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Thread: Concern about diffraction may not be aplicable

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    Concern about diffraction may not be aplicable

    Often one reads about people suggesting that one should not use smaller than perhaps f/16 becuase of the distortion caused by diffraction. usually in connection with close-up work where depth of field is minimal and the temptation is to use the smallest aperture available to one.

    As I understand it, diffraction is what happens when light goes through a small hole.

    When I started photography seriously I was told to work at f/64 and I got nice crisp results, all else being equal.
    But I am aware that P&S cameras rarely have apertures smaller than f/8.
    This puzzled me until I worked out that f/8 with a 6mm lens has an aperture of 1.3mm diameter. But that lens I was using at f/64 was a 200mm which despite being worked at f/64 had 'a hole' of 1/8" approximately.
    It was an 8 inch lens or 203.2 which gives 'a hole' of 3.175mm [ assuming the diaphragm mechanism was accurate ]

    From all that I suggest that when one uses a longer lens with say a close-up lens for tight framing one need not be afraid of the diffraction gremlin ... becuase as Sporting Life said "It ain't neccessarilly so".
    You would need to be stopped down to f/156 before you had the same aperture as the P&S.

    I suspect there maybe a fly in the ointment of my argument becuase of the difference between long focus and telephoto lenses and modern compact zooms .... my 8" was a Rapid Rectilinear. A pair of 15" lenses used together.

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    Re: Concern about diffraction may not be aplicable

    Always test your lenses - find where diffraction comes in. It will vary slightly even between lenses of the same make and design. Then, when you get a new body, test them again because the sensor matrix density will be different and the point of diffraction may have changed.

    Avoid the dogma. Work with the facts.

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    Re: Concern about diffraction may not be aplicable

    My Suggestion with regards to diffraction is to simply ignore it; a correct sharpening workflow will have a far greater impact on the final image, and diffraction usually causes far less damage to an image than the adverse affects of an incorrect shutterspeed (in terms of subject motion and/or camera shake) or aperture (in terms of DoF) or ISO (in terms of dynamic range).

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    Re: Concern about diffraction may not be aplicable

    It also helps to be a bit aware of subject matter. Diffraction does soften the image, but mostly in a very particular way. Strong light sources against a much darker background influence more than dark objects against a relatively brighter background. This was once a real issue, in optical enlarging in the darkroom, where diffraction or diffusion would spread dark areas into the brighter ones, due to the negative/positive process.

    So generally, diffraction has little impact on your image, and may in most cases be disregarded completely. When depth of field is more important than the softening effect of diffraction, stop down as much as you need. You might notice "diffraction stars" created of light bending at the edges of your diaphragm blades sometimes, as when you shoot against the sun, or with light sources in dim lighting, or when specular reflections are much brighter than surrounding areas. You aren't likely to see them on night shots of stars though, as those are mostly done with the lens wide open.

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    Re: Concern about diffraction may not be aplicable

    The size of the hole, in of itself, doesn't matter. What matters is the F-number. The F-number dictates the size of the Airy disk, and that dictates the effect of diffraction on your image.

    See CiC's article on Airy disks here...
    http://www.cambridgeincolour.com/tut...hotography.htm

    Here's the thing about Airy disks...the size of the Airy disk depends on the F-number and nothing else. Neither focal length nor size of the lens matters. This means that a compact camera at f/8 has the same size Airy disks being projected on the sensor as a full-frame camera at f/8. As the page above shows, the Airy disk may fit within the pixel of a full-frame sensor, while the same size Airy disk will cover several pixels of a compact camera. This is the reason that compacts don't go above f/8. At f/8, the blur caused by the Airy disks is about the same as the blur on full-frame cameras at f/32-f/40 (which, btw, is also the limit of apertures found on full-frame camera lenses.)

    On a top quality lens, there is a visible difference between f/4 and f/5.6, and I can see a huge difference in sharpness between, say, f/10 and f/40. On my Nikon D90, There's no reason to go beyond f/16 on landscapes because diffraction cancels any gain from increased DOF. f/11 gives sharper results for landscapes if I don't need anything within 10 ft of me to be sharp.

    The larger the format, the less the effect of diffraction for any given F-number. If you're shooting with large format film, then sure, go up to f/64 as it won't be that big of a deal. But with smaller formats, diffraction can be a problem if you're not careful.

    Nowadays I believe focus stacking is the answer to the DOF issue of macro photography. I'm surprised that Lytro cameras haven't become the camera of choice for macro.

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    Moderator GrumpyDiver's Avatar
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    Re: Concern about diffraction may not be aplicable

    Unless I am shooting under virtually ideal conditions; heavy tripod, mirror lock, shooting with cable release etc. and going into "pixel peeper" mode, I really have not found that diffraction is a limiting factor to my shooting. If I were to rank sharpness issues in the hand-held world, I would say the causes of lack of sharpness in my images, in order of frequency would be as follows:

    1. Focus is off;
    2. Camera movement;
    3. Lens softness, especially wide open; and
    4. Other causes, including diffraction, dirty lens, etc.

