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Thread: Do you do color corrections using Curves?

  1. #21
    Member fastshot's Avatar
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    Default Re: Do you do color corrections using Curves?

    Color correction for color casts can probably be best controlled using "Lab Color" mode in Photoshop. There's a good book on this written by Dan Margulis. It is quite technical and the methods can be pretty intricate. But it does produce good results for the important pictures you want to perfect.

    Everyone knows that color corrections are highly dependent on a well calibrated monitor that can cover a wide enough color gamut, at least AdobeRGB or better. So it is strange that people will spent $2~3K on a great lens but won't spend that kind of money on a great monitor, calibrator, tablet, pigment ink printer, software etc.

  2. #22

    Default Re: Do you do color corrections using Curves?

    I think some of you are missing out the purpose of using a curves to do correction.

    It is very true that one can correct colour cast in various ways within photoshop.

    Yes, one can use eye dropper in Level, or Channel Mixer, or Color Balance. But do remember that these tools will cause colour changes across the entire spectrum and light levels of your photos.

    If you use a colour test chart, you will realise that sometimes, at different percentage of grey shades there are different amount of colour cast. By using Level's eye dropper. Channel Mixer, or Color Balance techniques, you will find it impossible to correct these colour cast as the moment you get a correction for neutral 8 grey, your value for neutral 5 grey will change accordingly.

    This is when using curves, you are able to correct colour cast in the various greys.

  3. #23
    vince123123
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    Default Re: Do you do color corrections using Curves?

    Oh just to add on one bit which I may have omitted. I use Capture NX's area sampling tool to get a feel on the rough correction. After that I fine tune using their R and B sliders in the same dialog box (there is no G slider). And that works for me.

    Hence if we translate that to PS, I guess what is equilvalent is eyedropper then colour balance; although Capture is better because you can see how much the software adjusted and readjust, and refine accordingly.

    Quote Originally Posted by David View Post
    Yes, I don't like the eyedropper tools. It's quite controversial. Some gurus say don't use it, others used simple color problems (I suspect) and showed they are effective tools. I find they never gave me consistent results. Sometimes even after I click on blacks and whites, I get weird colors. I never know what the software is doing.

  4. #24
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    Default Re: Do you do color corrections using Curves?

    Quote Originally Posted by David View Post
    Thanks Plato. Yes, I did try the Eyedropper method but haven't found much success except for simpler cases. Hope I'm not doing anything wrong?

    Anyway, hope you can kindly check my method:

    1. Use 3x3 average or point average eyedropper. (Some photographers recommend one over the other. I find little difference as long as I click as accurately as possible.)

    2. Set the approriate black, grey and white point values before clicking.

    3. Find the black-most area (excluding complete blacks) then click with black eyedropper.

    4. Find a white-most area (excluding specular highlights) then click with white eyedropper.

    5. If still not optimal, find a midtone area then click with grey eyedropper.

    If what I did above is correct, hmm,, sadly, I still couldn't get the colors right. By the way, as with my earlier posting, what do you do if you can't find an approrpriate midtone to click on? Trial and error? I find this to be tedious. I find setting the black and white points alone is insufficient.

    Images taken with available lighting where you must take care of the human flesh tones, plus the surroundings, presents the greatest difficulty for me.
    Hi David,

    It's a bit difficult to explain with words alone, so I've attached a screenshot to illustrate the process that I use when working with JPEGs (my first time attaching an image. Hope it works!). Other people may have other methods, so pick the one that works best for you. This method is a much more subtle and less brute force approach compared to simply using the eyedropper to select black or white points with a single click.




    If you can't see the image, please go to
    http://www.flickr.com/photos/photokalia/3146952020/


    Step 1
    Open Curves or Levels (it's available in either function) and doubleclick on the white point button.

    Step 2
    Position your mouse cursor (eyedropper tool) over an area that you know should be white. For stronger colour correction, choose areas that are less well exposed, since they will display a greater degree of colour cast. Once you have chosen your point on the photo, click the left mouse button.

