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Thread: Diffraction and DSLR's?

  1. #1

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    Diffraction and DSLR's?

    If I read the Diffraction tutorial correctly diffraction occurs between the final lens element and the sensor.

    Maybe I have got it wrong but surely this means that the large distance necessitated by the mirror in DSLR's is going to aggravate diffraction errors.

    The blurb for my 7 year old Sony R1 says that its lens to APS-C sensor is 1.5mm. Since the lens is fixed I cannot confirm this. Its minimum aperture is F16 but, is there a message here somewhere?

    Incidentally its zoom lens has negligible geometric distortion from 24 - 120mm equiv (1% barrel at 24mm) though whether there is software correction I don't know. Could the very short lens/sensor distance help here?
    Last edited by Dave Humphries; 22nd June 2012 at 09:47 PM.

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    Re: Diffraction and DSLR's?

    As it is not the 1:st of April, I can perhaps rather safely assume that you are serious to some degree?

    The lens to sensor distance is not implied in the diffraction equations, but is an explanation to why it is not the physical dimension of the diaphragm orifice that governs the size of the Airy disk, but the f/ number. The exit pupil of the lens varies with actual distance from the sensor, so that a more telecentric lens has a larger exit pupil. A lens with its exit pupil close to the sensor will have a smaller exit pupil than one with its exit pupil farther from the sensor at the same f-number.

    And I don't understand why they would publish such a silly number, but I do believe that it is not correct. However the number bears no relation to the diffraction issue, and diffraction is not in any way connected to the rear element of the lens. It is the diaphragm that causes diffraction, assuming that the lens is stopped down. Seen from the sensor, this aperture is the exit pupil of the lens. It is certainly not as close to the sensor as you have stated here. Maybe you misread that blurb, or there is a flaw in it.

    Nevertheless, the rear element of the lens is disregarded when we look at diffraction; in fact all elements of glass within it. It is the actual diaphragm that is the source of diffraction, and it can be far from the sensor or closer, but it is only the actual f-number that governs the Airy disk size.

  3. #3
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    Re: Diffraction and DSLR's?

    Urban,

    Perhaps you don't intend it, but your postings sometimes seem insulting, at least to me. There is no reason to state sarcastically that you assume that the post you are responding to here is not an April Fool's joke, and there was no reason to refer to something I posted recently as a "childish" view. To an unusual degree, people on this form provide each other with a lot of useful help and information, and are, with rare exceptions, respectful to each other. You have a lot of information to offer. No reason to spoil it.

    Thanks

    Dan

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    Re: Diffraction and DSLR's?

    pjbw:

    The distance of the lens/aperture/mirror from the sensor is not part of the diffraction issue.

    Diffraction is the bending of light rays as they pass near an object. An interesting reference:

    http://www.exploratorium.edu/snacks/...ion/index.html

    As light passes through a lens, it also passes through the diaphragm - which is essentially a hole - in fact in the earliest cameras, small brass plates with different sized holes were used to change the amount of light passing through the lens and onto the photographic emulsion (film).

    The smaller the hole, the more the effect that diffraction has on the image. Another simple demonstration is to look through the small slit when two fingers are close together. By varying the size of the slit, the effect of diffraction becomes quite visible (as the fingers get very close together).

    Hope this helps.

    Glenn

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    Re: Diffraction and DSLR's?

    Quote Originally Posted by Glenn NK View Post
    pjbw:
    The distance of the lens/aperture/mirror from the sensor is not part of the diffraction issue.
    Glenn
    Thanks everybody. I had got the wrong end of the stick re diffraction. I had thought that the diffraction caused by the 'hole' manifested itself after the 'hole', not inside the 'hole'.
    No way am I going to dismantle my R1 in order to prove or disprove Sony's specified lens/sensor distance. All I know is that the obselete (from 2005, too big, too heavy, only 5x optical zoom, slow processor) mirrorless R1 takes superb competition standard photos without the trial and error/years of practice needed for similar photos in DSLR's.

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    Re: Diffraction and DSLR's?

    AS a newbie trying to upload a photo...

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    Re: Diffraction and DSLR's?

    Quote Originally Posted by pjbw View Post
    AS a newbie trying to upload a photo...
    Forgot to set a link to my photo, try again
    Diffraction and DSLR's?

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    Re: Diffraction and DSLR's?

    The Sony R1 is an APS-C camera with an equivalent wide angle end of zoom of 24mm ... assuming a 1.5 'crop factor' that gives me a 16mm actual lens and a guestimate of about 10mm from back element to sensor distance. I assume that the 1.5 that pjwb writes about is not mm but the crop factor. The specification page[s] of the manual could confirm this. The lens is actually 16-80mm focal length perhaps?

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    Re: Diffraction and DSLR's?

    Quote Originally Posted by Glenn NK View Post
    pjbw:

    The distance of the lens/aperture/mirror from the sensor is not part of the diffraction issue.

    Glenn
    Surely that is not what the tutorial says ... as the distance increases so does the diameter of the Airey disc at the sensor after the fashion of circle of confusion.[ CoC also mentioned in the tutorial ].

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    Re: Diffraction and DSLR's?

    Quote Originally Posted by jcuknz View Post
    I assume that the 1.5 that pjwb writes about is not mm but the crop factor.
    You could very well be right. I really have no idea. I bought my R1 in 2006 and have not checked the specs since. The uploaded photo above taken last month at the Floriade in Holland does show the resolution (if that is the right word!) of this zoom lens on a medium priced camera. The other members of the club missed this exhibit. It would have been interesting to see photos taken with top-end prime-lens Nikons, Canons.
    The original .SR2 image was 3888 x 2592 pixels rotated and cropped. It was processed in Photoshop CS5 via Camera Raw as a .TIF file. The background was posterized otherwise just leveled and sharpened. It is resized and saved as .JPG for this upload.
    I would be delighted to receive comments about this picture...

