Measure focal length of a lens

  • 31 October 2019
  • 2 replies

I am using a Zemax model of an Edmund Optics lens in my design. The lens has a focal point of 10mm at 587.6nm, but my instrument will use a 650nm laser. Is there a tool in Zemax that calculates the focal length of a lens, with consideration of wavelength? Alternatively, I would appreciate guidance on how to calculate this by hand. The best I have been able to do so far is eyeball the location where the light converges and measure that distance (which appears so be about 12.3 mm).

Best answer by David 1 November 2019, 15:08

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Userlevel 5
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I suggest inserting the lens in a sequential design file. You can then use Prescription Data under the Analyze / Reports tab to generate a report that includes focal length. In the attached file both wavelengths are in the wavelength editor. When I check only .5876 I get EFL of  10.0039 mm.  With 0.650 selected I get 10.04819 mm. This is often less useful than just optimizing to determine the back focal distance because the BFD is measured from the vertex of the last element, which is measurable, while the EFL is measured from the principal plane which must be determined separately.


Userlevel 1
You could also try the merit function operands EFLX and EFLY which give:


"Effective focal length in the local x or y plane of the range of surfaces defined by Surf1 and Surf2 at the primary wavelength."

This is useful if you just want the focal length of a contiguous part of a lens design but you are limited to the primary wavelength.  For symmetrical systems x and y are the same.