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>>102863 It doesn't really change the amount of light hitting the focal plane, but it changes the brightness in a given spot.
For example: Sunny day + magnifying glass + anthill. When the sun is not properly focused, you get warm ants--the light is distributed over a small area. When the sun *is* properly focused, the average light is the same, but the spot where you're raining down fiery death on the ant who has invoked your wrath is getting a shitload more than anywhere else in the area were it out of focus.
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