DOF question

Discussion in 'General Technical Discussion' started by Beezle, Apr 3, 2007.

  1. Consider the two following images. Both were shot at f/13 or so. One an 85mm lens on a DX camera, the other a 110mm lens on 6x7 film.

    What would I need to do in order to achieve the same DOF on that film that I did from the 85? Why are these different? Do I need a longer or shorter lens?

    The theory seems simple enough, but the reality is confusing.

    roll18-01-web.

    _D217928-web.
     
  2. Longer lens on film. 85mm x 1.5 = 127.5mm.
     
  3. jfrancis

    jfrancis

    May 8, 2005
    Orlando, FL
    Interesting. I wwent to www.dofmaster.com and looked up the DOF at a shooting distance of 2 feet (not sure what your distance actually was of course):

    APS sensor 1.95-2.05 feet

    6x7 film 1.94-2.07 feet

    Not much difference . . . :confused:
     
  4. davidzvi

    davidzvi

    Apr 30, 2005
    Massachusetts
    David
    Much longer. I took a class with 4x5 view cameras and the instructor noted that while ~50mm is the normal view for a 35mm film camera, it's closer to 85mm for a 4x5.

    I'm not sure what it would be for 6x7 film, but I'm guessing you'd need at least 200mm.
     
  5. Well, note that to achieve similar views on the subject above, the 6x7 was a 110mm lens, but a lot closer to the subject. About half as far.

    110mm for 6x7 is roughly equivalent to 53mm in 35mm, which would be about 35mm (focal length) for a DX camera.

    So I guess the equivalent of an 85mm on DX would be a 127.5 on 35mm, which would be > 200mm on the 6x7?

    Confusing as heck this is. :biggrin: :confused: :confused: :confused:
     
  6. jaymc

    jaymc Guest

    Here's what I found:
    Focal Length Multiplier Calculator shows that the 6x7, 35mm and DX are 110mm, 56mm and 37mm respectfully.

    This site also has a nice Depth of Field Calculator.
     
  7. I'm way confused now....
     
  8. This is what leads to just buying stuff until it works!

    :biggrin:
     
  9. Beezle,

    The 85mm @ 135 format has a FOV of 28 degrees, whereas at DX about 18 degrees because of the crop factor. The 110mm @ 6x7 has an FOV of 39 degrees, I think.

    So, it looks like you placed the cameras at different distances to get to the same size picture, or you cropped in PP. Might it boil down to changing the apertures on each lens to get the same DOF?

    Now, I am not sure either....
     
  10. No crop, just different distances to the subject.

    I could have tried a much smaller aperture with the 6x7, but I have no idea when and if that becomes a problem like it does for digital cameras.
     
  11. Iliah

    Iliah

    Jan 29, 2005
    nowhere
    > Consider the two following images. Both were shot at f/13 or so. One an 85mm lens on a DX camera, the other a 110mm lens on 6x7 film. What would I need to do in order to achieve the same DOF on that film that I did from the 85? Why are these different? Do I need a longer or shorter lens?

    You need to increase shooting distance. And shoot closer to f/22
     
  12. The 'standard' lens for a 6x7 camera would be around 80mm (that would equate to a 50mm on a 35mm film camera). A comparable lens for the 6x7 would be in the range of 150mm or so.

    Or, as Iliah pointed out, change your shooting distance and f stop.

    Paul
     
  13. Iliah

    Iliah

    Jan 29, 2005
    nowhere
  14. But if the theory (and that article) are correct, if you keep the same framing focal length is irrelevant. Which is what I thought.

    Then I saw these two images and wondered why the 85 on DX (that does use a little tilt, but not much) and 110 on 6x7 would produce such different results with the same framing and aperture.

    And framing matters. I don't want to have to crop.

    PS, I don't see the point of that article actually. He is testing DOF with a close object and a tower some hundreds of yards away? That makes no sense. He should do the test using a long line of fence posts or something like that.
     
  15. Iliah

    Iliah

    Jan 29, 2005
    nowhere
    > But if the theory (and that article) are correct, if you keep the same framing

    They are correct, while folks who argue that DoF changes with focal length are incorrect.

    But the article does not speak of framing, it speaks of the size of the subject.

    BTW, there are three subjects in that test, and you can try something different, like http://www.waynesthisandthat.com/dof.html
     
Loading...
Similar Threads Forum Date
About the DOF Master accuracy...... General Technical Discussion Sep 12, 2015
DOF question General Technical Discussion Nov 27, 2010
DoF technical question. General Technical Discussion Jan 28, 2009
DOF & Backfocus question. General Technical Discussion May 25, 2005
Question about DOF preview button on D2H/X General Technical Discussion May 6, 2005