Re: Sharpening when the lens is soft

Discussion in 'Digital Photography' started by Don Stauffer in Minnesota, Jul 10, 2008.

  1. On Jul 10, 4:10 am, "G Paleologopoulos" <>
    wrote:
    > "Alfred Molon" <> wrotenews:...
    >
    >
    >
    >
    >
    > > In article <1215638971.761781@athprx03>, G Paleologopoulos says...

    >
    > >> MY experience has been that a SMALL amount of sharpening (preferably
    > >> unsharp-masking) is acceptable in the stated situation. More looks
    > >> artificial and will introduce artifacts.
    > >> An unsharp lens is a LOSSY low-pass filter. Sharpening after the fact
    > >> will
    > >> do little.

    >
    > > What do you mean by lossy?

    >
    > > In some cases it makes a huge difference if you apply some sharpening
    > > during RAW conversion as opposed to not applying any sharpening.
    > > --

    >
    > > Alfred Molon
    > > ------------------------------
    > > Olympus 50X0, 8080, E3X0, E4X0, E5X0 and E3 forum at
    > >http://tech.groups.yahoo.com/group/MyOlympus/
    > >http://myolympus.org/photo sharing site

    >
    > OK, I meant that as a result of "unsharpness" some original information
    > (which would be recorded by a sharper lens) is irretrievably lost, no matter
    > what follow-on procedures one uses.
    > Rgrds, G.


    Also, the sharpening filter has no way to distinquish between a sharp
    edge that has been rounded off by optics, and an edge that was already
    gradual. It sharpens both.
     
    Don Stauffer in Minnesota, Jul 10, 2008
    #1
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  2. Don Stauffer in Minnesota wrote:
    []
    > Also, the sharpening filter has no way to distinquish between a sharp
    > edge that has been rounded off by optics, and an edge that was already
    > gradual. It sharpens both.


    Wouldn't applying the appropriate inverse MTF function restore the loss of
    sharpness (up to the spatial frequency when the signal is lost in the
    noise)? I accept that the so-called sharpening filters are not the
    correct approach - if they are non-linear operators.

    David
     
    David J Taylor, Jul 10, 2008
    #2
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  3. Alfred Molon wrote:
    > In article <5apdk.24338$>, David J
    > Taylor says...
    >
    >> Wouldn't applying the appropriate inverse MTF function restore the
    >> loss of sharpness (up to the spatial frequency when the signal is
    >> lost in the noise)? I accept that the so-called sharpening filters
    >> are not the correct approach - if they are non-linear operators.

    >
    > That was my idea. The assumption I made was that unsharp mask would
    > approximate this inverse MTF function.
    >
    > In any case, this inverse MTF filter would have to be tailored for the
    > lens and know for instance that the lens is more sharp in the centre
    > than at the edges (i.e. apply more sharpening in the centre) and know
    > the sharpness characteristics at different focal lengths and
    > apertures.
    >
    > I think there could be a generic "inverse lens blur" filter which
    > would have to be calibrated with a number of shots of a test target.


    Alfred,

    I think you may need to be careful exactly which "sharpening" method you
    use. Some are non-linear (and non-reversible) and are not purely MTF
    changes. Agreed about MTF changes over the image area.

    Having said all that, when people refer to a "tack sharp" image, I suspect
    that they actually mean over-sharpened, i.e. a net response which is
    higher in the spatial frequency region where the eye is most
    sensitive.....

    Cheers,
    David
     
    David J Taylor, Jul 10, 2008
    #3
  4. G Paleologopoulos wrote:
    > "Alfred Molon" <> wrote
    > news:...
    >>
    >> ....................................................
    >> I think there could be a generic "inverse lens blur" filter which
    >> would have to be calibrated with a number of shots of a test target.
    >> --
    >>
    >> Alfred Molon
    >> ------------------------------
    >> Olympus 50X0, 8080, E3X0, E4X0, E5X0 and E3 forum at
    >> http://tech.groups.yahoo.com/group/MyOlympus/
    >> http://myolympus.org/ photo sharing site

    >
    >
    > Excellent idea!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
    > I wonder why it hasn't been implemented years ago. Should not be too
    > difficult with digital based imaging.


    It's called deconvolution, and it's as old as the ([digital] image
    processing) hills.

    David
     
    David J Taylor, Jul 10, 2008
    #4
  5. Don Stauffer in Minnesota

    Guest

    On Thu, 10 Jul 2008 20:57:39 +0200, in rec.photo.digital Alfred Molon
    <> wrote:

    >In article <lasdk.24457$>, David J
    >Taylor says...
    >
    >> It's called deconvolution, and it's as old as the ([digital] image
    >> processing) hills.

    >
    >Well, perhaps deconvolution refers to the correction of motion blur.
    >What I was referring to is lens blur removal, quite a different
    >operation.


    Guess you never bothered to read anything about the post processing
    corrections made to the original Hubble photos needed before the corrective
    optics were installed.
     
    , Jul 10, 2008
    #5
  6. Alfred Molon wrote:
    > In article <lasdk.24457$>, David J
    > Taylor says...
    >
    >> It's called deconvolution, and it's as old as the ([digital] image
    >> processing) hills.

    >
    > Well, perhaps deconvolution refers to the correction of motion blur.
    > What I was referring to is lens blur removal, quite a different
    > operation.


    Alfred,

    The cause of the blur is different, yes, but the basic operation is the
    same, as others have kindly pointed out.

    David
     
    David J Taylor, Jul 11, 2008
    #6
  7. Hi Group,
    >
    > Well, perhapsdeconvolutionrefers to the correction of motion blur.
    > What I was referring to is lens blur removal, quite a different
    > operation.
    > --


    The term 'Deconvolution' does refer to the removal of any time of
    blur. When you can't measure or model the actual blur functions, we
    use the term 'blind deconvolution.' Using a 'sharpen' filter can be
    thought of as a blind deconvolution, but it is only effective for
    small amounts of de-focus blur. However, this is the case for the
    vast amount of images taken, so there is no harm in giving it a try as
    a first step. Many blind deconvolution methods do target specific
    types of blur by starting with an initial guess and iteratively
    improving the result. My technique, called SeDDaRA, uses another
    image as the model and is able to remove blur due to de-focus and/or
    motion in one application in a much faster time. You can see examples
    of the results at http://www.quarktet.com/Gallery1.html and also try
    out the software.

    There are two requirements for (just about) any deconvolution to be
    effective. First, the signal-to-noise ratio of the image needs to be
    high enough to retain the blur function. Second, the blur must be
    spatially-invariant, i.e. consistent throughout the image. For
    example, if you have an image of a landscape where far objects are in
    focus, and near objects are not, your blur function varies throughout
    the image.

    Hope that helps,
    Jim C
     
    JimAtQuarktet, Jul 11, 2008
    #7
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