Underexpose or Raise ISO?

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Today’s Question: All things being equal, with a raw capture is it better to raise the ISO rather than underexposing and brightening in post? A specific challenge for me is holding detail in moonlit scenes without blowing out the moon. Under-exposed holds more highlight detail while darks can be dug out in post. Brighter exposure cooks the high lights irretrievably.

Tim’s Quick Answer: Generally speaking, it is better to raise the ISO setting rather than under-exposing and brightening in post-processing. Note, however, that in the type of scenario described in the question, bracketing several exposures to assemble into a high dynamic range (HDR) result can improve quality in terms of noise in the image.

More Detail: As noted in Monday’s Ask Tim Grey eNewsletter, raising the ISO setting doesn’t increase the sensitivity of the image sensor in the camera. Rather, when you raise the ISO setting you are under-exposing the image and then having the camera apply amplification to the signal recorded by the image sensor. The result is generally an increase in noise in the image.

However, as outlined in an article that appeared in the July 2016 issue of Pixology magazine, if you under-expose without raising the ISO, you’ll generally get more noise in the final image than if you had instead raised the ISO setting in the camera.

As I’ve also covered before, making sure to expose an image as brightly as possible without actually losing highlight detail will help ensure minimum noise and maximum detail in the image. Therefore, in the example from today’s question you would want to make sure the moon is as bright as possible without losing any detail.

However, this is a good example of a scenario where you may very well end up clipping shadow detail by creating an exposure for the highlights. You may be able to bring out more visible detail by brightening up the shadows in the image, but that will also reveal noise in those areas.

To help ensure maximum quality, you can create a high dynamic range (HDR) image in this type of situation. This involves bracketing the exposures so you have a capture that retains full highlight detail, and capture that retains full shadow detail (to the point that there isn’t even a true black in the image), and additional exposures in between so there is no more than two stops of separation between the exposures. Those images can then be combined into an HDR image that will have less noise than a single exposure that requires brightening to reveal shadow detail.