What lens (collecting or diffusing) is used: a) for a magnifying glass; b) in the lens of the camera; c) in the projector lens?

In all cases, collecting. In cameras, objects are usually very far away, so the lens will give a scaled-down, upside-down, actual image. A projection apparatus produces an enlarged, upside-down, real image. Using a magnifying glass, you can get both enlarged and reduced images.

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