To make an accurate measurement on the image, you need to use a minimum of two pixels per smallest feature that you want to detect. The calculations can be done for each dimension separately but, for simplicity, this is often reduced to one dimension. This is in two dimensions for example 640X480. PS: One thing that I should really add to that page is a detailed calculator description/directions page as a pop-up window.The resolution of an image is the number of pixels in the image. Please let me know if there's still any confusion. I am always interested in feedback regarding real-world use/interpretation of this calculator. 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. 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. 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. 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 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. However, nowadays people are also concerned with whether their camera's megapixel count or its optical resolution is the limiting factor. (Taken from this site's tutorial on Depth of Field) 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.
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:Īn 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. Whether the camera is 10 or 100 megapixels will not change the diffraction limited aperture for a sharp 8x10 inch print, for example. 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. Confused more about the calculator and article than I am about the theory. Why have the input there if it does not impact the result?Ĭonversely, when I check the CoC = Twice Pixel Size setting to On, I can effect the the diffraction outcome by changing the resolution. 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. Surely, I reasoned, if resolution in the form of megapixels was required as one of the inputs, it has to have an impact. As one of my proofs, I presented the Cambridge diffraction calculator as evidence. 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.
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