Comparing two images

I am stuck in here with a dilemma. I need to compare two images for contrast enhancement in grey scale. But as the two images are taken in different settings, how can I calculate for the enhancement only excluding all other variables.

Hello @Muhtamim and welcome to the ImageJ Forum!

What is exactly the comparison you want to do? Can you maybe normalize or match the histograms of your images before the comparison? Have a look at this previous post.