Page 1 of 3 123 LastLast
Results 1 to 10 of 25

Thread: Here's a good color-correction Photoshop tip

  1. #1

    Join Date
    Mar 2001
    Location
    Seattle
    Posts
    626

    Here's a good color-correction Photoshop tip

    Hi guys, I thought I'd pass along a useful LAB color-correction technique that I use, that not many people know about. It's something that can't be done in RGB mode, at least without about a 15-step masking process that is not as accurate as a simple curves adjustment in LAB.

    The technique is to reduce the saturation of low-saturation colors, without affecting the high-saturation colors. In RGB this would be called a "saturation curve," which is a tool that does not exist in Photoshop.

    For example, in a photograph of a city, you want grey concrete things to be grey, not tinted with blue or purple or cyan or whatever, and you want your shadows to be neutral instead of blue or purple or red. But, you also want the blue things to be blue, and the cyan things to be cyan, etc. Using the standard RGB color tools, it can be difficult to remove the color from these neutrals while maintaining correct color balance and saturation overall.

    Being able to do this is especially important for users of transparency films, because of tranny films' high sensitivity to color. Tranny films will almost always put too much color in the neutrals and blacks, which can give the image an over-saturated look. If you reduce saturation overall in RGB mode, that will take color out of the things that are supposed to be saturated, making the whole image look soft and dead. The technique I'm talking about here will allow you to take the color out of the low-saturation greys while leaving the colored objects untouched.

    It's a simple curves adjustment in LAB, done only in the two color channels. Here it is: First go to the "a" channel, which controls the green-magenta spectrum. Lock down the center point (0,0), and then lock down a series of points up and down the line from 10 upwards, and -10 downwards. So you should have a point at 10, 10, then one at 14, 14, and one at 20, 20, and one at 24, 24. Then do the same thing with the negative values, placing points at -10,-10, -14,-14, -20,-20, etc.

    Now place a point on -4, -4, and change the "output" number to 0, so the point is now at -4,0. Place another point on -8, -8, and change that output number to 0. That will have the effect of taking everything in the image that is slightly green (green values of 0 to 8), and taking all the green out, without affecting the things that are very green (with green values of 10 or more, which are locked down).

    Now do the same thing to the positive side of the curve, which is the magenta side. Place a point on 4,4, and then change the output number to 0, and do it with 8,8 also. That will have the effect of taking everything in the image that is slightly magenta (with magenta values of 0 to 8) and neutralizing those to zero, without affecting any of the saturated magentas that have magenta values of 10 or more.

    Now anything in the image that had a slight green or magenta cast is perfectly neutral grey, while all the saturated greens and magentas are untouched. Because of the way that LAB works, this tweak will apply to the whole tonal scale at once-- it will take a tiny green or magenta cast out of the highlights, midtones and shadows simultaneously; everywhere in the image that has a slight amount of green or magenta will be neutralized to a color value of zero.

    Then you can do the exact same thing on the blue-yellow continuum on the "b" channel. The "b" channel adjustment is particularly useful because most tranny films throw too much blue into the neutrals and shadows. This tweak will fix that without taking any blue out of the things in the image that are supposed to be blue. It works especially well for images that have a lot of neutral greys, such as fog, cement, grey overcast skies, etc. Interestingly, after applying this curve, the colors frequently will appear more saturated, because you have neutralized the neutrals and now the colored objects will contrast more with the clean neutrals and the whole image will sparkle.

    And of course you don't have to do all the colors at once; you can correct only the blues this way if you want, or only the magentas or whatever. Just lock down the other side of the curve and work on the side you want to neutralize.

    If you do it as an adjustment layer you can fade it back to taste, and of course you also can adjust the curve to taste-- if 8,0 takes out too much color, then you can try 8,4 or so, which will be a softer version of this adjustment. You will have to flatten the image before converting back to RGB or the adjustment will be lost.

    I usually do this as a final step to make sure the neutrals are nice and clean before resizing, sharpening and printing.

    Cheers from Seattle,

    ~cj

    www.chrisjordan.com

  2. #2

    Join Date
    Feb 2005
    Posts
    2,947

    Re: Here's a good color-correction Photoshop tip

    Thanks Chris, that will save me much time in the future.

    I've played around a bit with LAB since reading "Canyon Conundrum", it is a powerful colorspace, but I have lots to learn.

  3. #3
    bob carnie's Avatar
    Join Date
    Jan 2004
    Location
    Toronto, Ontario,
    Posts
    2,679

    Re: Here's a good color-correction Photoshop tip

    very nice tip,
    Do you find and increase or decrease of contrast in these open shade areas with the colour sucked out of them?

