There have been a flurry of statements recently on photo fora that exposure to the right (ETTR) is an obsolete concept. The argument goes that, while it may have been useful in the past, modern cameras have so little noise that ETTR is currently a waste of time and brain cells. If you buy that line of reasoning — and who am I to question what image quality you think is good enough? — you can save yourself considerable money. Read on.
Photon noise dominates the noise of modern cameras at signal-to-(photon) noise ratios (SNRs) of greater than 10:1, which I consider the low end of photographically acceptable noisiness. Assuming similar sensor technology, shot/Poisson/photon noise is proportional to the square root of the pixel area. Thus, it’s proportional to the inverse of the pixel pitch. We can remove sensor resolution from the picture by noting that at identical pixel pitches, SNR is proportional to the inverse of the square root of the sensor area. Thus, it’s proportional to the inverse of any linear dimension of the sensor area.
That means (eliding differences in sensor technology) all of the following have the about the same resolution-adjusted SNR:
- A Nikon D810 6 stops underexposed
- A Sony alpha 7000 5 stops underexposed
- A micro four-thirds camera 4 stops underexposed
- A Leica D-Lux 6 2 stops underexposed
- An iPhone 5 exposed correctly using ETTR
Another way of looking at this is, if you have a full frame camera and regularly leave two stops of blank space on the right side of your raw histogram, you could get the same image shadow SNR — and the reason for ETTR is maximizing shadow SNR — by buying a MFT camera and practicing ETTR.
The above ignores many things, among them lens quality, sensor technology, read noise, PRNU, diffraction, depth of field… Still, I think it’s a useful way to think about ETTR.
While modern sensors produce remarkably low noise, practicing ETTR can either give you more ability to boost the shadows in post processing, or allow you to use a small camera and get results that are equivalent to a much larger, heavier, and more expensive instrument.