01-05-2015, 02:54 AM
Optically a lens can be reduced to test results though, just not ones used by photographers. Interferometry is the only method I know of that reduces a lens to pure aberrations in a way that mimics design as well. In design you see a transverse ray plot. The focusing error of the ray is aberration. Different aberrations have different focusing errors throughout the pupil. Here is spherical aberration:
[sharedmedia=gallery:images:1315]
An interferometer will produce an interferogram - the fringes are wavefront error in a number of waves (multiple of ~630nm). Like a RIM plot (transverse ray plot), different aberrations manifest in different interferograms. Here is a series of interferograms across different field positions:
http://i.imgur.com/DEIaigC.png
What you see here is field constant coma and asymmetrical linear astigmatism. These are effects you won't see outside freeform optics (my area of research).
The limits of an interferometric test is that you can't evaluate polychromatic aberrations this way, and the resutls are only for the wavelength of the laser. Typically interferometers contain an HeNe laser, but they can be made with other lasers as well. The machine can also only be run "at infinity", otherwise everything comes out wrong.
Other than that, interferometry is the method we use to evaluate produced optics for any factor. Different methods of interferometry can test wavefront error (aberrations), surface roughness (finer than any other method of physical measurement, might I add - SEMs barely scratch 1/1000th the precision), crystal structure, and perhaps other things I do not know about. Interferometry, done correctly, has *no* confounding variables. All you need is a reference optic with the same radius of curvature as the lens, which is simple enough to produce given the focal length of the lens, which can be physically measured to exacting precision.
Of course mechanical factors cannot be evaluated this way, but all optical factors can be analyzed independently through interferometry. You can even compute the coefficients of the aberrations in a merit function for the real lens through it, or the coefficients, normalization radius, etc, for aspheres.
My beef with lenscore's test is that any aberrations in their mysterious optic between the lens and sensor confound the results unless the optic is diffraction limited at an aperture faster than the lens being tested.
[sharedmedia=gallery:images:1315]
An interferometer will produce an interferogram - the fringes are wavefront error in a number of waves (multiple of ~630nm). Like a RIM plot (transverse ray plot), different aberrations manifest in different interferograms. Here is a series of interferograms across different field positions:
http://i.imgur.com/DEIaigC.png
What you see here is field constant coma and asymmetrical linear astigmatism. These are effects you won't see outside freeform optics (my area of research).
The limits of an interferometric test is that you can't evaluate polychromatic aberrations this way, and the resutls are only for the wavelength of the laser. Typically interferometers contain an HeNe laser, but they can be made with other lasers as well. The machine can also only be run "at infinity", otherwise everything comes out wrong.
Other than that, interferometry is the method we use to evaluate produced optics for any factor. Different methods of interferometry can test wavefront error (aberrations), surface roughness (finer than any other method of physical measurement, might I add - SEMs barely scratch 1/1000th the precision), crystal structure, and perhaps other things I do not know about. Interferometry, done correctly, has *no* confounding variables. All you need is a reference optic with the same radius of curvature as the lens, which is simple enough to produce given the focal length of the lens, which can be physically measured to exacting precision.
Of course mechanical factors cannot be evaluated this way, but all optical factors can be analyzed independently through interferometry. You can even compute the coefficients of the aberrations in a merit function for the real lens through it, or the coefficients, normalization radius, etc, for aspheres.
My beef with lenscore's test is that any aberrations in their mysterious optic between the lens and sensor confound the results unless the optic is diffraction limited at an aperture faster than the lens being tested.