RDKirk wrote in post #17943206
There are some particularly hard reasons why f/11 at 1/125 and ISO 100 won't produce the same image as f/4 at 1/30 and ISO 200; so there is
reason why a specific exposure index needs to be targeted for the intended optimum results in a given situation.
No processor can read your
mind. No processor can know that you want a particular tone in the image to reproduce at a particular brightness level and whether you care how other tones reproduce.
I don't think that you quite grasp what a RAW capture is. Barring any camera-specific defects, exposure affects only the amount of photons you collect; There is noise, too, and the higher the exposure, the less of it there is relative to the signal. If white and gray squares are in the scene, and there are 8x as many photons per pixel in the white square, there are also 8x as many if you "underexpose" by 2 stops, or "overexpose" by 1.22 stops, as long as the data doesn't clip. There are no qualities besides noise, clipping and posterization (only possible in the deepest shadows of lowest ISOs) at different tonal levels at a given ISO setting. Contrast and color saturation do not vary in the range, except as affected by noise, and we avoid noise by using a lower exposure index than intended for the ISO setting. If you really want it, you can add much nicer-looking noise in software.
Why shoot at ISO 43 when your scene allows for 100?
Less noise, of course. This allows you to display your image slightly larger before noise gets distracting, or to sharpen details more before noise gets distracting.