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Thread: Focus Shift For Infra-Red

  1. #21

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    Re: Focus Shift For Infra-Red

    I don't have any direct experience with UV photography. Pretty much every lens is going to have a focus shift in the UV and IR, and the sense of the focus shift will usually be shorter focal length in the UV, meaning when you focus the lens in the visible the value on the focusing scale will be "too close" in the UV. Stopping down helps because stopping down gives you greater depth-of-field / depth-of-focus to compensate for focus errors. I would try adjusting your focus to shorten the distance between lens and camera by a small amount, and stopping down more than f/11.

    The other thing is, clearly you should do a focus test with your UV setup and a static subject. If you try a yardstick at an oblique angle to the camera, you can measure the focus shift (like, lens focused in the visible at 1 meter on the center of the yardstick, but the part of the yardstick that comes out in focus was at 1.1 meters from the camera), and then you'll just know what the shift is for your particular lens.

    Split images darken when the lens is slower than f/5.6 or so, for geometrical reasons.

  2. #22
    Gary Beasley's Avatar
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    Re: Focus Shift For Infra-Red

    Quote Originally Posted by senderoaburrido View Post
    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?
    Thats backwards of the way it works. The ir mark sets the lens slightly farther away, so you should compensate an unmarked lens by focusing slightly closer (which moves the lens further away).

  3. #23

    Re: Focus Shift For Infra-Red

    Gary, I'm talking about shooting in the UV, which I presume is the opposite from the technique for shooting IR. Thanks reddesert, I had no idea about the split image. I can post some examples of the 35mm exposure I think are quite off:


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    Please ignore that the poses and subject matter are bizarre, these were mostly trials. At first I had thought that perhaps my scanning mount was out of focus, but even when corrected, the scans are still pretty blurry. These were all taken tripod-mounted, at 1/8 of a second or faster. I'm either over- or under-compensating, paired with perhaps a failure to I know that these two reels were off, because the reel before them produced results like this:

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    Nothing about my methodology changed significantly. I don't do anything to my scans in BW except make minor adjustments to contrast and brightness.
    Attached Thumbnails Attached Thumbnails UV-35mm-fomapan079.jpg  

  4. #24

    Re: Focus Shift For Infra-Red

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    Here's another example of the sort of fine focus I can achieve when stopping down on 35mm (and when all the unknown variables that I am trying to deduce go right).

    4x5 is even more difficult. I'd say I only get about 1/3-1/2 of those in focus, on average, when I'm shooting UV. Generally I fall short a bit, like here:

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    But I've also managed to nail it, like here:

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    I have no idea what is going on between these exposures, except that perhaps the models are moving ever so slightly in the interim where I install the filter and turn on the lamps. What makes this more frustrating is that generally these scenes are simple enough that there isn't enough material, either behind or in front of the subject, for me to discern whether I fell short or focused too far.

  5. #25
    Gary Beasley's Avatar
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    Re: Focus Shift For Infra-Red

    Have you tried laying a ruler out and shooting visible and uv without changing the focus? You might see the focus shift directly in the image and be able to figure out the problem.

  6. #26

    Re: Focus Shift For Infra-Red

    For 4x5 that would quickly become prohibitively expensive.

  7. #27

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    Re: Focus Shift For Infra-Red

    Quote Originally Posted by senderoaburrido View Post
    For 4x5 that would quickly become prohibitively expensive.

    You're burning film right now without knowing the focus shift.

    You need to set up a static subject - try a yardstick (meter stick if not US) laid out at say a 45 degree angle to the camera, with its center at your normal portrait distance, so that the left side is closer to the camera and right side is further away. Measure the distances at some fiducial points, for example say the 18 inch mark is 60 inches from the film plane, the 12 inch mark is 56 inches from the film plane, the 24 inch mark is 64 inches from the film plane. Focus in the visible on the center of the yardstick at the 18 inch mark. Expose the film in the UV at your normal working aperture. Examine the negative with a loupe to find the point that is in best focus. Now you know your focus shift for your combination of lens, filter, and film, rather than having to guess from unknown rules of thumb.

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