  4. #4

    Join Date
    Nov 2003
    Posts
    1,218

    Re: Here's a good color-correction Photoshop tip

    I just tried the basic idea with the Gimp, which I use under Linux. (I will try it under Phtoshop when I boot into XP.) I work almost entirely with negative film, so I don't usually have oversaturation problems, but it is sometimes difficult to remove slight color tinges from neutral areas. The method does seem to work, but I do have a question. In reality, there is seldom a true neutral color in a scene. There are just elements that one expects to be neutral. For example, concrete on buildings can be just a trifle yellow while asphalt can be slightly blue or some other cool color. My problem is that the color tinge may vary over a surface and that doesn't look right. It is a challenge maintaining the subtle differences in different shades of grays and whites while removing any variations in tint on any single surface. Your trick may help by desaturating all colors close to being neutral, but it would have to be applied very carefully. Alternately, I could apply the technique to selected regions separately.

    Any ideas about this?

  5. #5

    Re: Here's a good color-correction Photoshop tip

    Quote Originally Posted by Leonard Evens

    Any ideas about this?
    Another way of doing this is to use the iCorrect Editlab pro pluggin, which works in lab and is an easier workaround and should do what you want.

  6. #6

    Join Date
    Mar 2001
    Location
    Seattle
    Posts
    626

    Re: Here's a good color-correction Photoshop tip

    Bob, one of the nice things about LAB is that color and density are controlled on separate channels, so when you do something to the color it has no effect on image contast or brightness. WHen working in RGB mode you can duplicate that particular effect by putting your layers in "color" mode instead of "normal" mode. Every layer I make in Photoshop is always either in "lunimosity" mode (which affects the brightness but not the color), or "color" mode (which affects the color but not the brightness).

    However, the saturation curve I talked about above can not be duplicated in RGB.

    Leonard, this issue doesn't come up as much when working with neg film-- I have found that neutral tones in negative film tend to be much more accurate, and I usually like the fine variations in color that appear in the neutrals. Negative film will show a slightly yellow concrete with a slightly blue shadow, which can look really beautiful, so when working with neg film I generally don't do this LAB correction. And it always has to be used carefully; otherwise subtle tones get lost even working with tranny film. But it is a good way to soften oversaturated neutrals and make an image look more natural.

  7. #7
    Founder QT Luong's Avatar
    Join Date
    Aug 1997
    Location
    San Jose, CA
    Posts
    2,193

    Re: Here's a good color-correction Photoshop tip

    That's a great tip Chris, and it answers the question of why you needed to go LAB space.

  8. #8

    Re: Here's a good color-correction Photoshop tip

    I'm having a tough time getting the 4,4 and 8,8 (-4,-4 and -8,-8) points in. When I click inbetween 0 and 10 the 10 moves. How do I avoid this?

  9. #9

    Join Date
    Mar 2001
    Location
    Seattle
    Posts
    626

    Re: Here's a good color-correction Photoshop tip

    You can click some points higher up on the scale, and then move them down by entering the numbers manually (key in the actual numbers), or by using the arrow keys to get the points down to where you want them.

    Once you get the curve set up, you also can hit the "save" key and save that curve so you can call it back up again. It's enough of a pain to construct that I did that awhile back-- just saved the curve as "Lab Neutrals" and then click on "load" every time and it pops right up.

  10. #10
    bob carnie's Avatar
    Join Date
    Jan 2004
    Location
    Toronto, Ontario,
    Posts
    2,679

    Re: Here's a good color-correction Photoshop tip

    Chris

    Funny enough I have two interns from a French photography school on my premise using our Lambda.
    One of the fellows is a big fan of yours., in fact he is shooting large format negative with lots of concrete and funny enough , colour casts in the nuetrals.
    I printed your first post, scratched out and indications on who posted and gave it to them.
    I wanted to see if they got it. immediately they dissected the notes and were applying the method you described in LAB and applying it to their test prints for final prints.
    It is amazing how the world has flattened and a post in Seattle is being used three hours later in Toronto by two very young and talented individuals.
    Thanks for the tip.

    Now I will get the two kids to teach me how to use it. this will only take me two years.

Similar Threads

  1. Good Used Monorail 8x10 Camera?
    By Ron Whitaker in forum Cameras & Camera Accessories
    Replies: 12
    Last Post: 4-Apr-2007, 16:13
  2. photoshop technical introduction
    By Matus Kalisy in forum Digital Hardware
    Replies: 14
    Last Post: 28-Apr-2006, 02:36
  3. Metering with EOS5/85mm, good enough?
    By bmgmusic in forum Style & Technique
    Replies: 8
    Last Post: 5-Dec-1999, 22:59
  4. Where is a good source to buy 4x5 film on the Net?
    By Robert Butts in forum Resources
    Replies: 5
    Last Post: 19-Oct-1998, 01:46
  5. Is a Super Graphic a good 1st LF camera?
    By Martin F. Melhus in forum Cameras & Camera Accessories
    Replies: 3
    Last Post: 16-Mar-1998, 18:45

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •