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I would like to purchase my own aspherical lens for testing purpose. Then I need to define the required coefficients. But how? My known requirements are f=25mm F/# 2.0 and MTF 0.5 @ 120lp/mm with a wl of 850nM. The distance from the object plane to my lens is 10m. I will test it for an imaging application. Thanks

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You can purchase some aspheres designed for particular uses, like collimating a laser beam, from most optics supply companies (CVI, Newport, etc.). They will also have best-form spherical lenses that will work over a broader range of conjugates.

If you need to design an imaging system, there is really no simple formula to compute aspheric coefficients. You would use an optical design tool like CodeV to determine the ideal shape via nonlinear optimization.

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I'm actually a little concerned how you ended up in the position of needing to specify an imaging lens with those specs without having already been introduced to lens design software, or at least knowledge of it's existence. One does not usually know enough to ask that question before they know enough to answer it. – Colin K Feb 27 '12 at 14:14

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