    [IMPT] Be very careful not to move your mouse as you will need to click on the EXACT same point again later.

    Step 3
    Use the Tab key on your keyboard and tab your way until you get to the RGB values in the dialog box. You will probably notice that the R, G and B values are all different. RGB value of 255,255,255 would correspond to pure white, and 0,0,0 would correspond to pure black. In this example, the area I chose is not pure white, so I set the RGB values to 236 instead of 255 for all the 3 RGB values. This will correct the colour cast, but prevent the highlights from getting blown out (ie the top of the wall which is a lighter shade of white). Normally what I do is to either set all the RGB values to the lowest value of the 3, or the highest value of the 3. Eg if the RGB values turn out to be 100, 136, 157, I will either set all 3 values to 100,100,100, or else 157,157,157.

    [IMPT] Do not touch your mouse or use it to navigate to the RGB options.

    Step 4
    Still not touching your mouse, press the Enter key on your keyboard (Alternatively, use your Tab key on the keyboard and tab your way to the Ok button, and press space bar to activate the button).

    Step 5
    Very very carefully click the left mouse button (hopefully your mouse is still at the exact same location as before). You will see that the colour cast has been more or less corrected. If you are not satisfied with the result, redo the steps, but this time choose a different point in step 2. You can also do the same process using the Black point or Grey point button, but there is no need to do all 3 in the same image. Just use the one that is most convenient, ie because some pics may not have a white, or grey to use for correction, so you can use black. I normally use white. Once you have done this, you may get a pop-up asking if you want to set this as the default colour or something. Click No.


    Ok, I hope this helps. Just curious, is anyone else using this same method? I've tried various other methods, but this one seems to work best for me, though like I said, there are limits to how much you can colour correct an image that has a bad colour cast. Nowadays, I prefer shooting in RAW.
    Last edited by plato; 29th December 2008 at 03:39 PM.

  5. #25

    Default Re: Do you do color corrections using Curves?

    "Be very careful not to move your mouse as you will need to click on the EXACT same point again later."

    when using eyedropper just turn on the Caps Lock in your key pad - the eyedropper will become a cross hair. It will be easier to get the exact spot.
    Last edited by jopel; 29th December 2008 at 03:51 PM.

  6. #26

    Default Re: Do you do color corrections using Curves?

    I use Curves sometimes. Just drag slowly till I see it ok. A few trial & error needed. For me I try to do as little PP as possible.

  7. #27
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    Default Re: Do you do color corrections using Curves?

    For general Landscape/Architecture/Street shots, I don't use curve for colour correction. Only when I am doing skin tone adjustment then I will use curve to adjust the RBG (mapped to CMYK values). If you are not sure about WB, using Curve RBG can help you if you have real white and black points in the photo, once this is done properly, your WB will be superbly accurately.

  8. #28
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    Default Re: Do you do color corrections using Curves?

    Other than these, I will set the eyedropper tool to 5x5 instead of 1x1 for a more average sample.

    Quote Originally Posted by jopel View Post
    "Be very careful not to move your mouse as you will need to click on the EXACT same point again later."

    when using eyedropper just turn on the Caps Lock in your key pad - the eyedropper will become a cross hair. then use the up down arrow keys to move to the exact spot.

  9. #29
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    Default Re: Do you do color corrections using Curves?

    Quote Originally Posted by David View Post
    Just wondering... how many of you correct your images using curves? As in tweaking the individual Red, Green and Blue channels.

    I find this technique to be the most accurate but also extremely tough to master/control.
    If you're talking about white balance, it requires multiplying the three channels by individual adjustment factors. If your curves operate on linear RGB values, that is trivial to achieve using curves (all curves remain straight lines).

    However, if the curves operate on nonlinear colour coordinates (i.e. gamma-encoded data such as sRGB), this approach breaks down. To maintain uniform colour balance across all brightness levels would require applying carefully calculated curves. In other words, it's not practical.

    Correctly written "white balance adjustment" tools would have to take the nonlinear encoding into account. In this light, it's usually better to use them rather than mess around with the curves.

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