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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

ISO Test Progression
We shot this lens-cap-on noise test with the new Pentax K-5 DSLR, which received DxOMark’s highest APS-C sensor rating, and demonstrates a more orderly progression: Noise becomes more apparent as ISO setting increases. Interesting is the reddish cast that occurs starting at ISO 6400. These are cropped portions of JPEG images; note that actual photos shot at the various ISO settings don’t exhibit such exaggerated noise. Compared to the behavior of noise in Tony Lorentzen’s video, you can see why it’s important to test your own gear.
What Is Native ISO?
The sensor’s native ISO is the one requiring the least manipulation during processing by the camera. (All digital images are “processed.”) Theoretically, it should produce the best image quality. Camera manufacturers don’t specify a native ISO, but it’s generally in the ISO 100-200 range. The official line is that “we provide you with a range of ISO settings so that you can choose the one that best suits your needs for your photograph.”

The photodiodes in an image sensor have an inherent sensitivity to light, which in turn gives the sensor containing them a natural or “native” sensitivity. But photodiodes alone don’t produce a digital image. The camera’s A/D converter, image processor and processing algorithms all come into play. Thus, digital ISO speeds actually are for the camera, not just for the sensor.

Each manufacturer has its own proprietary (and confidential) methods of achieving the different ISOs. But here’s one possible explanation as to why a lower ISO setting actually may produce worse image quality than a higher one: Let’s say a DSLR’s native ISO happens to be 160. To achieve higher ISOs, gain is applied and this increases image noise. To produce lower ISOs, other manipulation is applied, also reducing image quality. To achieve ISO 320, gain is applied to the sensor’s native ISO 160 data. To achieve ISO 250, the amplified ISO 320 data is further manipulated down to ISO 250, reducing image quality from the ISO 320 data. Note that if the camera’s native ISO is 160, the default “full-stop” ISO settings of 100/200/400/800/1600 won’t produce the best possible image quality.

What Is Digital ISO?
ISO speeds are assigned based on criteria set forth in standards from the ISO (International Organization for Standardization). The ISO issues standards for pretty much everything.

As mentioned, the digital ISO ratings are for the camera, not just the sensor, since a digital image is the product of the sensor, processor, A/D converter, et. al. Film ISO ratings are based on the amount of light required to produce a specific density when the film is processed to a specific contrast in a specific developer. With digital, it’s a bit more complicated because digital cameras can provide variable gain and post-capture digital processing—and can save RAW sensor output or processed JPEG images.

The purpose of standards, of course, is standardization, and the purpose of ISO values—film or digital—is to provide the following: If you set your exposure meter to ISO 100 and load your 35mm SLR with ISO 100 film and make an exposure for 1⁄100 sec. at ƒ/16, that image should match in brightness an image shot with a digital camera with the ISO set to 100 and the exposure made for 1⁄100 sec. at ƒ/16. Were it not so, we’d have exposure chaos.


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