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Thread: DOF and diffraction

  1. #1

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    DOF and diffraction

    To follow up on the post I made about how rear tilt affects DOF and plane of focus, there was something else I noticed on my camping trip.

    Some of the compositions I had set up, when I measured the rail distance between near and far to calculate the required f-stop (I tried both x5 and using the Rody calculator), ended up with f45 or even higher in some cases.

    Having read about diffraction, I was a bit concerned this might be too small an aperture, on the other hand I really wanted everything in the shot sharp. I have yet to develop the shots, but will be looking for signs of diffraction when I do.

    Since it's an f-stop issue for DOF, and not simply a "too much light" issue (where I could use a ND filter or similar to bring it down), how are some of the ways people deal with this? Take the shot and accept any softness? Re-compose? What else could I do in these situations?

  2. #2

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    DOF and diffraction

    Diffraction is better than out of focus when striving for sharpness. The answer as to whether it will be acceptable depends on how much magnification there will be in the final print and what is acceptable. Iíve shot plenty of f64 in 4x5 and the diffraction didnít bother me.

    As far as I know there is no other way around it except recomposing in a way that you can increase the aperture without sacrificing sharpness. Sometimes that is not practical or possible.

    The ultimate resort would be to stack focus using multiple sheets and then combine digitally. Often times 2 shots is more than enough, one focused on the near and one on the far, with enough DOF on them to make sure the middle is adequately covered.

  3. #3
    Maris Rusis's Avatar
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    Re: DOF and diffraction

    For an exhaustive analysis of Depth of Field versus Diffraction go to the LF Home Page and tackle Depth of Field in Depth by Jeff Conrad. The mathematical derivations can be largely skipped if simple working solutions are enough.
    Photography:first utterance. Sir John Herschel, 14 March 1839 at the Royal Society. "...Photography or the application of the Chemical rays of light to the purpose of pictorial representation,..".

  4. #4

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    Re: DOF and diffraction

    IM-HO, depth of field trumps diffraction degradation every time. If I need to stop down to f/45, like you did, then I just plan on not printing large. I've found that I can get a more-than-acceptable 11x14-inch print from a 4x5 negative (not too much cropping...) made at f/45. If I plan on only printing smaller, I can get away with an even smaller aperture (most of my close-up work and still lifes end up being printed only 8x10 or maybe slightly larger).

    As far as work-arounds in the field: One tried-and-true method is to use a shorter focal length lens for more depth of field along with changing camera position to minimize cropping. You can keep the same camera position and just crop, but, although you do get more depth of field, you end up with more grain due to the crop and the resulting need to enlarge more.

    Of course, using movements to optimize the plane of sharp focus and depth of field often helps, but finding the right configuration can be tricky with subjects that aren't so straightforward. Often, I'll apply a bit of tilt or swing and do the near far focus and note the spread to see if it's better than the "zero" position spread. If so, then I'll try to find the exact right amount to optimize things. Sometimes I find that a bit of movement, that I otherwise would not have anticipated, gives me a smaller focus spread.

    There are some subjects where movements of any kind simply won't help and you just have to stop down. There are also some shots that simply aren't practical, however, and have to be abandoned.

    Best,

    Doremus

  5. #5

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    Re: DOF and diffraction

    Fight this reality and way of Nature or work with it, there are no other options.

    It is much about trade-offs, not all images made are best with everything in focus, not all images made are best with selective focus. To be dogmatic about one or the other tends to enforce unrealistic limitations as there is validity in expressive image making with:

    ~Selective focus coupled with out of focus qualities of any given lens.

    ~Every area of the image in apparent focus.. there is only one specific plane of true focus for any lens except when focused to true infinity.
    This is one of the reasons why doing "sharpness" test is often not as relevant to real world photographic images than most would want to believe.

    ~Combination of focused areas and selective focus in the same image.


    There are the options, all are valid depending on what the image goals are.... fully understanding what the enforced trade-offs are.

