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An ordinary slide viewer is turned around so that its lens
faces the ceiling light panel. An image of the light panel is
projected onto the back of the viewer by the lens. This neatly
demonstrates how lenses are used to make pictures.
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If you look carefully at the image on the slide viewer you will
see that it is really not very good. This would not make a good
camera lens, the image it has created is not evenly illuminated
and is only in focus at the centre. Camera lens' have extra
lens elements added to 'flaten out' the focus so that the image
is sharp from one corner to the other. The lens is also made
sufficiently large to create an image which is not darker at the
edges.
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