Switch to English Language Passer en langue française Omschakelen naar Nederlandse Taal Wechseln Sie zu deutschen Sprache Passa alla lingua italiana
Members: 70,287   Posts: 1,535,294   Online: 915
      
Page 1 of 4 1234 LastLast
Results 1 to 10 of 40
  1. #1

    Join Date
    Jan 2008
    Shooter
    35mm
    Posts
    298

    Does paper thickness change the critical focus?

    I'm just curious what you think in terms of the difference in the circle of confusion and critical focus. If I'm printing my negative and don't use a piece of paper under the grain focuser, would by focus be significantly off? Would it matter at all, considering the negative is not flat, but bent just slightly anyways?

    What is your opinion?

  2. #2
    BetterSense's Avatar
    Join Date
    Aug 2008
    Location
    North Carolina
    Shooter
    35mm
    Posts
    2,862
    How are you focusing in the first place? If you use a grain focuser, why don't you test it and see if it is within error margins? I personally test on a sheet of paper, but considering that I test at f/3.5 and expose at f/16, I doubt the thickness of the paper is significant. Plus, the paper often "bubbles" up at least a little bit so it's really probably several thicknesses higher than the grain focuser. Still, one less variable.

  3. #3

    Join Date
    Dec 2008
    Location
    Trinity, Alabama
    Shooter
    35mm
    Posts
    243
    I focus on a sheet of paper from the pack I'm printing on. I usually forget to change the aperture from 2.8 to whatever aperture I'm printing at at least once in the printing session, so I usually have some spare sheets lying around. I usually print at F/8 or F/11, so I dont think the thickness of the paper is going to make a noticable difference.

    The fact that I focus on a sheet of paper has nothing to do (for me at least) with the final print being out of focus because of the thickness of the paper. I've been doing that since my first photography class because my instructor said it might make a difference. It's just habbit for me. Better safe than sorry, I guess

  4. #4
    BetterSense's Avatar
    Join Date
    Aug 2008
    Location
    North Carolina
    Shooter
    35mm
    Posts
    2,862
    I usually forget to change the aperture from 2.8 to whatever aperture I'm printing at at least once in the printing session, so I usually have some spare sheets lying around.
    So I'm not the only one!

  5. #5
    Ken N's Avatar
    Join Date
    Oct 2004
    Location
    Creston and Newton, Iowa.
    Shooter
    Multi Format
    Posts
    389
    Images
    8
    I always put my grain focuser on a sample piece of paper to space it upwards a bit. But then again, I always place my paper face down in the developer, wear clean socks in the darkroom and have soft jazz playing on the stereo. Whether any of these things make a hill of beans worth of difference, I doubt very much as long as you aren't exposing with the lens set wide open.
    http://www.zone-10.com

    When you turn your camera on, does it return the favor?

  6. #6
    ic-racer's Avatar
    Join Date
    Feb 2007
    Location
    Midwest USA
    Shooter
    Multi Format
    Posts
    6,179
    Quote Originally Posted by Andrey View Post
    I'm just curious what you think in terms of the difference in the circle of confusion and critical focus. If I'm printing my negative and don't use a piece of paper under the grain focuser, would by focus be significantly off? Would it matter at all, considering the negative is not flat, but bent just slightly anyways?

    What is your opinion?
    I don't have an opinion on the matter, just the optical formula that predicts about a 2mm focus spread at f2.8 at 9x enlargement. This easily encompasses any photographic paper of which I am aware.

    Modular transfer function focusing equation (equation #38 in http://www.largeformatphotography.in...DoFinDepth.pdf) :
    N_max ~ 20 / (1 + m) sqrt(dv)

    N-max = F number
    m = magnification
    dv = focusing leeway on the baseboard
    20 = constant for circle of confusion about 0.15mm on the print

  7. #7
    raucousimages's Avatar
    Join Date
    May 2003
    Location
    Salt Lake
    Shooter
    Large Format
    Posts
    825
    I did a test once where I focused on the baseboard and then made two prints. One on single weight FB and one on premium weight FB. The premium weight was noticeably thicker than the single weight but both prints were equally sharp to my eye. That said I still focus with with magnifier on a piece of paper I am using at that time. Old habit but it reduces any variables in my printing process. In theory at least a shorter lens high above the baseboard will have a greater DOF then a longer lens close to the baseboard. So a 35mm neg with a 50mm lens making a large image will have less problem with paper thickness than a 4X5 neg with a 150mm lens making a small image. But even at that I doubt it makes a noticeable difference. Maybe I will do a test with a 150mm and a small image just to see what happens.
    DIGITAL IS FOR THOSE AFRAID OF THE DARK.

  8. #8
    MattKing's Avatar
    Join Date
    Apr 2005
    Location
    Delta, British Columbia, Canada
    Shooter
    Multi Format
    Posts
    12,276
    Images
    60
    I have easels that are yellow in colour. It's easier to evaluate the image using the (white) back of a discarded print, so I may as well focus using it too.

    Matt

  9. #9

    Join Date
    Dec 2008
    Location
    Trinity, Alabama
    Shooter
    35mm
    Posts
    243
    Quote Originally Posted by BetterSense View Post
    So I'm not the only one!

    good to know i'm not the only one


    I have noticed that i'm wasting less and less paper that way, so maybe one day I'll end up having to sacrifice a sheet instead of using a wasted one. Ok, that's not gonna happen. I doubt I'll ever get out of the habit of making careless mistakes :/

  10. #10

    Join Date
    Aug 2004
    Location
    Dunedin,New Zealand
    Shooter
    Multi Format
    Posts
    269
    I always focus at the aperture I will use to expose the print,on the assumption that any focus shift effect on stopping down will be negated. That said,none of my EL Nikkors show any shift.. Of course,if your focuser is pretty naff you won't have that option.

Page 1 of 4 1234 LastLast


 

APUG PARTNERS EQUALLY FUNDING OUR COMMUNITY:



Contact Us  |  Support Us!  |  Advertise  |  Site Terms  |  Archive  —   Search  |  Mobile Device Access  |  RSS  |  Facebook  |  Linkedin