Results 1 to 9 of 9

Thread: focus shift with infrared film

  1. #1

    focus shift with infrared film

    To all,

    I was wondering if those of you who are comfortable with shooting large format infrared film could comment on the need to incorporate a focus shift when exposing infrared film. The answers from previous posts have thoroughly confused me as the following have been recommened: 1) no need to use focus shift at all when using f16 to f32 because depfth of field compensates, 2) definitely use 1/100th of the lens' focal length and add that to the bellows extension, 3) no, use 0.0025% of the lens' focal length and add that to the bellows extension, 4) only need to use focus shift if shooting with a ratio of 1:10(2mm) to 1:1(4mm), and 5) no need to use focus shift when shooting with apochromatic lenses only, irrespective of the f-stop or focal length. Any clarification would certainly be appreciated.

    thank you,

    dan dumitru

  2. #2

    Join Date
    Sep 2003
    Location
    Kalamazoo
    Posts
    648

    focus shift with infrared film

    Dan,

    Ansel Adams recommends advancing the focus 1/70th of the focal length. (See page 53-54 of "The Camera". The amount works out to be 3mm with a 210mm lens, etc.)

    There shouldn't be any additional lens extension needed as you are simply compensating for the focal plane and wavelength and not trying to change the magnification. Just move boths standards forward equally.

  3. #3

    focus shift with infrared film



    The practical answer from experience in using IR in LF photography is that an adjustment of the focus seems to be unnecessary. That is probably the best answer that you are going to get. I don't know of any statements from LF lens manufacturers about the IR properties of their current lenses. With the variety of lens designs and glasses in use today, not all lenses will have the same difference between best visual and IR focus positions. The optimum compenstation won't be the same for every lens -- old rules of thumb about the compensation factor are probably no longer valid for every lens. Unless a manufacturer will answer your question, or someone is able to get the design specs of a lens that you use (perhaps patent data) and plug it into an optics program, there is no way to know the optimum focus shift.





    Rodenstock publishes curves of Longitudinal Color Aberration for their lenses, but only for visual wavelengths. With guessed extrapolations, these curves suggest that the focus shift between visual and IR should be very small: see http://largeformatphotography.info/l...ic/498338.html.
    This fits with my experience that the focus comes out as expected in my LF IR photos. I don't try to make any focus adjustment.


  4. #4

    Join Date
    Sep 2003
    Location
    Harbor City, California
    Posts
    1,748

    focus shift with infrared film

    Some of the information you have been given may have originated with Kodak. A just pre-WWII Kodak pamphlet says "As a general rule, better infrared pictures are obtained if the lens is extended by about 1/4% of its focal length after it has been focused for visible light To obtain additional sharpness, the diaphragm should be closed down as much as possible."



    .The problem is that the amount of compensation can be expected to vary from one lens to another. It is probabl]y true that an apochromatic lens could be expected to require less, though not necessarily no, compensation. To complicate matters,. there are apochromatic lenses out there that don't carry "Apo" in their names.



    I'm afraid testing of the particular lens or lenses to be used will turn out to be the only way to be sure unless the maker can provide data.

  5. #5

    focus shift with infrared film



    "Just move boths standards forward equally." -- For a subject at infinity, this is no compenstation -- moving both standards the same distance forward is moving your entire camera forward. For an object at infinity, the distance to the object isn't changed. The distance focused on isn't changed since both standards were moved the same amount -- if infinity before, infinity afterwards.





    If a lens has significant longitudinal chromatic abberation, it's position of best focus will vary with wavelength. If you focused in one color and took the photo in a different color, you would want to adjust the distance between the lens and film.


  6. #6
    Moderator Ralph Barker's Avatar
    Join Date
    Sep 1998
    Location
    Rio Rancho, NM
    Posts
    4,870

    focus shift with infrared film

    The IR focus issue is further complicated by the film and IR filter being used. That is to say, how much visible-spectrum light is contributing to the overall exposure. If the contribution of visible-spectrum light is significant (e.g. with a Wratten #25 or 29), you might lean toward that end of the focus spread.

  7. #7

    focus shift with infrared film

    I shot some Maco infrared with a #29 filter and some of their "aura" ir film- I asked them about focus shift. They said not to worry, the difference was quite small.

    In landscapes, I could not see any difference, although my work was NOT exhaustive

  8. #8
    おせわに なります! Andrew O'Neill's Avatar
    Join Date
    Sep 2003
    Location
    Coquitlam, BC, Canada, eh!
    Posts
    4,613

    focus shift with infrared film

    I used to stress over the focus shift issue...I found that by always focusing through whichever filter I was using (usually #25) my negatives were fine. Man, do I miss 4x5 Kodak IR...

  9. #9

    Join Date
    Sep 2003
    Location
    Kalamazoo
    Posts
    648

    focus shift with infrared film

    "If a lens has significant longitudinal chromatic abberation, it's position of best focus will vary with wavelength. If you focused in one color and took the photo in a different color, you would want to adjust the distance between the lens and film."

    Michael,

    Now that I think about it, you must be correct. Moving both standards forward would not bring the infrared image into better focus. It would just shift from something out-of-focus to some other subject plane being out-of-focus. You'd have to extend the lens away from the film more to bring those longer wavelengths to the focal plane. Sorry that I misspoke and for any confusion it may have caused.

Similar Threads

  1. Focus Shift For Infra-Red
    By Baxter Bradford in forum Style & Technique
    Replies: 26
    Last Post: 21-Nov-2019, 13:36
  2. How do you adjust focus for infrared film?
    By Yaakov Asher Sinclair in forum Darkroom: Film, Processing & Printing
    Replies: 4
    Last Post: 22-Apr-2004, 11:01
  3. focus shift
    By e in forum Lenses & Lens Accessories
    Replies: 3
    Last Post: 11-Oct-2001, 02:20
  4. Focus shift with infrared film
    By Dave Anton in forum Darkroom: Film, Processing & Printing
    Replies: 3
    Last Post: 4-Jun-2000, 09:58
  5. focus shift???
    By doc in forum Lenses & Lens Accessories
    Replies: 2
    Last Post: 18-May-1999, 22:39

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •