One of the most important visual cues when shooting for photo-real, is depth of field. There are a few things to consider if you are trying to mimic the lens effects of a camera when it comes to DOF, Focal length, sensor size (or film back size on the computer), and bokeh all make a difference in the image. Being able to simulate physically accurate depth of field becomes important.
Nuke Compositor Jed Smith offers some insights into simulating physically accurate depth of field in a composite. Jed talks through the nerd that is basic lens physics, including terminology, and also has a look at the variables that can affect the depth of field behavior in a lens.
Jed has built a couple of tools for Nuke that can help you visualize and simulate physically accurate depth of field. OpticalZDefocus helps to control the circle of confusion size based on actual lens geometry using a standard depth of field equation.
Another tool, DofCalc, lets you easily visually the near and far death limit for DOF in a given lens focal length, aperture, film back size, and focus distance.
Jed covers depth channels and how they work within Nuke, and how to use his two tools to simulate accurate DOF. Definitely worth the watch!