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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

Recent advancements in DSLR ISO capabilities have brought the topic back to the forefront of our collective photographic conscience. There’s a lot of confusion out in forums on the Internet because digital ISO isn’t as straightforward as film ISO, and also because there have been some interesting videos and images floating around that seem to indicate that digital ISO is even more mysterious than suspected.

Tony Lorentzen’s ISO Noise Test video sparked some serious online controversy with its surprising results: It shows that lower ISO settings don’t always produce lower noise levels. You can see the video yourself at www.vimeo.com/10473734.

By now many of you have probably seen the video that Tony Lorentzen made with his Canon EOS 7D. He shot at all the ISO settings with a lens cap over the lens to see how the noise varied at the different settings. His results were quite a surprise. Images didn’t get noisier in a linear manner as the ISO settings increased. In fact, the video shows ISO 160 being less “noisy” than ISO 100, and ISO 640 providing the same noise level as ISO 100. You can find the video at www.vimeo.com/10473734.

What’s going on here? Well, the camera manufacturers aren’t talking, but we’ll do some educated speculating. Keep in mind that the video is one test with one example of one camera model. It’s certainly an eye-opener and reason enough to consider doing your own tests with your own gear (our tests, using the procedure described later in this article, have turned up similar results with a number of DSLRs).

With film, it was straightforward: If you used a film with a higher ISO, your images would exhibit more grain; they would be “noisier.” When you needed maximum image quality—i.e., minimal “noise”—you’d use the slowest film available.

As Lorentzen’s video shows, this isn’t necessarily the case with DSLRs. This is due to the differences in the way film and digital cameras record images, and the way digital cameras process images.

Higher-ISO films employ larger silver-halide crystals, and that results in grainier images. With digital, higher ISOs are produced by amplifying the sensor output, and as with film, setting a higher ISO generally increases “graininess” (noise) in digital images. However, when you switch to a lower ISO film, you get finer-grained—“less noisy”—images. But with digital, ISOs below the camera’s native one also are manipulated, and this, too, results in reduced image quality (generally in terms of reduced dynamic range and even increased noise in dark areas). But as the cited video shows, even that isn’t the whole picture.

While film images are the result of light striking the silver-halide crystals and the developer/development time used to process the film, digital images are the result of light striking light-sensitive photodiodes (pixels) in the image sensor, the characteristics and size of those photodiodes, the RGB and low-pass filters on the sensor, the sensor’s circuitry, the A/D (analog-to-digital) converter, the camera’s image processor and the camera manufacturer’s processing algorithms. Even images shot at the camera’s native ISO are processed; images at higher and lower ISO settings are processed more.