While it's almost entirely subjective, a few factors play important roles. Angles, poses, lighting, theme, etc. Porn is made exclusively to arouse imo, erotic art not so much. You wouldn't mistake a good boudoir photograph, that might include penetration, for a porn still. I don't think I should post images here to make a point, but below are some generalisations, that while not ALWAYS applicable, are good enough guidelines.
1) Porn tends to be more dehumanizing to the subjects, be that males or females, than erotic photography.
2) In erotic photography often the subjects are framed in a way to tease the imagination, rather than lay it all bare. Especially so with the use of lighting techniques or by simply keeping some things out of frame.
3) Lighting is important. As an example for the above, you might have flat lighting during a gangbang scene in porn, or have the same exact scene with say bodyscapes in erotic photography, or rim lights and silhouettes.
4) Perhaps some extreme closeups that focus on expressions, leaving the action to the imagination (I certainly wouldn't suggest you do that in daz
)5) Posing too, a girl in a hot lingerie having a natural pose tends to lean on eroticism, compared to a pinup, or fuckme pose (though pinups have been used to great results in erotic photography too).
6) Focus. Often enough in erotic photography, the photographer might focus on something like the wrinkles of the fabric on say the raised skirt of the woman, leaving DoF to take care of leaving stuff up to imagination but clearly suggesting them. In porn usually they'd either have a very deep focus, or just zoom in to the cock entering her ass.
I could give more examples if I had my coffee first though. And as I said, these are generalisations. Not a guide, but things I've noticed. Even with all that in mind, the magic is in the execution. You could use ALL that and still look like porn, or none of it and be clearly some work of art. And above all, it's all subjective.
