Auto-ISO 1d4

Started Jul 9, 2011 | Discussions thread
bobn2 Forum Pro • Posts: 53,202
Re: Auto-ISO 1d4

John Sheehy wrote:

bobn2 wrote:

Unfortunately, Canon cameras are not designed like that, and there is an analog gain changed involved in producing the higher ISO's. On low end cameras that gain change goes in whole stops, on higher end ones in 1/3 stops.

Yes, but not at the photosites; the 1/3-stop analog gain is done separately, AFTER the late stage noise, serving no purpose but to standardize the meaning of RAW levels. Gain in vain, noise-wise, and destructive, DR-wise.

The "pro" cameras therefore butcher the 160/320/etc series more than the consumer cameras; the only thing positive one can say about the "pro" bodies' 1/3-stop ISO implementation is that highlight headroom is very consistent, as they all clip at about the same RAW level, and gray is the same level in all RAWs (except ISO 50 on some Canons).

The canon sensors have a variable gain stage on the sensor chip, which boosts the interchip signal to a more or less constant level (given the expected exposure for the set gain), and I suspect that feeds more or less directly to an ADC on the low end models. The high end ones probably use a more sophisticated analog front end with its own variable gain. My experience is that when engineers see that, they will often use it, even without a clear idea as to whether it will make things better.

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