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Thread: DoF comparison vs other formats

  1. #1

    DoF comparison vs other formats

    There is a project I've always wanted to do if I only I understood programing better to figure out to proper way of doing this.

    We've all seen DoF charts, I own a few myself and understand the concepts. Between formats (sizes) DoF changes accordingly. If my charts are correct it would appear the correlation is easily computable and equivalent between formats. Example, my charts say:

    35mm Film Standard 1.5, 50mm lens, subject @ 20ft = DoF of 5' 11" @ f/2
    4x5" format, 176mm lens (Horizontal Equiv.) @ 20ft = DoF of 5' 9.3" @ f/8+1/2

    My question is, is there anything else I'm not considering? To me it appears the DoF falloff is different in larger formats, and it is this characteristic that attracts me most. Is this just me or is there an explanation for this? The charts would leave me to believe DoF should be identical but to me it appears something is different.

    Any helpful feedback would be greatly appreciated. Thank you very much for your help!

  2. #2

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    Re: DoF comparison vs other formats

    There are already multiple iPhone apps an websites out there that do DOF calcs for all formats. DOF Master is one that I have.

    I find them quite useless since DOF calculations are based on an idea of "acceptable sharpness" that is usually assumed to be a value much blurrier than I can tolerate.

  3. #3
    Abuser of God's Sunlight
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    Re: DoF comparison vs other formats

    Whatever standard for acceptable blur you choose, the DOF relationship between format sizes will stay the same. So you can use the same calculations.

  4. #4

    Re: DoF comparison vs other formats

    Hey guys, thanks. Yes, I already have many different iPhone DoF calculators including the old charts before smart phones. My intention was not to recreate these charts, but to see if there are any anomalies not taken into account that I will notice when doing the calculations myself. I say this before despite all the evidence these charts suggest about DoF being the same between formats (when adjusting to the appropriate f/stop) I still can't help but feel the effects are different with larger formats.

    My goal is to find what changes with larger formats that make the DoF characteristic more pleasing to me. The sense I have is MAYBE the falloff is different, at least that's the way it appears sometimes. Does anyone know?

  5. #5
    Drew Wiley
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    Re: DoF comparison vs other formats

    As usual, I ignore the calculations are determine the effect I want simply with a loupe. After all, the final result is going to
    be determined by looking at a neg or print, and not a calculator screen. The bigger the groundlgass the better. During initial
    composition and focus I determine what the priorites of focus are going to be. I want something in the image to be dead on.
    Then I will work with tilts & swings to bring the rest of the image into the degree of sharpness I hope to achieve at the final
    working f/stop. Then I might view the effects of this halfway down. For instance, if my actual exposure is going to be at f45,
    I check critical areas thru the loupe at f/22 first. After awhile this all gets very instinctive. In the field, one rarely has the
    luxury of fully cooperative planes, like in tabletop photography. My own brother was all into hyperfocal forumlas etc, and then
    at first I used the Sinar line system for yaw-free corrections etc. Eventually it all seemed like a waste of time. And the last thing I need is another damn battery-dependent gadget that can go wrong in bad weather, or take up unecessary space in the pack. What is technically correct during focus might be esthetically irrelevant or even distracting. I want control over that
    variable too.

  6. #6
    8x10, 5x7, 4x5, et al Leigh's Avatar
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    Re: DoF comparison vs other formats

    There is absolutely no difference in DoF resulting from different formats.

    Think about it. How does a lens know what film is behind it?

    Take a photo on an 8x10 film. Note the DoF of various subject elements in a small area.

    Then cut a 24mm x 36mm rectangle from the film containing those subject elements.
    Look at them again. Has the DoF changed? Of course not.

    The differences in the calculated results are the consequence of using different fudge factors
    based on different "assumed" magnifications.

    - Leigh
    If you believe you can, or you believe you can't... you're right.

  7. #7
    Abuser of God's Sunlight
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    Re: DoF comparison vs other formats

    Quote Originally Posted by Drew Wiley View Post
    As usual, I ignore the calculations ...
    As usual you ignore the question.

    Marshallarts, there aren't any anomalies to consider. Defocus blur and diffraction both scale proportionally with magnification. If you have a sense of what apertures/focal lengths are working for you with one camera format, you can calculate the identical result with another format.

    If you're looking for equivalence that considers more than depth of field, there are other factors. This is somewhat confusing but thorough discussion on the topic.

  8. #8
    jp's Avatar
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    Re: DoF comparison vs other formats

    If you're using soft focus, DOF is different; usually greater, since it's a layering of sharp and out of focus images, then those out of focus layers are going to be in focus somewhere else.

    As Leigh says, in practical terms a 300 symmar-s 5.6 at f8 is going to have the essentially the same DOF as a Nikon 300/2.8 at f8. It's just going to be a narrower field of view on a small sensor.

  9. #9

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    Re: DoF comparison vs other formats

    The tables for depth of field are computed by plugging several values into a formula that has several input variables. You just need to look up the formula from reference books and do the substitution yourself with a scientific calculator. You will find that each published look-up table was obtained by making slightly different assumptions about the minimum acceptable circle of confusion. In addition to that inconsistency, the cut-off for the minimally acceptable circle of confusion is chosen differently between 35mm, medium format and large format. This is the reason why the equivalance you expected to find between 35mm and 4x5 is not confirmed exactly by the tables, but you have to agree that a DoF value of 5'9.3" is very close to 5'11" for a focus distance of 20 ft.

    Here's my take on the topic: In real life, the depth of field formulas (whether accepted on faith in the form of pre-computed look-up tables or diligently recalculated from scratch using your own personal choice for circle of confusion) don't matter as much as you'd expect. What counts is that you make a series of test exposures to find out how the images look for the varius possible choices of subject distance, subject patterns, focal length, aperture and film format. That's what you need to become familiar with. I'm sure that cinematographers test their lenses with those permutations by exposing film and looking at the images. I don't think that they sit in a room crunching numbers from an approximate heuristic formula and plotting graphs for various cut-offs. This is the epiphany I had about depth of field when I was first trying to understand it. You don't need to stress about the math. You just need to accumulate a few rules of thumb about the approximate depth of field you can expect to get for a couple of typical values of focal length, aperture and focusing distance. Yes, you need to understand depth of field, but there's no need to stress about the math.

  10. #10
    Abuser of God's Sunlight
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    Re: DoF comparison vs other formats

    Quote Originally Posted by jp498 View Post
    If you're using soft focus, DOF is different; usually greater, since it's a layering of sharp and out of focus images, then those out of focus layers are going to be in focus somewhere else.

    As Leigh says, in practical terms a 300 symmar-s 5.6 at f8 is going to have the essentially the same DOF as a Nikon 300/2.8 at f8. It's just going to be a narrower field of view on a small sensor.
    In practical use, you'd want to compare with a lens that gives the same field of view in different formats. A 90mm nikon lens at f1.7. would give the same depth of field at identical print sizes.

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