise in digital images. The primary ones are photonic noise and read noise. Read noise is produced by the camera’s electronics and processing pipeline, and generally, is visible only in the darkest areas of an image. Photonic noise is the noise carried by the light itself. As the number of photons increases, photonic noise increases, too—but by the square root of the photon increase. If 100 photons reach an area of the sensor, there will be 10 photons of noise (the square root of 100 = 10), for a photonic signal-to-noise ratio of 10:1. If 10,000 photons reach another area of the sensor, there will be 100 photons of noise, for a photonic signal-to-noise ratio of 100:1. The greater the number of photons that reach the sensor—the greater the exposure—the higher the photonic signal-to-noise ratio. When you set a high ISO, you give less exposure; fewer photons reach the sensor, so the photonic signal-to-noise ratio goes down and the image gets noisier. It’s not the in-camera processing that makes high-ISO images noisy; it’s the reduced amount of light used for the image. For optimal digital image quality, you want to give the most exposure you can without blowing important bright areas in the scene.
The Benefits Of High ISO
In general, using higher ISO settings allows you to use faster shutter speeds to minimize the effects of handheld camera shake and subject motion, and/or smaller apertures to provide more depth of field. You can shoot in dimmer light with the same shutter speeds and apertures a slower ISO would require in brighter light. That’s why available-light and action photographers especially like higher ISO settings.
It behooves you to test your own gear to see how high an ISO you can use and still get results you find acceptable. With today’s DSLRs and mirrorless cameras, you may be pleasantly surprised at how high that ISO setting is.