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Thread: What am I doing wrong?? (DOF question)

  1. #31
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    Re: What am I doing wrong?? (DOF question)



    Quote Originally Posted by Daniel Browning


    Once you've decided on a composition, there is no way to get deeper depth of field except by using a narrower iris diameter (i.e., stopping down). [There are some hard ways, such as using a tilt-shift lens or focus stacking.]


    If you move forward and zoom out (to get the same framing), the DOF will remain the same.


    If you move backward and zoom in (to get the same framing), the DOF will remain the same.


    So moving doesn't really affect the DOF. However, it does affect the perspective and background blur. Further away + longer focal length = flatter perspective and more background blur.


    So I chose to pull out my handy-dandy iPhone and bring up DoF Plus. It's set to the Canon 40D at the moment.


    Field of view, 35mm at 10m to subject, is 6.32m wide by 4.21m high.


    Field of view, 70mm at 20m to subject, is 6.32m wide by 4.21m high.


    Depth of field, 35mm at f/4 (8.75mm iris) at 10m to subject, is 6.17m to 26.19m (-3.83m and +16.19m).


    Depth of field, 70mm at f/4 (17.5mm iris) at 20m to subject, is 15.27m to 28.94m (-4.73m and +8.94m).


    Depth of field, 70mm at f/8 (8.75mm iris) at 20m to subject, is 12.35m to 52.38m. (-7.65m and +32.38m)


    So, if I keep the same aperture, I get different DoF. If I compensate for iris diameter, I get double the DoF. I get the same numbers using the link found elsewhere in this thread. Am I mis-interpreting something? I'm not seeing the DoF staying the same with unchanged iris diameter.
    We're a Canon/Profoto family: five cameras, sixteen lenses, fifteen Profoto lights, too many modifiers.

  2. #32
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    Re: What am I doing wrong?? (DOF question)



    I was wrong; thank you for the correction, peety3. I made at least two mistakes:

    1. I should have said f-number instead of iris diameter. ("Once you've decided on a field of view, it doesn't matter how close or far away you are: there is no way to get deeper depth of field except by using a narrower f-number.")

    2. Even that corrected version is inaccurate the closer you get to the hyperfocal distance. If you change your example from 10 meters to 1 meter, you will find that it is very accurate:


    Depth of field, 35mm at f/4 at 1m to subject, is 122mm


    Depth of field, 70mm at f/4 at 2m to subject, is 122mm


    My mistake was that I didn't point out this important weakness in the approximation. Even so, I think the approximation is still very useful because a lot of photography, including the OP, is not close to hyperfocal, but I will remember to point that out in the future. While I'm at it I might point out that extreme macro can throw off the approximation as well, thanks to bellows factor.

  3. #33

    Re: What am I doing wrong?? (DOF question)



    Madison,


    After wading through the fire swamp of posts on DOF I remembered a suggestions made by a gentleman name Scott Kelby in on of his books. Basically, go to someone in charge of the museum and see if you can arrange special access. I'm guessing if you talk to them for a few minutes and explain the situation you might be able to some help. If you can get in the museum either 30 minutes before public opening, or stay for 30 minutes after closing, they should have no problem with you setting up a tripod, and maybe even flash. You could offer to give them a copy of the finished picture as part of the negotiation.


    Now that my hero Daniel has been bested, I have to assume that this thread is mostly dead.


    Have fun storming the castle!


    David

  4. #34
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    Re: What am I doing wrong?? (DOF question)



    Quote Originally Posted by dmckinny


    Now that my hero Daniel has been bested, I have to assume that this thread is mostly dead.


    Whoo-hoo-hoo! Look who knows so much! It just so happens that your thread here is only MOSTLY dead. There's a big difference between
    mostly dead and all dead. Mostly dead is slightly alive. With all dead,
    well, with all dead there's usually only one thing you can do. Go through the posts and look for loose pixels.



    Quote Originally Posted by dmckinny


    Have fun storming the castle!


    Since the invention of the forum, there have only been five threads that were rated the most passionate, the most pure. This one left them all behind.

  5. #35
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    Re: What am I doing wrong?? (DOF question)



    Quote Originally Posted by Daniel Browning


    I was wrong; thank you for the correction, peety3. I made at least two mistakes:

    1. I should have said f-number instead of iris diameter. ("Once you've decided on a field of view, it doesn't matter how close or far away you are: there is no way to get deeper depth of field except by using a narrower f-number.")

    2. Even that corrected version is inaccurate the closer you get to the hyperfocal distance. If you change your example from 10 meters to 1 meter, you will find that it is very accurate:


    Depth of field, 35mm at f/4 at 1m to subject, is 122mm


    Depth of field, 70mm at f/4 at 2m to subject, is 122mm


    I still don't understand comment #1. I'll repeat the stats I found (with arbitrary numbers):


    Depth of field, 35mm at f/4 at 10m to subject, is 6.17m to 26.19m (-3.83m and +16.19m).


    Depth of field, 70mm at f/4 at 20m to subject, is 15.27m to 28.94m (-4.73m and +8.94m).


    Both of these have identical field of view according to DoF Plus. These do not have the same DoF.


    I think your guideline may only apply at compositions where the field of view is larger than the focal length, or perhaps where the DoF is within 2 x <focal length>, or something like that.


    Quote Originally Posted by Daniel Browning


    Another variable is print size. If you print it at 1.6X smaller size
    (7.5x11.25) the DOF goes back to being the same. In fact, it looks like
    you took a knife to the 5D2 print and just remove the center portion to
    make the 30D print:

    30D 50mm f/4 7.5x11.25 print DOF = 0.88m
    5D2 50mm f/4 12x18 print DOF = 0.88m


    So why isn't there a "print size knob" in ANY of the DoF calculators that I've found?
    We're a Canon/Profoto family: five cameras, sixteen lenses, fifteen Profoto lights, too many modifiers.

  6. #36
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    Re: What am I doing wrong?? (DOF question)



    Quote Originally Posted by peety3


    I still don't understand comment #1. I'll repeat the stats I found (with arbitrary numbers):


    Depth of field, 35mm at f/4 at 10m to subject, is 6.17m to 26.19m (-3.83m and +16.19m).


    Depth of field, 70mm at f/4 at 20m to subject, is 15.27m to 28.94m (-4.73m and +8.94m).


    It only applies to situations that are not hyperfocal, as mentioned in #2. Try with closer focus distances such as 1m.


    Quote Originally Posted by peety3
    So why isn't there a "print size knob" in ANY of the DoF calculators that I've found?

    Because most of them assume "same, but unspecified, print size". The best ones do have print size knobs:


    http://eosdoc.com/jlcalc/

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