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Thread: Vuescan: Is gamma correction 'lossy'?

  1. #1
    Join Date
    Sep 2009
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    Vuescan: Is gamma correction 'lossy'?

    I am using Vuescan Pro to scan BW negs on an Epson V700. As far as I understand it, I have turned off all processing in Vuescan such as curves (by setting the points to .25 and .75), sharpening etc and am trying to get a "straight" scan with as much data as possible and then to do all processing in PS.

    However, as Vuescan is scanning it displays a dark RAW image and then after scanning is done the image that is saved as a tiff or jpg file is much brighter. This is because the raw scan has a gamma of 1 and the final scan has a gamma of 2.2 (windows gamma, I think its the same for my mac). However, this image also seems to have, to my eyes, a shorter tonal range and less separation in the shadows and highlights. So I'm wondering if simply gamma correction can result in data loss in an image because as far as I can tell I have turned off all other processing.

    I am also ofcourse saving the raw output from Vuescan and while it is darker, its still a tad brighter than the raw display while scanning. Any ideas on why this might be since, again, I'm trying to turn of all processing by Vuescan?

    Thanks for any help and ideas.

  2. #2
    Join Date
    Oct 2009
    Copenhagen, Denmark
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    Re: Vuescan: Is gamma correction 'lossy'?

    I can not speak for Vuescan, but the following is a general.

    If "jpg" is ye olde JPEG, then you lose quite a lot of information. You definitely do not want gamma=1.0 (linear encoding) in that case.

    Something similar happens for an 8-bit unlossy TIF: if you use a linear encoding you will get very obvious artifacts, typically banding or (if dithering is used) lots of noise.

    With a 16-bit unlossy TIF you have enough range to cover everything and gamma should not matter for the image quality.

    I would go with a linear encoding personally, but I am a geek :-)

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