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Thread: Diffraction Limit Calculator - Independent of Megapixels?

  1. #1

    Diffraction Limit Calculator - Independent of Megapixels?

    I got into a cat fight at another forum regarding diffraction limits where I took the position that sensor megapixels played a role in diffraction. As one of my proofs, I presented the Cambridge diffraction calculator as evidence. Surely, I reasoned, if resolution in the form of megapixels was required as one of the inputs, it has to have an impact.

    Well, after playing with the calculator, it became obvious that if the CoC = Twice Pixel Size setting was checked as Off, then I could input 1 or 1000 megapixels and it wouldn't create a diffraction limited image with an f11. Why have the input there if it does not impact the result?

    Conversely, when I check the CoC = Twice Pixel Size setting to On, I can effect the the diffraction outcome by changing the resolution. Confused more about the calculator and article than I am about the theory.

  2. #2

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    Re: Diffraction

    Quote Originally Posted by rfkiii View Post
    Confused more about the calculator and article than I am about the theory.
    Hi rfkiii,

    Great to have you with us.

    To be honest, this is one area that pretty much goes right over my head, but Sean should be back tomorrow (the writer of the article and site owner) and I'm sure he'll be able to answer that authoritively for you. So until then, "sit tight" ... help is on the way!

  3. #3

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    Re: Diffraction

    I think the calculator tells you wether diffraction (if there is any) is visible or not.
    There might be diffraction with particular settings but it doesn't mean it's visible.
    So the calculator carry theory to practical use.

  4. #4
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    Re: Diffraction

    Hi rfkiii, welcome to the forums! I put together the difraction page that includes the diffraction limit calculator you'd mentioned. Please see below for some answers to your questions.

    Quote Originally Posted by rfkiii View Post
    I got into a cat fight at another forum regarding diffraction limits where I took the position that sensor megapixels played a role in diffraction. As one of my proofs, I presented the Cambridge diffraction calculator as evidence. Surely, I reasoned, if resolution in the form of megapixels was required as one of the inputs, it has to have an impact.

    Well, after playing with the calculator, it became obvious that if the CoC = Twice Pixel Size setting was checked as Off, then I could input 1 or 1000 megapixels and it wouldn't create a diffraction limited image with an f11. Why have the input there if it does not impact the result?

    Conversely, when I check the CoC = Twice Pixel Size setting to On, I can effect the the diffraction outcome by changing the resolution. Confused more about the calculator and article than I am about the theory.
    In the traditional sense, the megapixel count does not determine the diffraction limited aperture, since one is normally only concerned with whether diffraction related softness shows up based on a given print size. Whether the camera is 10 or 100 megapixels will not change the diffraction limited aperture for a sharp 8x10 inch print, for example. This is why the "Set Circle of Confusion = Twice Pixel Size?" is set to off by default, since it means that the CoC is defined only by the output print size, as follows:

    An acceptably sharp circle of confusion is loosely defined as one which would go unnoticed when enlarged to a standard 8x10 inch print, and observed from a standard viewing distance of about 1 foot. At this viewing distance and print size, lens/camera manufactures assume a circle of confusion is negligible if it is no larger than 0.01 inches at this size.
    (Taken from this site's tutorial on Depth of Field)

    However, nowadays people are also concerned with whether their camera's megapixel count or its optical resolution is the limiting factor. In other words: when your image is viewed at 100% on a monitor, will it appear soft due to diffraction, or is there still more detail to be obtained by going to a higher megapixel count (for the given aperture setting)? This is why the "Set Circle of Confusion = Twice Pixel Size?" was made available. When it is checked, it defines a custom size for the CoC based on (twice) the absolute size of each pixel (using the input boxes with gray text labels; otherwise the boxes with gray text are not required). In this case, as the number of megapixels increase (for a given sensor size), the size of the CoC decreases, so the diffraction limit is indeed related to the number of megapixels.

    The aim with the "Set CoC = Twice Pixel Size" setting was to settle a lot of the debates I was seeing on the forums relating to whether and when more megapixels actually improves image detail. There were a lot of people arguing that diffraction effects would mean that cameras with more than X megapixels were often times not producing any more detail.

    Hope this clears things up. I am always interested in feedback regarding real-world use/interpretation of this calculator. Please let me know if there's still any confusion.

    PS: One thing that I should really add to that page is a detailed calculator description/directions page as a pop-up window...
    Last edited by McQ; 9th February 2009 at 07:27 PM.

  5. #5

    Re: Diffraction

    Thank you so much for your time in answering my questions. Hope you will consider incorporating some form of this clarification into the original article.

    I respond well to mathematics and equations. Once I saw the equation for calculating the aperture at which diffraction will begin to impact an image, the bells went off that I may be mis-interpreting your calculator.

    N = anticipated viewing distance / desired print resolution for a 10-inch viewing distance / enlargement factor / 0.0135383

    As I have said, I respond well to formulas, but I recognize that others may stare glazed-eyed at it. Nonetheless, would it be helpful to write the equation into your article?

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