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Thread: Cast corrections using Photoshop

  1. #1
    Subscriber robsteve's Avatar
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    Cast corrections using Photoshop

    I am trying to figure out how to correct for lens shift color casts using Photoshop. I have shot a white target to get the cast, but I cannot figure out how to invert this cast from cyan to red and then apply it to an image.

    I am shooting a DMR with a 28mm PC lens. Imacon has not enabled the "Custom White" in Flexcolor for the DMR, so I need to do the correction in Photoshop. I asked Leica about this and after a few weeks they came back with "We don't know how to do it."

    Regards,

    Robert



    Last edited by robsteve; 17th June 2008 at 19:13.

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    Sr. Administrator Jack's Avatar
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    Re: Cast corrections using Photoshop

    It wont be perfect unless the correction frame was made with a translucent over the lens taken at the same time as the image, but you can try this:

    1) Drag your correction image directly on top of your building image. The correction frame will now be a layer on top of the image.
    2) With the correction layer active, go to Image>Adjust>Invert. You will now have a dark gradient on top of your image.
    3) Dial the opacity of that layer to 50% and set the blend mode to "Color".
    4) An optional step is to next increase saturation a bit if needed. (To get back to original, usually need around 36%.)
    The final image should be more neutral top to bottom. Here is what I got using your posted jpegs and steps 1 through 3 with no added saturation:
    Jack
    home: www.getdpi.com

    "Perfection is not attainable. But if we chase perfection, we can catch excellence."

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    Super Moderator Cindy Flood's Avatar
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    Re: Cast corrections using Photoshop

    I was doing it at the same time as Jack. I dialed the opacity of the inverted layer to 75% and color blend mode. I'm sure that you can probably get some improvement working on a raw file.

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    Re: Cast corrections using Photoshop

    part of the problem here is that the white part of the white exposure is desaturating the other parts of the image.

    I did something similar but more manually: I sampled your white exposure to get the color of the cyan drift, used that to fill a layer and inverted that. Put that layer over the image and set the blending to color as jack said, but them layer masked out the correction and just painted it back in.

    still it did not feel convincing, so I made a clipping color adjustment layer, you could use a curve or levels, same thing, which allows me to tweak the color of the blended layer. got it?

  5. #5
    rainer-v
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    Re: Cast corrections using Photoshop

    i really dont get it how anybody can be content with colorcasted images, does not matter if phase leica kodak sinar hasselblad or leaf stands on the tool.
    but it seems so that 99% of the users dont care, cause they dont see the casts. for me they often are visible even in normal motifs,- probably because i have trained my eyes to see them.
    if more shooters would realize them, since years would be out software solutions which work PRACTICAL ( means fast).
    i say this because the leaf and phase solutions are very complicate, although at least they offer a solution. the rest not. except brumbaer tools for sinar, which was designed to offer here a fast workflow and not only a basic functionality to calm down complainants.

    i came to this cast problem first time with my kodak 14n and with the kodak slr and searched a solution for this cameras. they used a kodak chip as leica ( and phase and hassy ) are doing now.
    this kodak chips are traditional the worthiest in terms of color shifts.
    for me the kodak slr simple wasnt usable ( and the dmr wouldnt be it either ) if i would not have figured out a workflow to remove them every time i wanted.

    so here it is ( sorry i cant say the exact names of the ps tools cause i have a german version only, but i think it should be clear what i mean ):

    1. you need a white plexi to make a white shot, just to capture in it the color shift , nothing more.
    you can buy it from sinar in form of 10x10cm or a round 72mm filter,or from leaf.
    or you have an old light-box and cut out a 10x10cm piece.

    2. before or after EVERY shot you take an exposure with the same setting as you shoot later,
    exposure should be two stops higher but use the same f.

    3. you develop this white shot in the same way as the motif in the raw converter and import it to photoshop.

    4. layer the white shot and make an overlay layer out of it.

    5. convert the file to LAB without reducing the layers

    6. set the tonal values of the luminance channel ( only) of the overlay layer to 128/128. no you see only the pure color shift in this layer without any vignetting.

    7. invert this layer. theoretical now the color shift will be removed to 100%. in practice you have to play a little bit with the layer. increasing the contrast of the layer will remove the shift more, decreasing it less. same happens if you higher or lower the saturation of the layer.

    ( 7b. in long time exposures denoise the layer or blur it. otherwise you multiply the color noise. you can blur it with a good amount, cause the color shift has no sharp corners or details. )

    8. after you are content with the result, merge the layers and go back to your RGB color-space.



    this method works perfect and with a bit of routine even fast.

  6. #6
    Subscriber robsteve's Avatar
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    Re: Cast corrections using Photoshop

    Rainer:

    I am not as proficient with Photoshop as others here and can't seem to follow your directions.

    How do I layer the white shot and make an overlay layer of it? Just open it and copy and past it onto my shot that needs correction?

    I think I can follow the rest of your directions, but I just can't seem to get it so the grad applies like a nd red. My background image does not show through.

