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Thread: Red Shift

  1. #1
    Sparky's Avatar
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    Red Shift

    It would seem quite likely to me that shooting B/W through a red No.25 filter would require a focus adjustment to maintain critical sharpness - much like using an IR filter when shooting IR film since a good part of the spectrum for which a lens may be corrected will now be largely missing. Any thoughts on this?

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    David A. Goldfarb's Avatar
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    You're still in the visible spectrum, so unless the film has some extended red sensitivity beyond the visible spectrum, and as long as you are focusing through the filter, there shouldn't be any focus shift.
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    I know that Red Shift was a big problem for Senator McCarthy in the 50s.

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    cymbal, please...

    Okay - let's put this another way. Since no lens is truly orthochromatic - and ALL lenses MUST perforce (my $5 word for the day) focus different wavelengths of light at different locations on the film, and since the red end of the spectrum is reasonably far from the 'average' set of circumstances for which lens designers work - it*seems to me likely that a focussing adjustment when using a red filter MIGHT be in order. Has anyone actually TRIED this? If not - perhaps I will...(!)

    Can you tell I've been hitting the LP Clerc this week?

  5. #5
    Sparky's Avatar
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    though I guess an easy way to actually TEST this would be to focus on something, make a ref. mark and then slap a No.25 on and see if your focus changes over a series of trials. Nuff said I guess.

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    Yes, I have a thought. Find something else to think about.
    Claire (Ms Anne Thrope is in the darkroom)

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    David A. Goldfarb's Avatar
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    If the lens has severe chromatic aberration, then red might focus at a different plane than other wavelengths, but it doesn't matter as long as you focus with the filter in place. With some older lenses it might be recommended to use a strong monochromatic filter all the time with B&W to correct for chromatic aberration.

    The reason that IR is a problem is that you can't see IR, and most lenses aren't chromatically corrected for IR, so you have to adjust for the focus shift based on a calculation, and not what you see with your eyes.
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    Sparky's Avatar
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    I just figured the glass doesn't know the difference between the visible and invisible portions of the spectrum... and since the red getting through a red filter is actually QUITE close in wavelengh - that the 'falloff' or required focus shift would vary according to wavelength... hence you would possibly need, say, 3/4 of the offset for IR in this circumstance...

    just be happy I'm not posting any questions about focus shift vs. temperature..!!!

    PS - enjoyed your website, David. I have respect for your opinions.

  9. #9
    Ole
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    Any lens of more complicated construction than a simple magnifying glass should not need any correction with a red filter. Even the simple "Landscape meniscus" could handle that - mine does, at least.

    The focus shift correction made by daguerrotype and calotype photographers was due to these materials being primarily sensitive to UV, which is further from the "average" green spectum than the red end of a panchromatic film's sensitivity.

    The IR focus mark is a confusing thing too - AFAIK it is set for the far end of KOdak HIE's sensitivity. If you use MACO IR 820 the focus will be worse at the IR mark than if the normal focus mark is used.
    -- Ole Tjugen, Luddite Elitist
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