Down sampling and Nyquist limits

Discussion in 'Digital Photography' started by Scott W, Nov 1, 2006.

  1. "Bill Tuthill" <> wrote in message
    news:...
    SNIP
    > Your use of the lens-blur sharpen filter seems to indicate that
    > the original image was taken with a poor camera lens.


    Not really, I just 'abuse' the filter to 'solve' a perceived issue ;-)

    The Smart Sharpen filter is based on one of two different assumptions
    from which one must be chosen. One assumption is that the image
    resolution is degraded by (convolved with) a Gaussian shaped Point
    Spread Function (PSF), the other is that it is convolved with a disk
    shaped PSF. The Gaussian shaped PSF resembles the kind of blur
    function that a combination of different sources of natural blurring
    provide. The disk shaped PSF resembles defocus, like a slice from the
    cone of focused light but not at the focal plane.

    The effect of the Smart Sharpen function resembles a process called
    deconvolution (the mathematical inverse of the blurring process), but
    it's much faster and only a bit less effective than the real thing (it
    does avoid some of the risks of deconvolution). The 'lens blur'
    variant exhibits only very minor halo tendency.

    In a properly bandwidth limited, down-sampled, image in a regular grid
    there is a gradual attenuation of the sharpness as we approach the
    limiting resolution at the single pixel level. There is then a
    trade-off between aliasing (in the form of stair-stepping and moire
    patterns) and blur, and the Smart Sharpening allows to fine tune that
    trade-off. It also allows to boost the modulation of other spatial
    frequencies, but that's beyond the scope of this topic.

    --
    Bart
    Bart van der Wolf, Nov 5, 2006
    #21
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