To ETTR or not to ETTR...?

Started 3 months ago | Discussions thread
The Ghost of Caravaggio Contributing Member • Posts: 701
Just Maximize Exposure
4

More exposure means more signal (and less relative photon noise). More signal means more information. More information means more perceived image quality.

So just maximize exposure.

Emil Martinec wrote[1,2]:

"... I would prefer "Maximize Exposure"; maximize subject to three constraints:

(1) maintaining needed DoF, which limits how much you can open up the aperture;
(2) freezing motion, which limits the exposure time;
(3) retaining highlight detail, by not clipping wanted highlight areas in any channel.

Note that ISO is not part of exposure. Exposure has only to do with aperture and shutter speed. Maximizing exposure guarantees that one captures as many photons as possible subject to photographic constraints, and therefore optimizes S/N."

This advice is based on the physics of digital imaging and produces in the highest possible analog signal-to-noise ratio. Dynamic range depends directly on SNR.

An important aspect of maximizing exposure is intentionally overexposing unimportant highlight regions. Not all highlight regions are "wanted".  Just two examples are specular reflections of sunlight in daylight and point-source lights in night scenes. Even if sunlight reflections are not overexposed, those regions have no color or detail. They are not informative. Overexposure produces white, uninformative regions as well.

This advice also implies one should use the lowest possible native ISO value (often called base ISO). [3]

One strategy to maximize exposure is to auto-bracket three exposures in 1/3 to 1 stop increments using different aperture or shutter times and raw files. You keep the image with an acceptable amount of highlight region over exposure and delete the other two. Of course, there are scenes where all the highlight regions are important. In this case intentional overexposure is impractical.

1. Martinec is currently a professor at the Enrico Fermi Institute, University of Chicago. He is the Director of the Kadanoff Center for Theoretical Physics.

2. The technical details are found here in three articles "Noise, Dynamic Range and Bit Depth in Digital SLRs". These are old articles and older cameras are used as examples. However, the science and engineering principles apply to today's cameras as well.

3. For newer cameras with dual-conversion gain sensors use the lowest ISO (low conversion gain) in bright light and the lowest ISO with high conversion gain in low light.

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