asepherical lens
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Spherical lenses (simple lenses) have a surface which is an 'arc' of a true sphere. The downside is that the focussed light rays from different parts of the surface will fall on a slightly different image plane.
Aspherical lenses (or aspherical elements in a lens) correct this by ensuring (or trying to!) that rays entering at any point on the surface will be focussed on a common image plane.
Wider aperture lenses have increased need to employ aspherical elements as the depth of focus decreases as the size of the aperture increases....for the same focal length.
Click on the tabs at the top of the diagram in this link to see what happens.
Bob
Aspherical lenses (or aspherical elements in a lens) correct this by ensuring (or trying to!) that rays entering at any point on the surface will be focussed on a common image plane.
Wider aperture lenses have increased need to employ aspherical elements as the depth of focus decreases as the size of the aperture increases....for the same focal length.
Click on the tabs at the top of the diagram in this link to see what happens.
Bob
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Woodsy
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^ This.
HoppyUK
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Good link from Bob. What it doesn't really mention is that the reason traditional lenses have spherical surfaces is because that's all you can do in mass production with conventional grinding and polishing machines.
Aspherics became economically possible with the introduction of computer controlled machines that can do different curves. They're still expensive though.
Aspherics became economically possible with the introduction of computer controlled machines that can do different curves. They're still expensive though.