On Wed, Dec 7, 2016 at 2:22 AM, Larry Colen <[email protected]> wrote:
> In simple terms, the definition of infinity focus would be to have parallel > incident light rays. Divergent rays from a point source are closer than > infinity. One could probably set up a calibration unit with parallel light > rays using conventional optics. I suspect that a low powered laser would > also work. In case it helps your googling, what you're trying to achieve is "collimation" of light. When I taught astronomical instrumentation lab, we achieved this on the bench with a white light shining onto a spatial filter (basically a pinhole), with the spatial filter located in the focal plane of a Nikon 200/2.8 or 300/4 lens (which was "locked down" to infinity focus). The light coming out the front of the lens was (reasonably) collimated. If you picture parallel rays coming from a star on the sky and being focused to a point on the sensor, it's exactly that, except the light is moving in reverse. We then sent the collimated light into rudimentary lenses (e.g. achromatic doublets) and viewed the spots produced, to compare the actual aberrations with predicted spot diagrams from optical design software. The big caveats are: 1) Your collimating lens has to be focused at infinity and the right distance from the point source, or the light that comes out isn't collimated (it will "look" closer or farther than infinity). So this just shifts the problem of finding infinity focus to another optic! You may be able to get calibrated collimating lenses, making it a mechanical positioning problem. 2) At least if you're trying to evaluate aberrations of the optics under test, the collimating lens has to be optically superior to the test lens, or its own aberrations will come into play. In our case, with good Nikon lenses against doublets or triplets, this was the case. -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
