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Thread: GRDII/GX100 RAW: pink highlights - and a solution

  1. #1
    simon_t
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    GRDII/GX100 RAW: pink highlights - and a solution

    Hi all,

    Over the past week or so, I have been researching which pocketable RAW-shooting camera I should buy. While playing with the raw files from a number of cameras, I noticed something strange in the Ricoh files: the highlights can take on a subtle magenta tint that gets stronger as you try to recover blown highlights. As an example, here is a crop from a GRDII DNG file taken from http://ricohforum.com/phpbb/viewtopi...d737bed7d5e71c
    [Exposure has been set to -4 to exaggerate the effect.]


    This effect seems to be caused by the following. The sensor outputs a 12-bit signal that is roughly proportional to the amount of light that has been recorded. If too much light is collected, the signal clips and the output maxes out at a value of 4095. The RAW converter detects this and corrects for any color shifts that may happen because one channel (usually green) clips before the others.

    In practice, however, the sensor's response first levels off before it hits the maximum value. Often, the green channel will reach this plateau first, which causes the green channel to become weaker relative to the red and blue channels. If the RAW converter doesn't know this is happening, it incorrectly assumes that there is less green in the original scene, leading to a magenta color cast - and this is exactly what's happening with the Ricoh DNG files.

    However, there is a fix for this: the DNG specification allows one to specifiy the point at which the camera's response is no longer linear by means of the LinearResponseLimit tag. It seems that Ricoh has left this tag at its (incorrect) default value of 1.0 (fully linear response). I definitely think Ricoh should re-assess this (and the defined WhiteLevel) in the next firmware upgrade.

    Luckily, users are not completely dependent on Ricoh for a fix. ExifTool allows you to manually adjust the LinearResponseLimit tag. Running it on the above DNG file with the "-LinearResponseLimit=0.9" option gives the following result:


    I hope this is of any use to you. It will be for me, as I have decided to get the GRDII once I have time to drop by my camera store.

    Cheers,
    Simon

  2. #2
    VladimirV
    Guest

    Re: GRDII/GX100 RAW: pink highlights - and a solution

    This is very informative. Thanks for pointing this out.

  3. #3
    David Paul Carr
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    Re: GRDII/GX100 RAW: pink highlights - and a solution

    Thanks Simon, that is very useful indeed. I had noticed the pink highlights when doing recovery and assumed it was a clipping problem with no solution. I will be downloading ExifTool as soon as possible...
    Last edited by David Paul Carr; 13th February 2008 at 01:37.

  4. #4
    Super Moderator Cindy Flood's Avatar
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    Re: GRDII/GX100 RAW: pink highlights - and a solution

    Thank you, Simon.
    It seems Phil Harvey's web site is down right now, so I can't download the file. I'll try later. How do you work the exif reader into your workflow? Do you run it only when you have a problem, or do you make the change on every file with some kind of a batch?

  5. #5
    simon_t
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    Re: GRDII/GX100 RAW: pink highlights - and a solution

    Cindy, I don't actually have a GRDII yet, so I don't have a workflow right now. However, if I would, I'd probably make a backup of all files first and then run ExifTool on all files at once.

    Also, after posting a link to this post on Ricohforum.com, it was pointed out to me this trick only seems to work with Adobe Camera Raw/Lightroom, which is what I used for my testing. Other raw converters apparently do not read this tag. A similar, but less elegant solution would be to change the WhiteLevel tag, discarding the non-linear output values. Unfortunately, ExifTool does not allow writing to this tag.

  6. #6
    Super Moderator Cindy Flood's Avatar
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    Re: GRDII/GX100 RAW: pink highlights - and a solution

    Simon,
    Thank you for this info. I use Lightroom and C1, so I will give it a try with Lightroom.

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