I need help....

here goes my problem:

I am measuring light intensities in pictures.
When I take a couple of pictures of the same object and with the same settings, I get slightly different intensities. That is fine, that is just a "Normal" variation between pictures.

Then I take some more pictures of another object. In these pictures I change the settings a bit between the different pictures.
I want to find out if the differences in these pictures are due to an actual difference in the intensity (because of the different settings) or if it is just the "normal" variation, I found above.

The intensity levels are completly different from object to object. The difference in intensity will therefore be much higher in pictures with high intensity than pictures with low intensity. Therfore the variation needs to be in relation to the intensity. It could be in % for example

SO should I calculate the standard deviation and then divide it with the mean intensity?
or should I calculate the variance and then divide it with the mean  intensity?
or should I do something completely different?

ravenslament: Do you have a dog? If yes, it was him. If not, it must have been one of your friends, the kids or your Mum.

Statistically, I am 100% sure you ate the chocolate.