Depth of field question

Discussion in '35mm Cameras and Accessories' started by michael_r, May 17, 2010.

  1. michael_r

    michael_r Subscriber

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    Hi. For a given film format (35mm), and a given plane of focus at a given aperture, will all lenses of a given focal length have the same depth a field? In other words, is depth of field at a given aperture and plane of focus determined by the focal length, or is it variable depending on the specific optical formula/design of the lens?

    The reason I'm asking, I have two 85mm Nikkor lenses. One of them, a standard AF Nikkor has a decent depth of field scale, the other, an 85mm tilt/shift Nikkor has a small and pretty useless depth of field scale. For a particular photograph I need the shift functionality. So I'm wondering, if I use the AF Nikkor to figure out the depth of field for a plane of focus of say 15ft at f:11, can I assume the other 85mm lens will have the same depth of field if also focused at 15ft at f:11? Or will the two lenses have totally different depth of field characteristics due to their different optical designs?

    Thanks
    Michael
     
  2. abuelo

    abuelo Member

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    I think at a same aperture both will have the same deph of field

    abuelo
     
  3. Lee L

    Lee L Member

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    The DoF scale is likely smaller on the tilt/shift lens because of a smaller throw in the focus adjustment. (i.e. the focus ring turns less for the same focus change compared to the AF lens you have.)

    DoF should be the same between the two 85mm lenses until you apply some tilt to one lens, at which point the Scheimpflug principal takes over when the central lens axis is no longer perpendicular to the film.

    http://en.wikipedia.org/wiki/Scheimpflug_principle

    Lee
     
  4. Christopher Walrath

    Christopher Walrath Member

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    The Scheimpflug rule states that the film plane, the subject plane (plane of sharp focus), and lens plane (the plane through the optical center and perpendicular to the lens axis) must converge along a single line. If the lens and/or the film plane is changed then the depth of focus will also change in regards to the degree of change. In tilting your change your plane of focus. The focus on farther objects due to tilt or other movements will have greater ODF while you will notice shallower DOF on objects that are in the closer portion of your focal plane. Using the DOF scale on the fixed lens will not help you except for determining the DOF in the portion of your planar area that is unaffected by movements.
     
  5. dnk512

    dnk512 Member

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  6. Christopher Walrath

    Christopher Walrath Member

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    There needs to be an addition here. By tilting you are effectively changing the focal length of the lens to certain portions of the subject area. And therein lies the difference between DOF locally in the subject area. I think I got that right.
     
  7. RalphLambrecht

    RalphLambrecht Subscriber

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    Yes!
     
  8. Alex1994

    Alex1994 Member

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  9. Q.G.

    Q.G. Inactive

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    You're not changing the focal length really.
    What you do is tilt the lens and film planes so that the conjugate distances in (ideally) all parts of the image are what the simple lens maker's formula says they should be to achieve focus for a given, fixed focal length.

    DoF doesn't really change, except as you described: less for the close-up parts, more for the far away parts of the subject.
    It still depends on f-stop and magnification only, just as it does with a fixed, untilted lens.
     
  10. Q.G.

    Q.G. Inactive

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    This calculator (like many) derives DoF from hyperfocal distance. That will get you close, but it's not quite correct.
     
  11. michael_r

    michael_r Subscriber

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    Thanks for the responses everyone. This is a head-on architectural shot of a building facade. So I won't be using the tilt function, just shifting to get the equivalent of front rise. So I guess the answer is I can assume the same depth of field charactersitics. Excellent news. It also makes more sense to me now why the depth of field scale is narrower on the PC lens - shorter throw. I should have thought of that before. Seems logical enough.

    Michael
     
  12. RalphLambrecht

    RalphLambrecht Subscriber

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    Michael

    One thing to keep in mind is that many DoF scales are rather optimistic (using a very liberal CoC). If you want your image to be more critically sharp, use an f/stop or two less on the scale than what your actual aperture is.
     
  13. Mainecoonmaniac

    Mainecoonmaniac Subscriber

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    The DOF will be the same since they're the same focal length. There is a cool Iphone app called Simple DOF Calculator.
    http://www.montanamedia.nl/apps/simple-dof

    I also like using the depth of field preview on cameras also.
     
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  15. RalphLambrecht

    RalphLambrecht Subscriber

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    That never worked for me.
     
  16. michael_r

    michael_r Subscriber

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    Me either. I can't see anything well once stopped down.

    Regarding optimistic DoF scales, I never really rely on them. They are sometimes useful for a rough idea and then I always stop down further for better sharpness, although I try not to go further than necessary to avoid too much diffraction.

    Thanks
     
  17. Sirius Glass

    Sirius Glass Subscriber

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    For a particular exposure [read; f/stop fixed] and the same image size on the film for close subjects, the depth of field is the same regardless of the focal length of the lens!

    I have posted the detail here of this before.

    Steve
     
  18. 2F/2F

    2F/2F Member

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    The short answer is YES.

    If magnification is the same (magnification being controlled by focal length and distance from the subject), and aperture is the same, depth of field will be the same. That is, if the lenses are focused at the same distance, and prints of the same size and quality are made and viewed at the same distance, the part of the picture that is "acceptably sharp" in front of and behind the plane of critical focus will be the same.

