ISO 1600 vs correcting exposure in photoshop

[Daniel Browning]

In a purely digital world, you may be right. In the days of film, f/4 and 1/500th might underexpose some film and overexpose other film. Otherwise, Bryan Peterson must be wrong in calling it the Exposure Triangle...and he's certainly sold a lot of books calling it that.

There's nothing wrong with calling it the Exposure Triangle: that does not imply that changing ISO (without changing f-number or shutter speed) is changing exposure. I do not recall Bryan Peterson ever using the incorrect definition of exposure in "Understanding Exposure".

This took me a long while to digest and interpret. What I finally figured out is that you're suggesting that we shoot in manual mode,

I did use manual for illustration, but it's possible to use the "ETTR then ITTR" technique with autoexposure as well.

starting at ISO 100 "for every shot", and choose "exposure" (using your definition of exposure) values such that the desired image is (hopefully) captured while also maximizing the light upon the sensor, without so much light that important highlights are lost. If it's impossible to achieve the desired image via aperture and shutter values (i.e. slowing the shutter will result in a blurry image, or opening the aperture will thin the DoF too much),

Yep.

it's best to increase the ISO to maximize the light upon the sensor, without so much light that important highlights are lost.

Increasing ISO doesn't change the light on the sensor at all. It increases brightness and decreases read noise.

My translation of all of that is that it's best to achieve a right-justified histogram (using appropriate margins for the details we wish to keep) by prioritizing the widest artistically-reasonable aperture and the slowest artistically-reasonable shutter, and then the minimum amount of ISO increase.

That will result in the same settings (Tv/Av/ISO) that I am suggesting.

My initial interpretation was that I should set my camera to ISO 1600 and lock that in, which I don't think is the desired interpretation.

Correct. ISO 1600 always has less noise for a fixed exposure, but that doesn't mean we should always use it. High ISO has a price: clipped highlights. That price is always far too high to pay, except when we are in low light, and we no longer need the normal amount of highlight headroom.