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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

Using DxOMark.com

DxO Labs is best known as the maker of the DxO Optics Pro lens-correction and RAW-conversion software. But DxO’s core business is providing the imaging industry with image-processing technologies and image-quality measurements via DxO Analyzer. A couple of years ago, DxO made available results of its testing of a wide range of popular DSLRs and compact digital cameras on their www.dxomark.com website, recently adding results obtained with a number of lenses as well.

There’s a treasure trove of information there, including overall scores of RAW sensor performance (before demosaicing), plus ratings in three specific areas (color depth, dynamic range and low-light ISO), along with tech articles and reviews. There’s lots of material, some of it a bit techy, but spend some time there, and you’ll learn a lot. Check out the “Learn More” and “Our Publications” tabs for some good information.

What does this have to do with native ISO? Well, the graphs and charts show you what each sensor’s best ISO is for various parameters. As DxO points out, there’s more to consider when contemplating a camera besides the sensor’s color, dynamic range and low-light ISO performance (resolution, camera speed, AF performance, ruggedness, ergonomics and lens lineup, for example), but the DxOMark data is a great camera-evaluation aid.

Pixel Size

All other things being equal, larger photodiodes collect photons more efficiently, just as a wide-mouthed bucket collects rainfall more effectively than a champagne bottle does. Thus, sensors with larger pixels tend to produce images with lower noise, especially at higher ISO settings. However, all other things aren’t always equal. Sensor technology, image processors and processing algorithms all continually improve, and today’s smaller-pixel DSLRs actually produce better image quality than larger-sensor DSLRs of a few years back. However, among contemporary cameras, those with the largest pixels tend to produce the best high ISO performance—Nikon’s D3S, with its relatively large pixels (12.1 megapixels on a 24x36mm full-frame sensor equals a pixel width of 8.45 microns), is the current high-ISO image-quality champ among DSLRs. Note that Nikon’s D700 has the same-size pixels and outstanding high-ISO performance, but it can’t match its sibling’s quality, due to improvements in sensor and processing technology in the newer D3S.
Normal ISO Range Vs. Extended ISO Range
Before processing, digital images are just photons, electrons or “data.” Even images shot at the native ISO must be processed in order to be seen.

However, images shot at ISO settings other than the sensor’s inherent one are processed more. In the camera’s “normal” ISO range, this extra processing still results in what the manufacturer considers good images for each ISO speed. Many cameras also offer “expanded” ISO settings above and below the limits of the normal ISO settings. These settings produce noticeably worse image quality and are segregated from the normal ISO values for that reason. If you must use ISO 102,400, it’s on the Nikon D3S and the Canon EOS-1D Mark IV, but don’t expect great image quality when using it—noise is excessive, although amazingly low for such incredible ISO speeds, and it’s great for surveillance work. While lower-speed films almost always have finer grain than contemporary higher-speed emulsions, digital speeds below the sensor’s native speed actually can be worse, due to a reduction in dynamic range, among other things. This is why some manufacturers use “low” or “lowest” settings for very low ISO values.

So What’s The Best ISO For My Camera?
Since the camera companies don’t provide this information, you’ll have to shoot your own test. (Bear in mind that this “optimal” ISO setting is useful only when conditions permit. For example, sometimes you need to shoot at a high ISO setting to get the required shutter speed and aperture.)


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