The basic setup is very simple.

Load a one-sided square. Scale it up so it is like a screen hanging in front of your whole scene, between the camera and the subject. This is your "lens". You cannot see the scene directly anymore, only through the lens.

Set the square properties to no shadow, and not visible in raytracing.

Go into the material room for the lens (the square).

Set Diffuse_Value=0, Specular_Value = 0.

Add a Refract node and plug it into Alternate_Diffuse. (Or if you prefer, use one of the two-knob channels such as Ambient or Refraction. With such channels, you can have separate controls (and potentially nodes) on the color and the brightness.)

Set the Refract IOR = 1.0.

At this point you can render and your original scene should appear unchanged. You have a neutral lens. Save this in your library for future use as a starter.

Then you go from there playing with the effects.

For example, put an HSV node between the Refract and the root node. Now you can adjust hue, saturation, and brightness for the entire scene. Not only does this simple setup give you many artistic options, but you can also adjust your LIGHT LEVELS all from this one place: HSV.Level.

After you get the hang of that you can start doing more tricky stuff between the Refract node and the root node.

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