ISO Invariance


Today’s Question: What about the new ISO-invariant cameras? Does you answer [about optimal night exposures in yesterday’s edition of the Ask Tim Grey eNewsletter] apply to them too?

Tim’s Quick Answer: In general I would still say that increasing ISO in the camera is preferred over adjusting in post-processing, even with a sensor that has been labeled as being “ISO Invariant”.

More Detail: The term ISO Invariant in this context refers to a sensor where you can achieve the same results by increasing the ISO in the camera, or under-exposing the image and then brightening in post-processing. That doesn’t mean you’ll have a result with low noise levels necessarily, but rather that the results will be the same with the two approaches.

It is important to keep in mind that, as I pointed out in the article “ISO Illustrated” in the December 2013 issue of Pixology magazine, raising the ISO setting really represents underexposing a photo (perhaps severely) and amplifying the signal recorded by the image sensor in an effort to compensate.

In other words, raising the ISO setting can be thought of as brightening a photo in much the same way that dragging the Exposure slider in Adobe Camera Raw or Lightroom will brighten the photo.

In other words, in the context of ISO the real question is whether the camera can do a better job of amplifying the signal recorded by the sensor or if software in post-processing is able to do a better job.

In general I have found that the camera does a better job of amplifying the signal compared to using software after the capture. This makes sense considering the camera has the benefit of analog data to work with from the image sensor, rather than digital values in the RAW capture file after the capture.

Some cameras perform better than others, of course, both in terms of baseline noise thresholds as well as amplification quality. What I would say in general though is that based on what I’ve seen and have been able to test, there isn’t a clear advantage to ignoring the ISO setting in the camera, even with an “ISO Invariant” sensor.

As such, my recommendation is still to expose properly in the camera, even if that involves increasing the ISO setting to achieve the desired overall settings. That still represents underexposing the photo and using ISO to brighten in the camera, but I have found that this provides a superior result in most cases.