Sequence viewing > Index - Cameras - lenses - optics - Resource - © Lloyd Godman

Film - Digital

Why do cameras have lenses?

A lens is used to reconverge the rays of light reflected from an object and focus them on a given point. This point is most usually where the film is positioned. If we compare focusing the camera  to starting a fire with a magnifying glass, rather than concentrate the rays of light on a single point, the rays of light through the camera lens fan out past this point and project onto the film. Note that because the rays of light cross over, the image of the tree focused on the film in the diagram below is back to front and up side down. This happens in all cameras but we don't usually see it as it is not evident in the view finder and then it is corrected at the printing stage. 

In the case of transparencies or slides, we view them to make visual sense not how the camera saw them.

Move over image to view roll over image

Move over image to view roll over image

How much light gets through the lens depends upon:
the aperture the lens is set at 
and with auto lenses the speed of the lens

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