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Thread: Strange artifact

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    Strange artifact

    Hello and this is my first post. I have a Hasselblad 503 CW and the CFV II (16MP) digital back. I was trying to take some shots of a few musical instruments using an 80 CF lens but the strings have a strange color artifact that I cannot remove in Phocus or Lightroom. I tried it again with my Leica MP 240 and there is no artifact. Is this due to CCD vs CMOS sensors? Using one Broncolor Siros 800 studio strobe in a soft box. ASA 100, F8, 1/250 on the Hassy/CFV. Leica: ASA 200, F11, 125. Any thoughts would be appreciated. Hasselblad on top and Leica below. Attachment 117324Attachment 117325
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    Re: Strange artifact

    Could it be some uncommon form of Moire?

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    Re: Strange artifact

    Looks like moiré from the grooves in the string. Try the moire slider on Phocus. Also try retaking the shot but move the camera slightly closer or further from the subject and compare.

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    Re: Strange artifact

    Quote Originally Posted by masud View Post
    Looks like moiré from the grooves in the string. Try the moire slider on Phocus. Also try retaking the shot but move the camera slightly closer or further from the subject and compare.
    I tried the moire slider in Phocus and it just added more blue, especially to the higher strings. If anything there was a marginal improvement when moving only the color slider under the Noise Filter, all the way to the right. Otherwise, nothing else helped

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    Senior Member aztwang's Avatar
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    Re: Strange artifact

    http://photography-on-the.net/forum/...d.php?t=862422

    Very common issue...Ive seen it on my guitars in the past but paid little mind to it. It is a tightly wound continuing pattern. Check out a anti moire filter, something like a Softner I think is what they call it from Zeiss.

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    Re: Strange artifact

    Try to recapture with the Hassy and close down the aperture to f/16.

    Regards, Udo

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    Senior Member ErikKaffehr's Avatar
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    Re: Strange artifact

    Hi,

    It is a form of colour aliasing. Lens outresolves sensor, so guitar strings don't cover more than a pixel. So, the demosaic algorithm cannot reproduce the correct colour.

    Stopping down, so diffraction reduces resolution helps.

    This kind of phenomena is very frequent on system not having an anti aliasing filter. The task of the AA-filter is to suppress these kinds of artefacts.

    Best regards
    Erik


    Quote Originally Posted by osroubek View Post
    Hello and this is my first post. I have a Hasselblad 503 CW and the CFV II (16MP) digital back. I was trying to take some shots of a few musical instruments using an 80 CF lens but the strings have a strange color artifact that I cannot remove in Phocus or Lightroom. I tried it again with my Leica MP 240 and there is no artifact. Is this due to CCD vs CMOS sensors? Using one Broncolor Siros 800 studio strobe in a soft box. ASA 100, F8, 1/250 on the Hassy/CFV. Leica: ASA 200, F11, 125. Any thoughts would be appreciated. Hasselblad on top and Leica below. Attachment 117324Attachment 117325

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    Re: Strange artifact

    Quote Originally Posted by ErikKaffehr View Post
    Hi,

    It is a form of colour aliasing. Lens outresolves sensor, so guitar strings don't cover more than a pixel. So, the demosaic algorithm cannot reproduce the correct colour.

    Stopping down, so diffraction reduces resolution helps.

    This kind of phenomena is very frequent on system not having an anti aliasing filter. The task of the AA-filter is to suppress these kinds of artefacts.

    Best regards
    Erik


    Thanks so much for your comments. I tried re-shooting at F16 and it did improve the problem. It actually improved quite a bit when I stood closer to the guitar. Not everything needs such a tight crop though. If the sensor were more current, such as the Sony 50 MP CMOS, would this still be an issue? I also tried shooting the same scene again with my Leica at F4 and there was no artifact. It is my understanding that the 240 does not have an AA filter either. So what is the difference? Thanks again

  9. #9
    Senior Member ErikKaffehr's Avatar
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    Re: Strange artifact

    Hi,

    It's about feature size relative to the pixels. If feature size (projected on the sensor) is similar to the pixel size, this kind of artefacts will arise.

    The new CMOS sensors also show these effects, but they probably have gapless microlenses that sort of increase the fill factor and that reduces this kinds of artefacts.

    If aliasing artefacts are visible depends on feature size, focusing accuracy and lens resolution. That is the reason moving in or out may help. Stopping down to f/16 smears the image and that eliminates the aliasing.

    Here is some diskussion:

    http://echophoto.dnsalias.net/ekr/in...ixels-are-good

    Best regards
    Erik


    Quote Originally Posted by osroubek View Post
    Thanks so much for your comments. I tried re-shooting at F16 and it did improve the problem. It actually improved quite a bit when I stood closer to the guitar. Not everything needs such a tight crop though. If the sensor were more current, such as the Sony 50 MP CMOS, would this still be an issue? I also tried shooting the same scene again with my Leica at F4 and there was no artifact. It is my understanding that the 240 does not have an AA filter either. So what is the difference? Thanks again

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