This is one of the most useful iPhone tricks I have latched on to. It is so good it has made me skip getting contact sheets or prints on my last few rolls of negative film.
PetaPixel described how to use your iPhone as a quick and easy negative viewer. It works like an absolute charm!
Simply enable the "invert colors" accessibility option and assign it to a triple-click of the home button. Then while using any camera app you can triple-click the home button to magically turn any negative image into a positive.
It works well with both B&W and color negatives. With color negatives the orange mask isn't perfectly translated but it is good enough to review the images.
This is both a time and cost saver if your workflow is oriented around digitally scanning your film.