    Robert

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    Re: Cast corrections using Photoshop

    Improving Photoshop skills is on my to do list. I'm heading down to Ben Willmore's one day seminar in Miami. A bargain at $99 ($79 if you're a member of a qualifying organization). Also, Lynda dot com, is an excellent source for tutorials on a variety of editing software.

  8. #8
    rainer-v
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    Re: Cast corrections using Photoshop

    Quote Originally Posted by robsteve View Post
    Rainer:

    I am not as proficient with Photoshop as others here and can't seem to follow your directions.

    How do I layer the white shot and make an overlay layer of it? Just open it and copy and past it onto my shot that needs correction?

    I think I can follow the rest of your directions, but I just can't seem to get it so the grad applies like a nd red. My background image does not show through.

    Robert

    you copy the white shot and paiste over the image which contains the motif. than you change the layer to "overlay" mode in the little pulldown menue at the left top of the layer palette. in overlay mode the image shines through.

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    Subscriber robsteve's Avatar
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    Re: Cast corrections using Photoshop

    Ranier:

    I have the overlay figured out now. How do I set the tonal values of the luminance channel?

    What I need is a grad without the white in it, just clear going from red to cyan.

    Robert

  10. #10
    rainer-v
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    Re: Cast corrections using Photoshop

    Quote Originally Posted by robsteve View Post
    Ranier:

    I have the overlay figured out now. How do I set the tonal values of the luminance channel?

    What I need is a grad without the white in it, just clear going from red to cyan.

    Robert
    convert the image to LAB mode:
    image/mode/LAB color ( option : dont flatten )

    than select the overlay layer and go to :
    image/adjustments/levels
    select lightness in the pull-down menue
    than bring the minimum and the maximum output value to 127.

    no just remains the color- layer.
    you can now subtract the shifts.

    there is one inconvenience:
    if the color temperature is far away from 5500kelvin ( daylight ) this method works not so good- because you always will end up with neutral color temperature. it is subtracted not only the color shift over the sensor field, also general color casts are neutralized, which will result in a changed color temperature.
    #so best is to convert the images with a fixed color temp. as the "daylight" setting or similar and make the color temperature/ grey point setting later in ps. everything has its price..... unfortunately.

  11. #11
    Subscriber robsteve's Avatar
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    Re: Cast corrections using Photoshop

    I am figuring out that they all have some drawbacks. The best I found was to use the selective color tool in Phocus and just change the color of the casts in the top of the image. The other methods seem to be killing the color in the rest of the image.

    I am going to try an early version of Flexcolor and see if I can shoot tethered and use the Custom White function. On the newer versions, Custom White is not available with the DMR.

    Robert

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    Re: Cast corrections using Photoshop

    Quote Originally Posted by Jack Flesher View Post
    It wont be perfect unless the correction frame was made with a translucent over the lens taken at the same time as the image, but you can try this:

    1) Drag your correction image directly on top of your building image. The correction frame will now be a layer on top of the image.
    2) With the correction layer active, go to Image>Adjust>Invert. You will now have a dark gradient on top of your image.
    3) Dial the opacity of that layer to 50% and set the blend mode to "Color".
    4) An optional step is to next increase saturation a bit if needed. (To get back to original, usually need around 36%.)
    The final image should be more neutral top to bottom. Here is what I got using your posted jpegs and steps 1 through 3 with no added saturation:
    I missed Jack's directions about adding saturation, so hadn't tried his method. Here is Jack's method, using a different file and with the saturation boosted.



    Last edited by robsteve; 18th June 2008 at 09:39.

  13. #13
    Sr. Administrator Jack's Avatar
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    Re: Cast corrections using Photoshop

    The critical step is taking the translucent white capture AT THE SAME TIME as the original image and processing it identically to the original. Using a "generic" correction is going to be at best a generic solution...

    PS: What we don't know is if the natural color of the building includes the pink cast at the base of the building and the blue cast at the top. Pink could be the original color of the stone and the blue could be from a copper roof shedding and staining the top portion over the years...
    Jack
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    "Perfection is not attainable. But if we chase perfection, we can catch excellence."

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    Subscriber robsteve's Avatar
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    Re: Cast corrections using Photoshop

    Jack:

    I balanced the camera to the light using a grey card. It was getting near dusk with the fog rolling in and I wasn't sure if it was the cast or the lighting. In the second picture of my last post, the colors look almost perfect on the sign on the lamp post and the other buildings around. Is suspect the stone was a bit pink and the stone higher up just may be stained and dirtier.

    Thanks for the help with the steps. I now need to go get a piece of white plexiglass so I can do the cast correction shot on the spot with the same lighting as the shots I am trying to correct.
    Robert

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    Sr. Administrator Jack's Avatar
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    Re: Cast corrections using Photoshop

    Capture Integration sells a nice little WB unit for pretty cheap.
    Jack
    home: www.getdpi.com

    "Perfection is not attainable. But if we chase perfection, we can catch excellence."

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    Subscriber robsteve's Avatar
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    Re: Cast corrections using Photoshop

    Quote Originally Posted by Jack Flesher View Post
    Capture Integration sells a nice little WB unit for pretty cheap.
    There is a small glass shop not far from my house. I think I will take a walk up there and see if they have any scraps small enough.

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