    Tilting the lens, however, will change the orientation of the plane of critical focus, to which the lowly "acceptably sharp" areas are slaves. Therefore, when tilting, you do not change the "depth" of field of the image measured either way from the plane of critical focus, but you do change which parts of the composition fall within by the "acceptably sharp" field.

    So, there are some semantics involved, the definition of D of F being subject to numerous debates. The way I read it, by tilting, you don't actually change the depth of the field of acceptably sharp focus in the image, but you do apparently change it in the print, by way of reorienting it. If D of F is simply defined as "the depth of the acceptably sharp area in a print," then you do change it. If D of F is defined as "the depth of the acceptably sharp area surrounding the plane of critical focus in the image," then you do not.

    However, while the short answer about transferring the D of F from one lens to the other is YES, the more important answer is that you do not want to rely on the D of F scales on lenses to tell you what your D of F will be.
     
    Last edited by a moderator: May 17, 2010
  19. Lee L

    Lee L Member

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    Others have refuted this often repeated claim, and shown their work.

    http://theonlinephotographer.typepa...er/2009/06/depth-of-field-hellthe-sequel.html

    Lee
     
  20. Q.G.

    Q.G. Inactive

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    That's not a refutation, Lee. Just a demonstration that it is extremely easy to confuse the amount of blur outside the DoF range with the size of the DoF range.

    What changes with focal length is the degree to which blur increases with distance.
    The actual range that falls within DoF however always is the same, given the conditions mentioned by Steve (though he should drop the "for close subjects" bit. It always is. The "for close subjects" thing stems from the confusion of hyperfocal distance with DoF.)
     
  21. Sirius Glass

    Sirius Glass Subscriber

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    Lee will not give it up. Physics and optics be damned according to him!

    From:http://en.wikipedia.org/wiki/Depth_of_field


    This requires repeating since we have the nay-saying-chorus:
    For close-up work, the hyperfocal distance has little applicability, and it usually is more convenient to express DOF in terms of image magnification. Let m be the magnification; when the subject distance is small in comparison with the hyperfocal distance,
    [​IMG] so that for a given magnification, DOF is independent of focal length. Stated otherwise, for the same subject magnification, all focal lengths give approximately the same DOF. This statement is true only when the subject distance is small in comparison with the hyperfocal distance, however.

    I properly stated a summary of this when I posted my statement before.

    Just because Mike Johnston [http://theonlinephotographer.typepa...er/2009/06/depth-of-field-hellthe-sequel.htm] has a pretty website, does not make him right. For example: See "This guy thinks film is dead" APUG thread at http://www.apug.org/forums/forum214/76293-guy-thinks-film-dead.html

    See what 2F2F said:

    Steve
     
    Last edited by a moderator: May 18, 2010
  22. Lee L

    Lee L Member

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    The article I referred to is by Ctein, not Mike Johnston. An opinion that Johnston's blog is "pretty" doesn't make Ctein wrong.

    I posted this: http://www.apug.org/forums/forum206/63724-more-analog-dof.html#post824775 to a similar thread earlier, which was a way of checking the stated 'rule' against the depth of field spreadsheet provided by Schneider Optics. It supports Ctein's observations.

    There are apparently a couple of folks posting to this thread who are on my ignore list, so I should bow out and let you guys carry on. No need for me to create a lot of confusion and cross-talk. People who are concerned can research things for themselves.

    Lee
     
  23. Lee L

    Lee L Member

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    OK, here's one more explanation, cited in the wikipedia article sirius glass cites:

    Here's the full article with charts and graphs:
    http://www.dofmaster.com/dof_imagesize.html

    Lee
     
    Last edited by a moderator: May 20, 2010
  24. Q.G.

    Q.G. Inactive

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    Once again, DoF calculations are derived from considerations concerning hyperfocal distance.
    Then yes, you get the impression that there is a double dependency on focal length.
    But it's plain wrong to confuse hyperfocal distance with DoF.

    What depends on focal length (besides image scale) is the degree of increase of blur. The DoF range however is the same at same image scale, no matter what lens.
     
  25. Sirius Glass

    Sirius Glass Subscriber

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    "It is not exactly true ..." because they used the wrong basis for their approximation. When magnification [same size image] is used then it is true, because that is the algebraic substitution that was made. By changing it to one quarter of the focal length the derivation does not apply. That would be like saying that your car got X mpg [or Y km/l], and then using that information to determine whether or not one should have orange juice for breakfast. The efficiency of the vehicle will have no bearing of the choice of beverage - of course it would not be exactly true.

    Steve
     
  26. Lee L

    Lee L Member

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    The cited web page does use the same image size [magnification of an object on the film] for both the short and long focal length lenses. The change is not to make a given object in the image one quarter of each focal length, the statement is that the 'rule' you've stated only holds approximately when the shorter lens is focused closer that 1/4 of the hyperfocal distance for that lens at a given aperture. At longer subject distances the 'rule' you've stated doesn't even hold as an approximation. Both long and short focal lengths are used at a distance from the plane of best focus in the image that sets that plane at the same magnification.

    Lee