Today’s Question: In a recent email you suggested doing some testing in order to get a sense of the impact of ISO on noise levels for a given camera. How do you recommend doing that testing?
Tim’s Quick Answer: I recommend capturing a series of test exposures that enable you to see the impact of various camera settings on the noise levels in your photos. The key is to create a series of simple captures, focusing on changes to on variable at a time (to the extent possible).
More Detail: There are a number of variables that affect noise levels in digital captures, but one of the most significant is generally the ISO setting. This will therefore typically be the key variable to focus on, but you’ll want to consider other factors as well.
To begin with, I would generally use a subject with a solid color and minimal texture, such as a gray card. A blank backdrop or other simple surface can be used as well. Then, with the camera on a tripod, configure the camera for a normal exposure. I would typically use Aperture Priority mode with the lens aperture wide open, to ensure the fastest possible shutter speeds. This will help ensure that the impact of a long exposure is not affecting your test results.
Then capture a series of images at varying ISO settings, starting at the base ISO for your camera and increasing in perhaps one-stop increments. So you might capture an image at 100 ISO, then 200, then 400, then 800, and so on, until you reach the maximum ISO setting (or the maximum setting that you actually want to test).
As you raise the ISO setting, you may find that the wide-open aperture causes you to be unable to achieve a proper exposure with the available range of shutter speeds, so you may need to stop down the lens as part of the capture sequence.
You might also want to capture longer exposures in a similar manner, both with in-camera long exposure noise reduction turned on and turned off.
When it comes time to review the results of this testing, keep in mind that in most cases the software used to generate previews from raw captures will apply some noise reduction by default. You therefore may want to process all of the captures in a batch with default adjustment settings but with noise reduction disabled.
By reviewing the series of captures (and the metadata for each of them) with no noise reduction applied, you’ll get a clear sense of how much noise you can expect at different camera settings. Again, in particular this would mean evaluating results at various ISO settings, but you’ll also want to see how your camera performs with long exposures of varying durations.
After reviewing your results, you’ll have a much better sense of the noise behavior of your camera, as well as what sort of limits you might want to employ to help ensure minimal noise whenever possible.