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Thread: Diffraction?

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    Subscriber Member TRSmith's Avatar
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    Diffraction?

    I confess to being something of a techno-phobe. By that I mean it doesn't take long for detailed engineering and technical discussions to sponsor a brain glaze in me. Some of that is a remnant of my past experience when photography was my "job" and the nature of the work I did was extremely technical and had the end result of ruining the fun of photography for me.

    With all that as sort of a disclaimer, I do have a technical question. I understand the principle of diffraction but am not that clear about the point at which it becomes a factor on a lens by lens basis. Generally, I don't stop down much further than f11 and figure I'm safe. But is there more I should know? Is the effect of diffraction different depending on the lens design? The focal length? Is it even an issue that needs to be considered?

    Do you factor diffraction into your decision-making process when shooting?

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    Senior Member LCT's Avatar
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    Re: Diffraction?

    Couple of links below. I don't shoot slower than f/11 personally but it is more an habit than a deliberate choice.
    http://www.cambridgeincolour.com/tut...hotography.htm
    http://hyperphysics.phy-astr.gsu.edu...cirapp.html#c1

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    Re: Diffraction?

    I consider diffraction whenever I want critically sharp images.
    The effect of diffraction is related to numerical aperture, which causes it thus to be affected by aperture only and independent of lens focal length.
    The cause of diffraction related image degradation is that the Airy disk projected on the sensor gets to be larger than a pixel in diameter. Traditional use of diffreation limitation involves the calculation of the circle of confusion which is based on the finished print size, the viewing distance, and the acuity of the viewer's vision. If you are a pixel peeper (as I tend to be), then use a circle of confusion of twice the pixel size in your calculations. For the M8, the diffraction limit (for pixel peepers) is about f/9.
    -bob

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    Subscriber Member TRSmith's Avatar
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    Re: Diffraction?

    Thanks LCT and Bob. Some good info there and while I don't pretend to grasp it entirely in one quick read, it seems to confirm my general understanding and MO. I guess I was under the impression (or hoping?) that Leica's slimmer AA filter might allow for another, smaller stop before reaching the trade-off point. But I'm fine with f9-11 as a rule of thumb.

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    Re: Diffraction?

    Thanks for the info Bob... this is pretty much what my practice has been but it's good to know there's sound foundation for it.

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    Administrator Bob's Avatar
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    Re: Diffraction?

    Actually, I would de-rate this number a bit since there are other effects that cause the Airy disc to diffuse, such as slight haze on the sensor filter, lens, or whatever. Note that AA filters make this worse on-sensor.
    A good practical guideline on the M8 might be about f/5.6-f/8
    -bob

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    Re: Diffraction?

    So that's; 'f8 and be there!'.

    Good to know some old folklore still holds true.

    .............. Chris

  8. #8
    wparsons
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    Re: Diffraction?

    The literature says that difr starts for 35 mm as soon as you start stopping down.

    With my M8, I can see that the images beyond f8 are inferior. In fact, it is visible on the 2-inch viewing screen.

    With my M4 and M6, images at f16 were never satisfactory, tho I often shot at f11 (full frame as apposed to 3/4-frame on the M8).

    Unless I need a lot of DOF, I stop at f5.6 on the M8.

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    Re: Diffraction?

    So to understand this better, if I want to shoot landscapes on the M8 I should shot at F8 and focus on infinity? Can someone assist the best way to get the optimal results fir shooting landscapes/cityscapes or anything where you want as much detail.

    Thanks

    Rodney

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    Subscriber robsteve's Avatar
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    Re: Diffraction?

    Rodney:

    If you want the infinity parts of the scene to be really sharp, you need to focus on them. If you use DOF, they will be less sharp. A good compromise is to use the DOF scale far an aperture one or two stops wider than the aperture you are shooting. In other words, if shooting at f8, use the f4 or f5.6 DOF scale for hyperfocal focusing.

    A couple years ago I made some tests with the M8 and the 15mm Voitlander and to get the objects at infinity the sharpest, the lens had to be at infinity, even if shooting at f6.3. The compromise was to use the DOF scale of f4, which gave pretty decent foreground sharpness and good infinity sharpness, but still not as sharp as the lens on infinity.


    Robert

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    Re: Diffraction?

    Hi Rob,

    Thanks, I am starting to get it but still struggle to understand how to get the best out of the M8 system, as some people have advised that at F8 and set the DOF 5ft and most things will be sharp, something I always wondered about street photography was how did they focus?

    Any other hints or examples to help beat this into my head would be great. As I love the M8 files and want to use more and get more from.

    Rodney

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    Re: Diffraction?

    Quote Originally Posted by hobbsr View Post
    Hi Rob,

    Thanks, I am starting to get it but still struggle to understand how to get the best out of the M8 system, as some people have advised that at F8 and set the DOF 5ft and most things will be sharp, something I always wondered about street photography was how did they focus?

    Rodney
    Most things will be out of focus doing that. When I shoot on the street, I put the rangefinder to my eye and align the images on the subject I want in focus.

    In digital, not just the M8, there just isn't as much DOF as there was with film. The F8 and be there will not work as well.

    Robert

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    Subscriber Member TRSmith's Avatar
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    Re: Diffraction?

    Quote Originally Posted by robsteve View Post
    Most things will be out of focus doing that. When I shoot on the street, I put the rangefinder to my eye and align the images on the subject I want in focus.

    In digital, not just the M8, there just isn't as much DOF as there was with film. The F8 and be there will not work as well.

    Robert
    I think it will depend on the focal length of the lens you're using. A wider lens will allow for more apparent sharpness when "guestimating" focus for each new scene. Lately, I've been practicing a little with a 21mm on the M8, estimating distance to the subject I want in focus, then setting the distance using the focusing ring without looking through the VF. At f4 - 5.6 I can get pretty close.

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    Administrator Bob's Avatar
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    Re: Diffraction?

    The issue with diffraction is that no lens will be sharper at f/11 than it was at f/8 on the M8 for subjects in the plane of focus.

    DOF is another subject entirely, and unlike diffraction is heavily influenced by focal length and subject distance.

    On the issue of absolute sharpness, there is only one plane of sharpest focus (actually a complex surface these days).
    The issue is that if image blur caused by lack of focus at some other plane is hidden by the circle of confusion or not. The circle of confusion, for those who do not pixel peep, is ultimately connected with the print size, the viewing distance, the viewer's vision, sensor size and so forth.
    -bob

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    Re: Diffraction?

    Composite DOF in PS. Focus on the near point and shoot at f/8. Do the same at the mid-point and far point. Now composite in PS masking the OOF areas in each layer. I used to do this with my Canon 1DsMII, works great, but I got tired after a while spending all my time at the computer. Then I bought a LF camera......one shot gets it right every time. Of course now I have to wait for my Mac to chew through 1GB drum scans

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