Why can’t Auto ISO be applied after the photo is taken?
Asked 8/26/2021
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2 answers
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I often shoot macro with flash and light modifiers using manual settings like f/18, 1/200 s, and 1/4 flash power. When the light changes, I end up adjusting ISO manually. Auto ISO doesn’t work well for me at close focusing distances because the camera’s metering isn’t accurate with my flash setup.
Why can’t the camera just apply the equivalent of Auto ISO after the image is captured to make the photo the right brightness? Is that different from brightening the image later in post-processing?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
4y ago
2 Answers
3
Because ISO adjustment changes the gain applied to the analog sensor signal before digitization.
Whether that results in better performance vs. a brightness adjustment after digitization is not necessarily given, (see steven-kersting's answer about ISO invariance), but in many cameras it is different.
If you have an ISO invariant camera, you can indeed forget about setting ISO if you're going to post process anyway.
Addendum: I should add that the setting "Auto ISO" at least in some cameras can set the ISO value to "non-natural", i.e. non full stop (doubling/halving steps) values, eg. 125. These values may not be available for manually selection and are actually digital expansions from the nearest real value. (I'm talking from experience with my entry level Canon EOS here, YMMV)
Originally by user32110. Source · Licensed CC BY-SA 4.0
user32110
4y ago
0
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Auto ISO isn’t really something the camera can intelligently “apply later” in the way you mean.
ISO changes the gain applied to the sensor signal during readout, before or around digitization. That can matter because once the data is digitized at a given level, some information may already be limited by the camera’s analog-to-digital conversion and noise performance. So it is not always fully reversible afterward.
Also, Auto ISO normally depends on exposure metering before the shot. After capture, the camera has no reliable way to know what brightness is “correct” artistically.
That said, on many modern cameras that are close to ISO invariant, brightening a low-ISO RAW file later can give results very similar to using a higher ISO in camera. In those cases, you can often shoot at low ISO and adjust exposure in post with little penalty.
With flash, Auto ISO can also interact with other exposure decisions depending on mode, and shutter speed, aperture, or flash output cannot be changed after the shot. So post-capture brightening is only equivalent to raising ISO, not to recreating the camera’s full Auto ISO behavior.
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