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    Re: Diffraction and DSLR's?

    Quote Originally Posted by pjbw View Post
    The original .SR2 image was 3888 x 2592 pixels rotated and cropped. It was processed in Photoshop CS5 via Camera Raw as a .TIF file. The background was posterized otherwise just leveled and sharpened. It is resized and saved as .JPG for this upload.
    I would be delighted to receive comments about this picture...
    Hi Peter,

    OK, you asked for it

    Nah, only joking

    The EXIF data reveals: Sony DSC-R1 at 25.7mm (about 38mm FFE at 1.5 crop factor), Auto exposure, Program AE, 1/200 sec, f/3.5, ISO 160

    The exposure looks to be a good choice, I can see the subjects clearly, although they are in a very busy environment - one I tend to avoid because I'm never happy with my own efforts. You have used a fairly wide perture to help with separation from background.

    What can I suggest?

    Well, a vignette might help diminish some of the adjacent foreground/background.

    As has been said in other theads on diffraction, for display on web, it is a 'non issue' because so much of the data is discarded in the reduction process, therefore it appears adequately sharp, even when viewed in the Lytebox full size (778px 1,000px).

    Welcome to the CiC forums from ....

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    Re: Diffraction and DSLR's?

    Is there not some missing information in the preceding posts?

    The statement that the size of the Airy disk is not dependent on the distance of the [effective] aperture from the image plane is only true if the lens is focused to infinity. As is an equivalent statement that the size depends only on the f/number. At 1:1 magnification the Airy disk is somewhat larger. Using a good old theoretical "thin lens", ​ I believe it's double the size?

    Ted
    Last edited by xpatUSA; 27th June 2012 at 02:29 PM.

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    Re: Diffraction and DSLR's?

    Quote Originally Posted by Dave Humphries View Post
    Well, a vignette might help diminish some of the adjacent foreground/background.
    Thank you Dave. Vignette; I must try that. Another interior photo I took recently could probably benefit as well.
    Another step up the learning curve...

    PS. I like the way this forum inverts the thread so that one enters one's reply at the top.

  14. #14

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    Re: Diffraction and DSLR's?

    Quote Originally Posted by pjbw View Post
    PS. I like the way this forum inverts the thread so that one enters one's reply at the top.
    Um, it doesn't (normally) - but it's one of the display options if the user selects it.
    Last edited by Colin Southern; 28th June 2012 at 06:17 AM.

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    Re: Diffraction and DSLR's?

    PJBW ... You must be fully familiar with the camera by now but a copy of operating instructions and Spec is available at
    http://pdf.crse.com/manuals/2654494122.pdf. and maybe of interest

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    Re: Diffraction and DSLR's?

    Quote Originally Posted by Colin Southern View Post
    Um, it doesn't (normally) - but it's one of the display options if the user selects it.
    Another step up the learning curve...

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    Re: Diffraction and DSLR's?

    Quote Originally Posted by jcuknz View Post
    PJBW ... You must be fully familiar with the camera by now but a copy of operating instructions and Spec is available at
    http://pdf.crse.com/manuals/2654494122.pdf. and maybe of interest
    Thank you. I still have Sony's printed manual from May 2006, identical to the PDF above.
    Having used a film SLR with optical viewfinder I tried out the R1 and fell for its electronic viewfinder and swivelable screen and then the review at http://www.dpreview.com/reviews/sonydscr1/ . It does seem that EVF's (not to be confused with those LCD screens on the back of cameras now and which can't be seen in sunlight) are making a comeback; keeping my fingers crossed in case my R1 gets lost or dies of old age.

  18. #18
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    Re: Diffraction and DSLR's?

    I love my 2 R1's i have....just the small 2" Screen is sometimes a bit challenging to see what you shoot (i have good eyesight btw) and i never use the EVF, because i see the actual pixels...it's a mess compared to a good OVF....but back in 2005 EVFs or Displays with 230.000 to 235.000 Pixel Size have been pretty much Standard. Apart from this, i never shot a smaller aperture as F11 with my R1. Mostly F2.8 to F8.

    greetx
    marc

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    Re: Diffraction and DSLR's?

    Quote Originally Posted by doomed forever View Post
    I i never use the EVF,
    Funny thing that. I have come across so many users and reviewers who have not realised that the RI's EVF has three modes triggered by a tiny button just below the command dial. Most people are stuck in the 'show everything' mode (a bit like that LCD screen on top of DSLRs) and complain that the view is too cluttered. Push the little button once and the view switches to exposure mode. Here there is a histogram and overexposed areas are highlighted. Press the button again and you get a plain view like an optical viewfinder on a DSLR. I live in the exposure mode with spot metering. Having a good idea of what will get to the memory card without being so dependent on the electronics and years of experience as with DSLRs or single mode EVFs is to me apparently priceless so far (2012).
    (The 24 to 120mm equivalent optical zoom lens (f2.8 to 4.8) with no barrel or fisheye distortion beyond about 6 feet, full aperture, wide angle seems to be impossible to replicate these days (2012, I think I know why!))

  20. #20

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    Re: Diffraction and DSLR's?

    Urban #2 And I don't understand why they would publish such a silly number, but I do believe that it is not correct.
    Since the lens is probably a 6mm at WA and being a zoom with a prime and other lens the rear component could well be a very short lens and just 1.5mm from the sensor .... no idea if I am correct just surmiseing the 6mm is only an effective angle of view and not a focal length I'm guessing ?
    Is this a nice idea but complete balderdash?

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