    Bernice

  6. #6

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    Re: DOF and diffraction

    Yes, physics is physics but doesn't mean one can't compartmentalize things to get away with more :-)
    Take this image for example. Even at f64 I couldn't get both the foreground and the background in focus, I would have had to go to f128 theoretically (my shutter wouldn't support it and obviously it would almost look like a pinhole image it terms of diffraction). Nothing much you can do with movements either. A bit of tilt improves things but to get fore and back in focus the tree would have had portions out of focus. Also I did not want to sacrifice the composition, say by foregoing the foreground as in my mind it would have created an imbalanced image.


    My solution, because I wanted to have everything in focus (but others may be fine having some parts out of focus in cases like this), was to shoot 2 more slides, one with the fore in focus, the second one focusing on the background. It's then trivial to overlay them in photoshop to achieve full sharpness.


    Click image for larger version. 

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  7. #7
    Land-Scapegrace Heroique's Avatar
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    Re: DOF and diffraction

    Quote Originally Posted by Kiwi7475 View Post
    ...My solution, because I wanted to have everything in focus (but others may be fine having some parts out of focus in cases like this), was to shoot 2 more slides, one with the fore in focus, the second one focusing on the background. It's then trivial to overlay them in photoshop to achieve full sharpness.
    A beautiful image and interesting solution. As Doremus suggests, did you consider a wider lens and moving closer? I'm also curious if front-forward tilt, moving the focus plane through the middle of the tree and rocks, might work with a reasonable aperture.

  8. #8

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    DOF and diffraction

    Quote Originally Posted by Heroique View Post
    A beautiful image and interesting solution. As Doremus suggests, did you consider a wider lens and moving closer? I'm also curious if front-forward tilt, moving the focus plane through the middle of the tree and rocks, might work with a reasonable aperture.
    Although it doesnít show in the picture the foreground is actually a big rock and thereís a gap and a drop of 10 ft quite near to (before) the photographed edge. In other words, I was as close as I could get without levitation capabilities :-)

    I tried tilting and it did improve some but couldnít get everything in focus in the tree and the foreground at the same time. In addition the light was about to change quickly so I couldnít fiddle with the tilt for more than a minute or two, and so then I decided to shot the other additional slides.

    In the end, thereís so many constraints in the field, we just need to use the tools that will get the shot you envision; in my opinion is about getting the shot, not how to get it, although it is of course interesting for us photographers to share/discuss the process and tribulations :-)

  9. #9

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    Re: DOF and diffraction

    Quote Originally Posted by Kiwi7475 View Post
    Although it doesn’t show in the picture the foreground is actually a big rock and there’s a gap and a drop of 10 ft quite near to (before) the photographed edge. In other words, I was as close as I could get without levitation capabilities :-)

    I tried tilting and it did improve some but couldn’t get everything in focus in the tree and the foreground at the same time. In addition the light was about to change quickly so I couldn’t fiddle with the tilt for more than a minute or two, and so then I decided to shot the other additional slides.

    In the end, there’s so many constraints in the field, we just need to use the tools that will get the shot you envision; in my opinion is about getting the shot, not how to get it, although it is of course interesting for us photographers to share/discuss the process and tribulations :-)
    What did you focus on?

  10. #10
    Land-Scapegrace Heroique's Avatar
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    Re: DOF and diffraction

    Quote Originally Posted by Kiwi7475 View Post
    …The light was about to change quickly … In the end, it's about getting the shot, not how to get it.
    Yes, when the desired light is about to change or disappear, this is an approach that has allowed me to take home a satisfying (if not perfect) shot, rather than none at all.

    Other times, the "Chance favors the prepared" proverb has led me to the "how," just in the nick of time, in the face of fleeting time.

    Quote Originally Posted by Bob Salomon View Post
    What did you focus on?
    I'm going to go out on a limb and say it was a limb.

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