Exposure basics, lesson two point one (& ISO)

Started Mar 19, 2013 | Discussions thread
Detail Man
Detail Man Forum Pro • Posts: 17,020
ETTR - The Quantization Levels Meme That Would Not Die

richarddd wrote:

In fact the original motivation for ETTR (giving more raw levels) was completely bogus.

The motivation for ETTR has always been to use the entire range of the sensor to record the image. No more and no less. Which is why the "optimal" exposure if you are shooting Raw (ETTR) is different from the "optimal" exposure if you are shooting JPEG (expose to minimize PP adjustments).

Take a look at the seminal article on ETTR: http://www.luminous-landscape.com/tutorials/expose-right.shtml He sure seems to spend a lot of time talking about available levels

A common maxim in digital photography is that image quality is maximized by "exposing to the right" (ETTR) -- that is, raising the exposure as much as possible without clipping highlights. It is often stated that in doing so, one makes the best use of the "number of available levels" in the raw data. This explication for instance can be found in a much-quoted tutorial on Luminous-Landscape.com. The thinking is that, because raw is a linear capture medium, each higher stop in exposure accesses the next higher bit in the digital data, and twice as many raw levels are used in encoding the raw capture. For instance, in a 12-bit file, the highest stop of exposure has 2048 levels, the next highest stop 1024 levels, the one below that 512 levels, and so on. Naively it would seem obvious that the highest quality image data would arise from concentrating the image histogram in the higher exposure zones, where the abundance of levels allows finer tonal transitions.

However, the issue is not the number of raw levels in any given segment of the raw data (as measured e.g. in stops down from raw saturation point). Rather, the point is that by exposing to the right, one achieves a higher signal to noise ratio in the raw data. The number of available raw levels has little to do with the proper reason to expose right, since as we have seen the noise rises with signal and in fact the many raw levels available in higher exposure zones are largely wasted in digitizing photon shot noise ...

... The end result is that exposing to the right at the lowest possible ISO provides the highest image quality, but not for the reason usually given .

- Emil J Martinec


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