This is a quick technique for introducing subtle imperfections to a rendered image in post.
The image on the right is the clean render. It has grunge on the surfaces and a bit of grain but it is still very sharp and clean.
This image has a vignette, lens distortion, chromatic aberration (redshift) and a slight blur added in Photoshop.
The effect is subtle but it helps the image appear more realistic and visually interesting.
Open the image in Photoshop and turn on the Channels Palette.
Open the Lens Correction Dialogue.
We want to make the image look as if it was photographed through a real lens.
Click the "Custom" tab and set "Remove Distortion" to a small negative value like -5.
This introduces a little bit of barrel distortion, wich is normally unwanted, but since we are photoshopping in reverse is a good thing.
Then set the "Vignette Amount" to a negative amount by dragging the slider to "darken".
Vignetting is the lensflare of this decade, we can be subtle or we can go all in..
Select the Red Channel only in the Channels Palette.
Select all pixels in the red channel.
Select the Move Tool and "shift" the Red Channel a few pixels up and to the right with the Arrow Keys.
This emulates the realworld phenomenon of the red part of the light being refracted by an imperfect lens (called "redshift" allthough technically that is not what is happening).
With the selection still active, turn all channels back on by clicking on the RGB button in the Channels Palette.
Then hit Ctrl + T to transform the selection and drag the bottom and left edge back to the extents of the image to cover the red gap.
This restores the original size of the images and introduces a slight blur from the resampling.
Deselect and we have a new version of the image that looks a little less computer generated thanks to subtle imperfections.
This technique can be recorded as an action, and run on a sequence of images for animations.