Camera ISO is often taught as a part of the Exposure Triangle, along with Aperture and Shutter Speed. It basically determines how much brightness you want to add to your exposure. When you get more technical, photographic experts will debate the meaning of ISO in relation to exp…
My friend, you’ve done a wonderful job explaining the historical conventions devised to measure the image-capturing capabilities of light-sensitive substrates, even down to their engineering aspects. But including ISO as part of what some schmo made up out of whole cloth and called an “exposure triangle” is photographic nonsense. It has no basis in academic theory or teaching.
The idea of an “exposure triangle” misleads novice photographers toward poor creative technique.
ISO does not “brighten” or “darken” photographs per se. You are confusing the subjective interpretation of an under- or over-exposed image as, respectively, too dark or too bright. That’s a judgment made after the fact, not before exposing — making — the photograph. No matter what ISO one deliberately chooses before making a photograph, it can still result in either under- or overexposure. Again, that’s strictly a subjective call. I’m afraid you are conflating a convention of language usage with the practice of a photographic technique. That’s dangerous for creativity.
ISO is an arbitrarily determined measurement of, or a means to explain, the relative sensitivity of a light-sensing device (whether it’s film or silicon) inside of a camera. No more, no less. That is its only relationship to exposure, technically or in an engineering sense. One can surely brighten or darken an image after the fact — after an exposure is made. But exposure happens at the outset of creating a photographic image, not after. The higher the ISO is set before exposure, the greater the (usually) adverse affect on the ultimate photograph; i.e., digital “noise,” film “grain,” and respective saturation of pixels or silver halide crystals.
Photographers will generally, or colloquially, refer to ISO adjustments by using the term “stops” (either stops up or stops down as a convenient reference to f-stops, of course). But it is imperative to note that any such reference to ISO numbers in the context of aperture settings, i.e., the f-stops, merely alludes to how little or how much ambient light exists in the first place; i.e., what is necessary and sufficient to exposure an image with a viable combination of f-stop and shutter speed to suit the photographer’s creative vision. If the photographer determines that the existing ambient light is impracticable for the f-stop/shutter speed combination of choice (that will be the exposure), s/he can change the ISO to reflect those conditions, lending to the use of the f-stop/shutter combination preferred for the task at hand.
It is imperative for every photographer to think of ISO adjustments as a fallback, as a compromise, in preparation for shooting under existing low-ambient-light circumstances. It should never be thought of as a creative aspect of exposure, with one exception: a deliberate decision to add noise or grain. Otherwise, the optimal setting is always the lowest setting.
One cannot adjust the gain (to use the engineering term) after the fact, any more than one can change a camera’s shutter speed after the fact. Yes, you can “brighten” or “darken” a digital image with Photoshop®, but it’s nonsensical to think of brightening or darkening an image before it is exposed.
Exposure is determined by the combination of f-stop and shutter speed one chooses. Still, the relative tonality of the myriad objects depicted in any given photograph and the relative detail revealed in each one have megatons more to do with exposure than how bright or how dark it is. The bottom line is, setting a camera to a higher ISO rating before exposing an image has absolutely nothing to do with making its exposure “brighter” or “darker.”