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Thread: Diffraction and DoF

  1. #1
    CJK's Avatar
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    Diffraction and DoF

    I just read the tutorial on lens "diffraction & photography" and now have a couple questions since Im forever confused.

    When shooting landscapes and wanting the max in dynamic range and depth of field, I've read to use a smaller aperture. After reading the tutorial, would it be better to use a middle range one, say I have F4-F32, so I would use around a F16 or so?

    I guess I just had the misinformed idea that a smaller aperture meant a greater DoF and sharpness.

  2. #2
    Moderator GrumpyDiver's Avatar
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    Re: Diffraction and DoF

    Everything in photography is about tradeoffs; higher ISO = more image noise, lower shutter speed = greater risk of motion blur / reduction in sharpness, wide open aperture = shallower DoF, but lower resolution.

    Diffraction and smaller aperture is just another one of those tradeoffs. You are indeed going to get greater DoF, but you will be paying for it with a slight loss of sharpness. It is really in the same order of magnitude as you will get shooting almost wide open to get shallow DoF, but the lens is not in the sharpest part of the range. It is up to you the photographer to decide which tradeoff to live with; more of the image in sharp focus versus a slightly less sharp overall image.

  3. #3
    CJK's Avatar
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    Re: Diffraction and DoF

    Thanks Grumpy, makes sense about the trade off like everything else in photography, guess I just got confused and caught up in all the technical writing of the tutorial and didn't think of it the same way as the other trade offs.

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    dje's Avatar
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    Re: Diffraction and DoF

    Quote Originally Posted by CJK View Post
    When shooting landscapes and wanting the max in dynamic range and depth of field, I've read to use a smaller aperture.
    Hi Chris

    The aperture won't directly affect dynamic range but ISO setting will. Generally speaking, the lower the ISO, the better the dynamic range.

    If you haven't already done so, have a look at the tutorial on Understanding Depth of Field, and in particular the DOF Calculator that is linked in that tutorial. If you are using a 1.5 or 1.6 crop factor camera and a fairly short focal length, you'll find that something like f/11 will you give you all the DOF you need for landscape work.

    Dave

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    kdoc856's Avatar
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    Re: Diffraction and DoF

    Hi, Chris

    Everything you and Manfred have pointed out regarding the trade-offs is absolutely true. From a purely practical perspective, the impact your aperture will have on your DoF will definitely be apparent, and is predictable and measurable. The impact of the diffraction effect on your IQ is much less definite and will depend on what you are shooting.

    Most often, a shot will be degraded more obviously by an inappropriate DoF, than by what little you give up in IQ by stopping down your aperture. So I'm encouraging you to go ahead and shoot with a narrow aperture if you need that DoF, because you may well not even notice the diffraction effect. I routinely shoot both ways, even F22 as well as at my sweet spot at F8 or F11. Then just compare the results- our "film" is really cheap

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    Re: Diffraction and DoF

    Chris,

    I second Kevin's suggestion: try it and see for yourself. Set your camera up on a tripod if you have one, set if to aperture priority, and take a series of pictures, closing down one stop between each one. It's best to use a scene that has lots of stuff at different distances. You will see the effect of DOF, as well as aspherical aberrations when the lens is wide open and diffusion when the aperture is very small.

    The effects of DOF on the image can be very large. In my experience, the effects of diffusion are really quite modest unless you close the lens down a great deal. Depending on the lens, the distortions from opening the aperture all the way can be larger. You will see lots of postings of people who complain that a lens is soft when opened all the way. However, nothing beats seeing it, which is why I suggest trying it out, as Kevin said.

    Dan

    PS: The image below was one I shot when I first started doing macro, with a good lens (EF-S 60mm macro) but a very cheap body (XTi). I shot it at f/20. The area I focused on would have been a tad sharper at, say, f/11, but the image as a whole would not have seemed sharp at that f-stop because of insufficient DOF. I don't think anyone looking at this would say, 'damn, that is soft--he must have closed the aperture down too far.'

    Diffraction and DoF

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    Re: Diffraction and DoF

    Hi Chris,

    In real-world terms, just forget about diffraction - it's really only visible if you pixel-peep an image at 100% ... you won't see it in a photo that's down-sized for internet display (because it gets sampled out with the other 95% of the information that's thrown away) - you won't see it in a small to medium sized print because it's too small for the eye to resolve - and you'd have to be really close to a BIG print (talking several feet across here) to be able to see it, and that's just not how people observe big prints (they stand back and look at them from a distance -- and of course then you can't resolve the diffraction).

    For landscapes, a narrow aperture will give you a deep DoF - but - past about F11 you pretty much get into "the law of diminishing returns" stuff - it's only an issue when you want distant and really close things to both be in focus. If the "near things" are more than a few feet away then it's not going to be an issue.

