Exposure basics, lesson two point one (& ISO)

Started Mar 19, 2013 | Discussions thread
Anders W Forum Pro • Posts: 21,468
Re: Pushing ISO in order to ETTR

Great Bustard wrote:

Certainly. Remember that my entire post is written under the assumption that exposure can't be increased (as implied by the low-light scenario described by Jack Hogan). If exposure can be increased without negative side-effects, it is of course better to stay at ISO 200 and increase exposure to reach the ETTR criterion than to keep exposure unchanged and go to ISO 400. However, if exposure cannot be increased, it is better to go to ISO 400 and ETTR rather than to remain at ISO 200 and stay one EV short of ETTR and that was the point I was making here.

True from ISO 200 to ISO 400, as the improvement in DR is greater than the increase in noise. Not true from ISO 1600 to ISO 3200, as the noise increase is now greater than the DR increase. "Pushing" ISO in order to ETTR only works in the linear noise region of the sensor -- up to ISO 400 or so for CMOS.

Not quite. Increasing ISO so as to ETTR while keeping exposure constant is an advantage in that region where the relationship between camera ISO and DR is nonlinear, i.e., the region in which read noise (as measured in electrons) keeps falling with increasing camera ISO. On the E-M5 it falls up to about ISO 1600 although the main read-noise reduction occurs between ISO 200 and ISO 400.

I thought the EM5 had relatively constant read noise from ISO 800 on up:


I prefer to rely on black-frame read-noise data rather than on DxO via Sensorgen in this particular case. And these data suggest that the read noise keeps falling up to 1600. But the difference between 800 and 1600 is small.

By the way, probably better to refer to "ETTR via higher ISO" as "BTTR" (brightness to the right) or "GTTR" (gain to the right), as the exposure is unaffected.

I see your point although I can't say I felt a strong urge to introduce unfamiliar terminology in the midst of this particular exchange.

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