Most are familiar with how ISO controls a camera’s sensitivity to light and susceptibility to film grain. However, ISO has other consequences with digital, and its implementation often varies.
With film, one had to select and load a specific magazine in advance, so the ISO speed had to remain constant. If the exposure needed tweaking afterwards, the film could be “pushed” or “pulled” during development. However, doing so was only practical within a limited range of effective sensitivities—otherwise colors, contrast, grain and other qualities would suffer.
With digital, ISO functionality has improved substantially. Camera hardware no longer has to be swapped out, so ISO has effectively become an in-camera exposure setting along with aperture and shutter speed. Similar to push/pull with film, high-end cameras with RAW capabilities can also have their exposure tweaked in post-production. However, depending on the camera, doing so may only be practical within a limited range, and can compromise image quality more than if ISO had been set optimally in the first place.
See full details and examples of approaches to ISO on the RED 101 Article: ISO SPEED REVISITED