Depth of field (DoF, http://en.wikipedia.org/wiki/Depth_of_field ) would be cool.
Maybe not so hard to implement:

• object distance must be known for other reasons too,
• let the people define over a slider the lens
• give them a slider for the position of the focal area
  and optional
• a slider for the camera characteristics, which will have a impact on the camera-typ (35mm or digital) or circle of confusion ( CoC http://en.wikipedia.org/wiki/Circle_of_confusion )

or just a simple set of sliders which defines:

• the length of the focal area in the deep,
• the stron of the blur-effect in the front and
• the stron of the blure-effect in the back of the focal area.