The Norton’s excerpt from Marx’s German Ideology contains the classic passage comparing “ideology” to a “camera obscura.”  What, you may ask, is a camera obscura?  Here’s an image:

[description from the Cabinet of Wonders blog]: The camera obscura works under the same principles as the pinhole camera: you make a small hole in the side of a box (either a real box or a room-sized box) and the light outside will get in through the hole and project itself onto a piece of paper or a wall, showing you a perfect image of the scene on the outside of the box. Because light travels in a straight line, and because the hole is small, the light on one side of the scene will have to come through at an opposing angle from the light on the other side of the scene.

As we will discuss, the metaphor points at the way cultural representations preserve a kind of fidelity to social reality (i.e., the representation issues from the real thing) but in a distorted manner.  So the work of “ideological criticism” is to re-establish the relationship between reality and representation, a job that’s much more complex in most cases than the simple two-dimensional “flip” in a camera obscura would suggest.