Low-Light Options

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Today’s Question: When one is confronted with extreme low-light conditions, could you discuss the pros and cons of shooting at high ISO with in-camera high ISO noise reduction engaged versus purposely underexposing a photograph at lower ISO and subsequently correcting for the underexposure and reducing the noise during post-processing?

Tim’s Answer: When you raise the ISO setting in your digital camera, you are effectively under-exposing the photo, possibly to an extreme degree. Therefore, it is worth considering (as suggested in today’s question) how to minimize the risk of noise associated with that under-exposed photo.

The basic choice here is how to compensate for an under-exposed photo. Your two options are to either increase the ISO setting in the camera, or to leave the ISO setting at a low value (with a photo that is therefore underexposed) and save all adjustments for your image optimization workflow after the capture.

Put simply, you will generally get better results (often much better results) by raising the ISO setting as compared to simply under-exposing. To be sure, it is best to use the lowest ISO setting possible for the conditions in order to minimize the amount of noise in a given photo, because raising the ISO setting translates into amplification of the signal being gathered by the image sensor. That amplification translates to increased noise in the image.

However, while a high ISO setting increases the amount of noise in a photo, severely underexposing the image will produce (in most cases) far worse results. This was actually the subject of an article called “ISO Illustrated” that I published in the December 2013 issue of Pixology magazine.

I most certainly recommend keeping the ISO setting as low as possible to minimize noise. However, that doesn’t mean using a shutter speed that is too slow, or underexposing the image. When the situation requires a higher ISO setting to achieve a proper exposure, by all means raise the ISO setting. You may need to mitigate the noise in post-processing, and even with noise reduction the photo may not exhibit optimal quality. But the quality will still be better than if you had simply kept the ISO setting at a low value and under-exposed the photo.