Thread: F-ratio myth
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Old 21-02-2018, 10:56 PM
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Shiraz (Ray)
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Join Date: Apr 2010
Location: ardrossan south australia
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maybe this will help. from An image requires that the light come from different angles.

"A lens which could only focus light rays striking the glass head-on .... would be fairly useless for astronomy. Fortunately, most lenses can also accept rays which come in at a slight angle to the optical axis, and bring them to a focus as well. This focal point is not the same as the focal point for rays which are parallel to the optical axis; depending on the angle of the incoming rays, their focus lies on one side or the other of the optical axis, as shown in the diagram below. But if the lens is well-made, all these focal points will lie on a plane which is parallel to the face of the lens; this is called the focal plane."
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