    I'd suggest reading up on hyperfocal distances - they'll tell you where to focus for optimal DoF at a given aperture.

    Here's a couple of examples:

    This first one was shot at F4 - the water was just a few feet away from the lens. The background is a LITTLE soft, but it's not too bad.

    Diffraction and DoF

    This 2nd shot was shot at F22 - where you'd think there's be a lot of diffraction - how does it look?

    Diffraction and DoF

  8. #8
    CJK's Avatar
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    Re: Diffraction and DoF

    Quote Originally Posted by dje View Post
    Hi Chris

    The aperture won't directly affect dynamic range but ISO setting will. Generally speaking, the lower the ISO, the better the dynamic range.

    If you haven't already done so, have a look at the tutorial on Understanding Depth of Field, and in particular the DOF Calculator that is linked in that tutorial. If you are using a 1.5 or 1.6 crop factor camera and a fairly short focal length, you'll find that something like f/11 will you give you all the DOF you need for landscape work.

    Dave
    Thanks for that tip Dave, I will keep that in mind. Nikon D5000 which I think has the 1.5 crop factor.

  9. #9
    CJK's Avatar
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    Re: Diffraction and DoF

    Thanks Kevin, I do love how cheap our film is.

  10. #10
    CJK's Avatar
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    Re: Diffraction and DoF

    Dan and Colin,

    Thanks for the tips and photos to view, you put my mind at rest, I'll just keep on shooting and experimenting and try not to worry about everything, specially things that aren't really noticed!

  11. #11

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    Re: Diffraction and DoF

    Quote Originally Posted by CJK View Post
    Dan and Colin,

    Thanks for the tips and photos to view, you put my mind at rest, I'll just keep on shooting and experimenting and try not to worry about everything, specially things that aren't really noticed!
    No worries Chris.

    In reality, a correct sharpening workflow (or the absence of one) has a far bigger impact on an image.

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    Re: Diffraction and DoF

    This is one of the most reasonable discussions of diffraction and dof I have read. Many times people say they don't shoot smaller than f 8 to avoid diffraction. Much better, in my opinion, to do as Kevin suggested, and shoot with your eyes open and compare the results of different apertures on a scene. I shoot a lot of selective focus--get close to the subject at max zoom. If I shoot wide open, I get a completely blurred background. Sometimes, I stop down to get more detail and pattern in the background. The closer I get, the more I have to stop down to get the desired effect. It is the effect I am after. The result takes priority over worries about diffraction. On the other hand, if I am not after a specific effect (in this case requiring a small aperture), I will shoot using that notorious sweet spot. I think Bryan Peterson laid it out very well in Understanding Exposure.

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    Re: Diffraction and DoF

    Quote Originally Posted by Brev00 View Post
    This is one of the most reasonable discussions of diffraction and dof I have read. Many times people say they don't shoot smaller than f 8 to avoid diffraction. Much better, in my opinion, to do as Kevin suggested, and shoot with your eyes open and compare the results of different apertures on a scene. I shoot a lot of selective focus--get close to the subject at max zoom. If I shoot wide open, I get a completely blurred background. Sometimes, I stop down to get more detail and pattern in the background. The closer I get, the more I have to stop down to get the desired effect. It is the effect I am after. The result takes priority over worries about diffraction. On the other hand, if I am not after a specific effect (in this case requiring a small aperture), I will shoot using that notorious sweet spot. I think Bryan Peterson laid it out very well in Understanding Exposure.
    Hi Larry,

    I think Jay Maisel said it best when he said "many people ruin their photos trying to save the pixels".

  14. #14
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    Re: Diffraction and DoF

    I had a similar problem with a series of images I took in one particular garden. I have put a white hibiscus in full bloom, a side view of the same bloom 8 days earlier, and a yellow day lily in full bloom.

    The DoF is particularly evident in the full hibiscus on the petal pointing down and to the right (approximate 18 minutes past the hour) which is off the back of the defined depth of field. The side view of the hibiscus is completely within my defined DoF. Pictures are below (having trouble with putting three images into the thread note; will try the other two in a little bit).

    virginia

    Hibiscus, full bloom

    Diffraction and DoF

    Hibiscus, bud

    Diffraction and DoF

    Day lily, full bloom, note the small shadow on the petal pointing towards the ground and the larger shadow on the same petal, both of which are still within my DoF and in focus.

    Diffraction and DoF
    Last edited by drjuice; 28th April 2013 at 03:08 AM. Reason: Add two images

  15. #15
    Glenn NK's Avatar
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    Re: Diffraction and DoF

    Roger Cicala of Lens Rentals wrote an interesting blurb on diffraction:

    http://www.lensrentals.com/blog/2013...bia#more-12845

    Glenn

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