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Identical scene and photometric exposure, otherwise a comparison is void, meaningless and totally without statistic merit or validity.
That's why ABX testing exists, mainly within audio, but also when you compare human perception evaluation of image quality in a scientifically valid way. We used to do that when comparing different screen pattern types for print.
It's quite hilarious how few "audiophiles" actually hear the difference between a good quality mp3 , a high-rate AAC or a raw uncompressed signal, even on a 3000$ set of heads on a good headphone amplifier. They're usually very sure of themselves in the beginning, but after a few ABX rounds (when they still have the same hit/miss rate as an average random generator) confidence always tumbles.
The power of induced self suggestion on perception should never be underestimated. And it shouldn't be scoffed at either - since it has the same base workings as a placebo medicine. As long as the patient / subject is convinced, you get physiological effects. As long as the customer is convinced that the [very expensive] equipment he bought is world class audiophile material, it will sound beautiful - regardless of how crappy it really is.
You can have an OPINION or THINK that one thing is true - but until you have a relevant and valid base of comparison, that's all it is. Opinions and guesses.
So if you want to have a case saying that "this is different from this", then:
1) take two shots with all circumstances as identical as you can - that means, if you're not in a studio with constant light setup: try to take the pictures as close in time as possible.
2) Make SURE that both shots are taken at the exact same photometric exposure. That means Manual Mode, same ISO, same aperture, same shutterspeed.
Going into a menu and changing that setting shouldn't take more than five seconds tops. I've owned and used about ten different Nikon bodies, and if it takes you longer than that - RTFM.