Criteria to judge the degree of photorealism of an image rendered by different pieces of software



Maybe I am in the wrong section. I apologise for that.

I am a PhD student and I am working with Blender for my thesis project. I would like to know what criteria are used to determine if a render is photorealistic or not.

I ask this because in my thesis project we want to create a scene with different levels of photorealism. What I mean by this is a scene that looks more or less like a picture.

I know that there are photorealistic rendering competitions. Do you know the criteria used to judge them?

Thank you for your answer.