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Old 21st of June 2008 (Sat)   #50
Bill Boehme
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Join Date: Jan 2007
Location: DFW Metro-mess, Texas
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Default Re: "Native" ISO speeds for sensors?

Although the term “native” gets bandied around a lot, I really do not think that it is applicable when talking about digital camera sensors, whether CCD or even CMOS. In my opinion, it is closer to being a marketing type of description. I encountered the term used much more frequently in the software engineering department where I worked before retiring and it seemed to usually be in the context of relating to the basic instruction set of a particular microprocessor ... in other words, if you wrote an applet for a processor using the machine level instruction set which is even more fundamental than writing in assembly language, that would be referred to as writing in the native language of the processor. And, of course, any assembler or compiler must be able to convert human readable code to the native language of the processor that will be running the code.

References to the native resolution of a computer monitor were made and that is something that makes sense in describing digital monitor parameters, but no rationale was established to make a case for using the term “native” with respect to digital camera sensor ISO values other than the implication that everything has a “native” state of being.

A camera sensor, by itself, doesn't have any particular ISO equivalent value associated with it. The charge that the sensor accumulates during an exposure is infinitesimally small and without much more amplification than is on the CMOS chip itself, not capable of having any ISO equivalent. Noise (in its various forms) would be major part of the sensor limitations when deciding on the operating range of the sensor. It is the job of the design and development engineers to see how much amplification ought to be applied to get a decent output. Several causes of noise exist -- amplifier noise, thermal noise, variation in light sensitivity between individual sensor sites, and quantization noise. Digital sensors react differently to light than film does, but equivalent ISO standards for digital systems have been developed to define the characteristics of a particular ISO in addition to the basic exposure value that a light meter would calculate (for example, with an EV of 12, a “correct” exposure could be obtained from using a shutter speed of 1/250 second, aperture of f/4, and an ISO speed of 100 whether using film or digital).

Once the engineers determine the sensor’s usable range of equivalent ISO values that can be synthesized for a particular sensor/processor set, they make a design decision on what range of ISO values is "right" for that particular camera. Of course, "right" is a very imprecise term and I can imagine that it involves the subjective evaluation of many people, both technical and non-technical along with considering the target user category.

Anyway, this long roundabout response hopefully explains why I feel that “native” is an irrelevant term for use with digital camera sensors. To my mind, the term “native” would describe the physical characteristics of something and not be a performance descriptor.
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