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    Mustafa Umut Sarac's Avatar
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    Cooke Chief Engineer writes on colors

    I asked few questions about Cooke XV famous Ansel Adams lens and Cooke Chief Engineer sent me archive papers and impossible to find Chance Brother Glasses dispersion characteristics.

    It may interest you.


    Your question about designing lenses for different colour temperatures is not so easy to answer simply. Frankly we do not really address the issue of photographing in different colour temperature light because any lens may be used with any colour temperature by different photographers at different times in the day, and at different seasons etc etc
    (b) But (and this is not really answering your question, but just trying to give you some background), at Cooke we design our lenses to be somewhat "warm". This is not so much about the colour of different glass types or anti-reflection coatings (although it sometimes is!), as you might think, it is more about the correction of the chromatic aberration. When you correct chromatic aberration, in effect you "fold" the visible spectrum so that red and blue light focus at the same focal plane, or, off axis, at the same image height. But because of what is known as the irrationality (a better word would be cantankerousness) of the spectral dispersion charactaristics of most optical glasses you get left with what is called "secondary spectum". On axis (that is, at the centre of the image) this gives a green focal plane closer to the lens than the red+blue focal plane. Somewhere between those two focal planes there is an achromatic focal region where all the colours mix nicely to give a neutral image. At Cooke we play with exactly which red and which blue wavelengths are superimposed, to try (in principle at least) to maximise the range of the achromatic focal depth....This results in a rather warm image, which some cinematographers say they like, but of course once the images get to the lab (whether it is film or digital) anything can happen.....
    (c) In the case of TV and Video, where there are usually three separate colour chips this secondary spectrum can to some extent be removed by placing the chips at carefully selected focal depths from the lens, but that all gets messed up when different focal length lenses are used. This is because the magnitude of secondary spectrum is more or less proportional to focal length on axis and more of less inversly proportional to focal length off axis.

    Best ,

    Mustafa Umut Sarac

    Istanbul
    Last edited by Mustafa Umut Sarac; 07-28-2010 at 04:59 PM. Click to view previous post history.



 

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