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Color correction filter?
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PostPosted: Wed Jun 17, 2015 3:39 pm    Post subject: Color correction filter? Reply with quote
What kind of filter would be suited for a lens corrected for 4861 - 6563 Angstrom?


PostPosted: Wed Jun 17, 2015 5:43 pm    Post subject: Re: Color correction filter? Reply with quote
Opticus wrote:
What kind of filter would be suited for a lens corrected for 4861 - 6563 Angstrom?


This is 486 - 656nm, basically from longer wave blue to red (usually lenses transmit at least 400 - 700nm), so it misses a bit on the short and on the long side.

But modern DSLRs anyway have internal cut filters (ICF) in front of the sensor looking like the following one, so you'll see that you're not missing that much:

(C) beyondvisible.com [modified by me]

You don't need any filter for normal photography, a custom white balance will nicely do the job and adjust the blue and red channels accordingly, so to even that out.

P.S.: Funny enough these numbers are Mercury lines, the so called F and C wavelengths (4861A and 6563A) often used by optical designers to make an achromatic and if they use F, e and C wavelengths (4861A, 5461A and 6563A), an apochromatic lens.

P.P.S.: Are you sure it is 4861 till (-) 6563A and not 4861A AND 6563A? BIG difference, as it would be a lens being designed and corrected for two (narrow) spectral bands only. I have seen such lenses...

What lens is that if I may ask??

I have a feeling you talk about an older aerial reconnaissance lens, as the USAF used that term 4861 - 6563A often in their requirements towards manufacturers for required color correction.


PostPosted: Wed Jun 17, 2015 6:45 pm    Post subject: Reply with quote
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Last edited by Opticus on Sun Aug 21, 2016 7:23 pm; edited 1 time in total


PostPosted: Wed Jun 17, 2015 9:32 pm    Post subject: Reply with quote
Nope, has nothing to do with it. It is the (lacking) spherical aberration correction and the lacking correction of longitudinal chromatic aberration - very common with very fast lenses. Stopping massively down helps...but I guess you have it because it's very fast.

Processing images afterwards is what some people do to salvage such images.
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