Re: spherical aberration as a function of conjugate ratio
- From: JK <johnknuhtsenSP@xxxxxxx>
- Date: Wed, 19 Apr 2006 11:22:29 +0200
On 18 Apr 2006 19:54:18 -0700, david_a119@xxxxxxxxx wrote:
Suppose I have a really good camera lens or microscope objective that
gives diffraction-limited performance if I use it at the conjugate
ratio that it's designed for. If I know the prescription for the lens,
I can use Zemax or Oslo to calculate the amount of spherical aberration
that I'll get if I use the lens at some other conjugate ratio. But
there's no way I can get the prescription from Nikon or Zeiss. If one
doesn't know the prescription, to calculate spherical aberration as a
function of conjugate ratio one needs to know whether the lens
satisfies Abbe's sine condition, Herschel's sine condition, Helmholtz's
tangent condition, or some other condition. (I personally can't
calculate spherical aberration even if I do know the equations of the
principal surfaces, but I know that some people can.)
Camera lenses are normally for infinite conjugate.
Microscopes are for finite conjugates.
I can model the lens as a "perfect lens" in Oslo (not as a "paraxial
lens" in Zemax) and presumably get accurate results IF the lens
satisfies Abbe's sign condition. Some people say, if it's a really
good lens of course it satisfies Abbe's sign condition, otherwise there
would be coma. How safe is this assumption? According to a paper that
I've seen (CJR Sheppard and Min Gu, "Imaging by a high aperture optical
system," J Modern Optics vol. 40, p. 1631-1651), "the experimental
results for microscope objectives seem to agree better with the
predictions of a system obeying the tangent condition rather than the
sine condition," but they don't provide any details. If the lens is
used at the "wrong" conjugate ratio, spherical aberration for a lens
that satisfies Abbe's sine condition has the opposite sign from
spherical aberration for a lens that satisfies Helmholtz's tangent
condition, so Sheppard and Gu's statement is alarming.
If I need to find a company or consultant who understands this sort of
thing and can measure the aberrations of the lens and develop an
accurate Zemax or Oslo model, is there anyone who you recommend?
I would recommend to look in patent litterature for a similar
construction. It doesn't have to be of the same brand.
You don't tell anything about your specific lens.
Patents describes thorougly all details of a construction, so it can
be set up in an optics program. Sometimes glasses and dispersions are
difficult to match, so colour aberrations might be difficult to copy.
Oslo and other programs does also have optional construction databases
where you can look for a similar construction.
A perfect lens does not exist. It is just a question of which scale
the aberrations have.
I am not so familiar with your 'conditions talk'. I use the numerical
approach.
But it should be fairly easy by using a test poster to test how
aberrations change by changing conjugate. If you test on axis you have
just spherical aberr. involved (and colour).
best regards
John Knuhtsen
.
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