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Thread: Focus Shift For Infra-Red

  1. #11

    Re: Focus Shift For Infra-Red

    Have a question about focus compensation: do I compensate towards or away from the lens? Do I want to focus to infinity, or closer inwards?

    And would anyone know how to calculate the same compensation for UV? I bought an old 135mm EL-Nikkor and B+W 403 filter six months ago and finally am ready to give it a try.

  2. #12
    Jac@stafford.net's Avatar
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    Re: Focus Shift For Infra-Red

    Quote Originally Posted by senderoaburrido View Post
    Have a question about focus compensation: do I compensate towards or away from the lens? Do I want to focus to infinity, or closer inwards?

    And would anyone know how to calculate the same compensation for UV? I bought an old 135mm EL-Nikkor and B+W 403 filter six months ago and finally am ready to give it a try.
    Closer for infrared, away for UV. The correction is more critical for close-up work. For distance scenes the depth-of-focus and smaller physical aperture for depth-of-field usually makes up the difference.

    May we ask what film and lighting you are using?

  3. #13

    Re: Focus Shift For Infra-Red

    EB/RA Carestream film is what I'm using for the UV stuff. Figured to use it in full winter daylight.

  4. #14

    Re: Focus Shift For Infra-Red

    I'm back, years later, struggling with focus compensation for UV. I'm supposed to be focusing as if the subject is slightly further away from the camera, right? I'm using an enlarger lens, which means that the lens should have less distance between it and the film than when the subject is in focus in solely visible light, correct?

  5. #15
    Cor's Avatar
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    Re: Focus Shift For Infra-Red

    I have never bothered about changing focus for IR film (Maco), I usually stop down to f16 f22, and I have not noticed a focus shift, the DOF takes care of it in my case, I mostly shoot landscape. I guess the same will hold true for UV, the detectable wave lengths will be close to visible I assume.

    good luck,

    Cor

  6. #16
    (Shrek)
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    Re: Focus Shift For Infra-Red

    I have also never compensated, using the opaque IR filters and Konica imagesetter film. Never been an issue with normal landscapes, I imagine critical focusing on a portrait with a wide-open f3.6 Petzval might require a minute correction. When I initially tried, I checked my old Canon FD lenses of similar focal length to what I was using in 4x5, because they had a red dot on the focus scale to help adjust. So I figured that if the 200mm lens required focusing at 15ft to get 18ft in focus, I could simply focus my lf camera on a measured object accordingly. It didn't work, my initial experiments were all failures, so I stopped down to f16 and just shot normally.

  7. #17
    Jim Jones's Avatar
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    Re: Focus Shift For Infra-Red

    Rudolf Kingslake recommended moving the lens closer to the subject by about .5% of the lens focal length in his 1950 Lenses in Photography. However, this is a generalization. The corrections for specific lenses published at that time by his employer, Eastman Kodak Company, sometimes very considerably from that general formula, even among lenses of one design but various focal lengths.

  8. #18

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    Re: Focus Shift For Infra-Red

    Every lens has a different shift for IR, some are corrected for ir not requiring a shift

    A solution is stopping more to extend DOF

  9. #19

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    Re: Focus Shift For Infra-Red

    Every 35mm and medium format lens I have looked at with an IR focusing mark requires the lens to move further away from the film for the IR, ie the lens has a longer focal length in the IR. Presumably the lenses would have a shorter focal length in the UV.

    This is sort of understandable because dispersion makes the index of refraction of glasses decrease at longer wavelengths, so at longer wavelengths a simple glass lens has less refractive power and a longer focal length. https://en.wikipedia.org/wiki/Dispersion_(optics) However, multiple element optics may not follow any simple formula. if you look at the IR focusing mark for a very complex lens like a zoom, it may have a much larger IR focus shift than a fixed focal length lens. Fixed focal lengths are more usable at the extremes of wavelength, and it's not possible to simply transfer the distance from one lens to another of different design, so educated guesses, stopping down, and experimentation are your friends. Some lenses are more suitable than others - it also depends on the glass types and coatings.

  10. #20

    Re: Focus Shift For Infra-Red

    So stopping down should work, because I need to compensate by focusing the image as if it was further away, since technically it is focused "too close" when focused in the visible range?

    I just had some really disappointing results with two reels of 35mm, and I'm trying to understand whether my models are moving, or whether I'm missing focus compensation. I always shoot stopped down to f/11. It's easily possible for the models to move, because I have to first focus in visible light, and then throw the visible range-blocking filter on the lens, after which I turn on the LEDs. There's ample time for them to shift, even as they try as hard as they can to stay still. However, I'm also wondering now if the split-image focusing screen is causing problems, because I've noticed that it doesn't work very well with the lenses I use for UV photography- one half of the split image is always a little dark, making it very hard to see whether the two halves are aligned, and the image surrounding the split image doesn't always seem to be in focus when the split image is.

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