On Mon, 17 May 2004, Rod Wynne-Powell wrote: > Subject: Re: [PRODIG] Purple fringing comparisons between cameras
> Hi Tony, > In the analogue days of film a lens may well have been classified as > apochromatic, but unless I am much mistaken, the description would have > been based upon setting these lenses geometry to focus on colour film > (probably Kodachrome or Ektachrome). The true apochromaticity (my > extrapolation!) would thus surely have taken into account the different > depths of the coloured layers in such film when deciding upon the > focus. Hi Rod Yes, I hadn't thought of this, you could be right, although I don't recall having seen selective depth focussing described in relation to film. I just reread a paper by Schneider on digital lenses, http://www.schneideroptics.com/info/white_papers/optics_for_digital_photography.pdf, and they say (I think about 1998) - '...we must also take into account that color fringes (resulting from chromatic aberrations) are of course disturbing (as in classical photography). The color fringes should be significantly smaller than a pixel size, which can only be achieved by using top quality lenses made with special glass having so-called apochromatic (high color correction) construction. ……..as a result of their (Digitar lens) MTF values and the apochromatic correction, they are also largely suitable for image sensors of the next generation.' This is not to say that bringing three or more spectral areas to a common focus (apochrom. correction) is now done without selective depth. I just don't know. I was under the impression that silicon had an inherent change of colour sensitivity with depth, as exploited by the Foveon Sigma system. > A modern chip be it CCD or CMOS is essentially one layer deep in terms > of where it is focussed, which presumably accounts for why > manufacturers have recomputed lenses for digital capture, The paper above describes other reasons for recomputing lenses for digital based on the difference between pixel width and film grain size in relation to the resolution of the eye. > In short, I do not see you avoiding chromatic aberration in using a > C-mount lens from a bygone era, however good it was for film. Again you could be right. I didn't explain very well, but my search is for an unfiltered ccd camera with a monochromatic response, on which I can experiment with my own filters. C mount seems to be the most common scientific mount, and I thought I could also try some of my apochromatic lenses, although it is a side issue. I like to fiddle about and try things! Thanks for your response though. Tony www.photosightdesign.co.uk www.sharpedgegallery.co.uk =============================================================== GO TO http://www.prodig.org for ~ GUIDELINES ~ un/SUBSCRIBING ~ ITEMS for SALE