    Although on any given day, depending on what and how I am shooting, points 1 and 2 are sometimes reversed. In practice, other than landscapes and city scapes, I tend to shoot towards the wide open part of the range. Why would pay for a fast lens to shoot at f/16 or below anyways, and if I'm really worried about it, a 0.3 or 0.6 ND filter gets one up to f/11 or higher?

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    Re: Concern about diffraction may not be aplicable

    Quote Originally Posted by Graystar View Post
    The size of the hole, in of itself, doesn't matter. What matters is the F-number. The F-number dictates the size of the Airy disk, and that dictates the effect of diffraction on your image.
    The Airy disk issue is relevant regarding resolution, and thus it is more noticeable when you pixel-peep an image from a sensor with sensels smaller than the Airy disk. However in most real life situations, we are not into pixel peeping, and often we need far less resolution than the maximum from the sensor. We should also remember that resolution and sharpness are not exactly the same thing, but that sharpness depends on various factors, one of them being resolution, another relative contrast, i.e. MTF, But sharpness is also formed by subject matter. There sometimes can be too much detail in an image for the eye to recognise sharpness, and there is no way to convey a sensation of sharpness other than enlarging more, while an image with less detail may be sharpened, due to the effect of contrast enhancing at low resolution.

    So still, even though the image may be softened by stopping down, sharpness, in the form of the rather fuzzy concept of acutance, may be enhanced in image processing. Acutance is the factor we usually alter in computer programs as unsharp mask or other sharpnes enhancing algorithms. Acutance is enhanced by increasing contrast at the edges of objects.

    Quote Originally Posted by Graystar View Post
    Nowadays I believe focus stacking is the answer to the DOF issue of macro photography. I'm surprised that Lytro cameras haven't become the camera of choice for macro.
    And no, there is no way Lytro could compete for sharpness in macro images. Lytro makes it possible to stack images taken in one single shot, but Lytro images are very low resolution images compared to what we are used to, and no Lytro image that I have seen so far is superior to any compact p&s image regarding image acuity, sharpness or resolution. Light field images need much more resolution to compete with real world images taken with our traditional cameras, and it can only be accomplished by either cramming far more sensels into that small area or using a larger camera with longer focal length and a larger sensor. In many cases the tilting technique is superior, and when insufficient, we resort to stacking. When taking inanimate subjects, stacking is the way to go still. If camera makers would automate lens tilting, it could also be used to enhance focus. If for example the photographer could choose three points to focus upon and the lens automagically would focus all three by tilting, the technique could be used more conveniently and without the help of a tripod. You could imagine why they don't do that. Tilt lenses haven't been a great success anyway, and they have gained popularity only for the opposite effect. Googling tilt effect will give scores of unsharp images where tilting is used to decrease the apparent DOF instead of increasing it.
    Last edited by Inkanyezi; 14th June 2012 at 12:18 PM.

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    Re: Concern about diffraction may not be aplicable

    >>>The size of the hole, in of itself, doesn't matter. What matters is the F-number. The F-number dictates the size of the Airy disk, and that dictates the effect of diffraction on your image.<<<

    Firstly people who write F rather than f do not command my respect, perhaps I am an old fogie who within the limits of his knowledge was trained properly, and sticks to how he was trained rather than indulge in modern carelessness which I note creeping into blogs these days.

    The f/number is a way of describing the size of the hole, rather inaccurate becuase it alters with focal length, which is why I wrote in the actual diameter which from the article seems to be the critical factor for whatever you care to call it. The point of my message was to illustrate the inappropriateness of working from the f/ number. Since the problem starts to be a concern from f/8 with a short P&S camera lens but is of no concern when using a longer lens, say 200mm for example.

    What nobody has answered is the possible difference between long focus/telephoto/compact zooms which I was hoping somebody would know and could advise us. Does the size of the aperture change between the three lens types.

    This was a concern to me in my ignorance of this aspect that my advice to the OP in the other thread could be wrong due to a difference in construction of a telephoto or compact zoom.

    I still have an interesting photo from my training days where I used a tilt back to get a darkroom timer in focus and of course got a rather peculiar shaped clock So when I saw the narrow focus example in my latest version fo PSP I was a little puzzled for awhile Actually it looks like I turned the camera on its side and got an inappropriate swing back Camera was a wooden Thornton Pickard half-plate with quarterplate adaptor for poor student ... which I don't think had a swing back like you could find on a monorail.
    Concern about diffraction may not be aplicable
    10Mp copy of my 1952-3 quarterplate negative. Reversed and some curves attention.
    Last edited by jcuknz; 15th June 2012 at 08:50 AM.

