Digital Light & Color

I have always thought the best way to determine how significant the difference between an image and its derivative (when making very subtle changes in tone/hue etc) was to use Composite with Absolute Difference blend mode. In the black result image, white suggests the difference. At least that is the way I have always done it.

However, I had an image that the only change was Colour Balance. The resultant image visually looked a bit different than the original but only when each was clicked on repeatedly back and forth.

The difference was so slight that I did not know if my eyes were tricking me, so I used the a/n method. The result image from A.D. was completely black, and as I have done in the past, I ran it through Levels and Colour then maxed the Dynamic Range (in the past this has brought out any white, so I know there is some, albeit slight, change). The result was pure black again.

The weird thing is, if I click the Histogram Tool on both images, there is a slight change to its shape, which does suggest there is something different between the two.

So, my questions - Is the a/n method the best to determine extremely slight differences in an image (especially to identify the exact location within the image)? - Is there some floor, below which the A.D. cannot detect a difference? - If A.D. is not the best, which method is?

I read the Help file on Composite, A.D. but it is way above my pay grade, so it was not much help.

When doing Absolute Difference you may need to increase the Strength slider all the way up to 100% to amplify very small differences. If that doesn't work, try using Autorange (not Levels and Color) on the absolute difference image with black and white thresholds set to 0%. Also, double check that you are not computing the difference between the image and itself.

Jonathan,
Thanks for the reply. I am sure I was not computing the difference of the image with itself. And I will give a go with your suggestions to see what happens.