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    Re: Concern about diffraction may not be aplicable

    OT:
    Quote Originally Posted by jcuknz View Post
    /.../
    I still have an interesting photo from my training days where I used a tilt back to get a darkroom timer in focus and of course got a rather peculiar shaped clock So when I saw the narrow focus example in my latest version fo PSP I was a little puzzled for awhile Actually it looks like I turned the camera on its side and got an inappropriate swing back Camera was a wooden Thornton Pickard half-plate with quarterplate adaptor for poor student ... which I don't think had a swing back like you could find on a monorail.
    Concern about diffraction may not be aplicable
    10Mp copy of my 1952-3 quarterplate negative. Reversed and some curves attention.
    Straying away a bit from the main topic, but swings and tilts are a bit different depending on which standard you tilt/swing, To alter the focus plane without altering reproduction scale or perspective, the movements are done with the front standard. Ideally, the lens should be a perfectly symmetrical construction with rectilinear reproduction, preferably with a large field of view, a wide angle. It should be mounted with the tilt and swing axis centered in the diaphragm plane of the lens.

    Adjusting sharpness (focus) at an angle with the rear standard, exaggerates perspective, as closer parts of the image become more enlarged than parts that are farther away.

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    Moderator GrumpyDiver's Avatar
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    Re: Concern about diffraction may not be aplicable

    Quote Originally Posted by jcuknz View Post
    What nobody has answered is the possible difference between long focus/telephoto/compact zooms which I was hoping somebody would know and could advise us. Does the size of the aperture change between the three lens types.
    By it's very definition the f/stop (or focal ratio) is simply the focal length / entrance pupil diameter. Any lens type, regardless of fixed or any type of zoom must have the same entrance pupil diameter, or physical hole diameter at the same focal length to have the same f/stop number.

    The only thing that might fool us, when looking at different lenses is that the aperture will look to be different sizes as we look through the front element of the lens. This is simply because different lenses will magnify the iris opening differently. Take the lens apart, and the aperture opening will be the same for the given f/stop.
    Last edited by GrumpyDiver; 15th June 2012 at 12:02 PM.

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    Re: Concern about diffraction may not be aplicable

    Quote Originally Posted by jcuknz View Post
    Firstly people who write F rather than f do not command my respect
    Officially, it's a hooked f, and it's been written and "F-number" and "f-number" for many decades. As lowercase "f" is equally wrong, I'd say your position is pretty retarded.

    Quote Originally Posted by jcuknz View Post
    The point of my message was to illustrate the inappropriateness of working from the f/ number.
    Right. And the point of my message and some others was to inform you that you're wrong.

    Quote Originally Posted by jcuknz View Post
    Since the problem starts to be a concern from f/8 with a short P&S camera lens but is of no concern when using a longer lens, say 200mm for example.
    Like I said...you're wrong.

    Quote Originally Posted by jcuknz View Post
    What nobody has answered is the possible difference between long focus/telephoto/compact zooms which I was hoping somebody would know and could advise us. Does the size of the aperture change between the three lens types.
    It was already explained. If you don't want to accept it because it doesn't fit with your view, that's a different issue entirely.

    Quote Originally Posted by jcuknz View Post
    This was a concern to me in my ignorance...
    You'd think that someone concerned about their ignorance would be a little more open minded.

    Good luck. I hope someone can explain it to you. It won't be me.

  12. #12
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    Re: Concern about diffraction may not be aplicable

    Quote Originally Posted by jcuknz View Post
    The f/number is a way of describing the size of the hole, rather inaccurate becuase it alters with focal length, which is why I wrote in the actual diameter which from the article seems to be the critical factor for whatever you care to call it. The point of my message was to illustrate the inappropriateness of working from the f/ number. Since the problem starts to be a concern from f/8 with a short P&S camera lens but is of no concern when using a longer lens, say 200mm for example.

    What nobody has answered is the possible difference between long focus/telephoto/compact zooms which I was hoping somebody would know and could advise us. Does the size of the aperture change between the three lens types.
    The absolute aperture diameter changes with focal length for the same f/number, so if the focal length used on the telephoto vs compact is different, then the aperture diameter will be different too. However, when it comes to diffraction, all that matters is the f/number. From the diffraction tutorial:
    Since the physical size of an aperture is larger for telephoto lenses (f/4 is has a 50 mm diameter at 200 mm, but only 25 mm diameter at 100 mm), why doesn't the airy disk become smaller? This is because longer focal lengths also cause light to travel further before hitting the camera sensor -- thus increasing the distance over which the airy disk can continue to diverge. The competing effects of larger aperture and longer focal length therefore cancel, leaving only the f-number as being important (which describes focal length relative to aperture size).
    Now, about using f/8 on a compact camera vs f/64 on a medium/large format camera; yes, the airy disk will be smaller at f/8 compared to f/64 regardless of sensor size or focal length. However, the airy disk has to be enlarged more with the smaller sensor, so the two effects roughly cancel. In other words: for f/numbers giving the same depth of field, the diffraction-limited resolution will be the same with both the large and small sensor sizes (presuming sufficient pixels for each). There's a lot more on this here (in the section on diffraction and in the conclusions at the end).

    As other have said though, this is all really academic, and only for those who want to know the "why" of diffraction. It's always best to gauge things visually by experimenting with your particular camera.

    I'm going to close this thread since it seems we've drifted away from the thread starter's original questions. jcuknz: if the above didn't address what you were wondering, please feel free to PM me for more.
    Last edited by McQ; 16th June 2012 at 05:40 AM.

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