Tone curve info/articles?

Discussion in 'Image Processing' started by jloden, Dec 10, 2012.

  1. jloden

    jloden Mu-43 Hall of Famer

    May 15, 2012
    Hunterdon County, NJ
    Jay
    I've been monkeying with Lightroom and post-processing a while but without any real guide or plan to follow - which means some areas I don't know about, don't understand, or just haven't used. One of those is the Tone Curve panel and it's various options. I can just play with the sliders of course but I'm not sure I fully understand the mechanics of what it's doing under the hood and why/when you would want to apply certain types of adjustments.

    Anyone know of any good reference articles/videos or books that address this topic? I googled up some but figured I'd ask here too. Ideally I'd like to understand *what* I'm actually doing and why it matters, rather than just tips on good settings.
     
  2. Vincen77o

    Vincen77o Mu-43 Regular

    123
    Nov 3, 2012
    St. Albans, Herts
    As no technical wizard has replied (curves can frighten people off) I'd ask if you also use Photoshop. I bought a very good book a few years ago, Adobe PS (CS3) for Photographers by Martin Evening. It has some very good guides on using curves, in fact he states that most adjustments can be made with Curves and Hue/Saturation.

    I use LR 3.5 where the curve panel is simpler than that in PS, not sure if LR4 is more advanced. I always try the Strong Contrast adjustment in most images to see if it makes a difference and if so I might manually adjust from there.
     
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  3. savvy

    savvy Mu-43 Top Veteran

    714
    Sep 28, 2012
    SE Essex, UK
    Les
    This is what Scott Kelby recommends as a first step in his book "The Adobe Photoshop Lightroom 4 Book for Digital Photographers". This book has lots on using the curves, and is definitely a good buy.

    Alternatively, you could search the resources/tutorials on adobe.tv
     
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  4. clockwise

    clockwise Mu-43 Regular

    38
    Apr 23, 2012
    New York, NY
    Brian R
    Sorry, I don't know of any online resources, but I do know what you're doing when you play with the curves. It's easier to explain if we imagine a grayscale image rather than a color one.

    Think of the tone curve chart as a Cartesian coordinate system, because that's what it is. The horizontal axis of the tone curve graph represents the input value for the pixels in your image before you make any curves adjustments. This means that each point on the horizontal axis corresponds to a value between zero (black) on the left side and 255 (white) on the right. The vertical axis represents the output value, which is the luminance value that Lightroom will assign to a pixel after you apply the curve to the image. Again, the values run from zero at the bottom of the axis to 255 at the top. In its starting position, the actual curve is a diagonal line running at a 45-degree angle from the origin (0, 0) toward the upper right corner, and ending at (255, 255). At any point along the curve as it exists in its initial position, you will find that the x and y values are the same. Middle gray is (128, 128). That's because you haven't made any changes. The output value will be the same as the input.

    When you place a point on the curve and relocate it, you are altering the output value for all pixels with that input luminance value. If you place a point on the curve at middle gray (128, 128) and drag it upward to (128, 150), you are telling Lightroom that you want all pixels with an input value of 128 to be brightened to a value of 150. In order to keep the image from looking crazy, Lightroom also increases the brightness of other pixels that have input values similar to the value of the pixels you've altered. In the previous example, pixels that were initially located at (120, 120), for instance, might be automatically remapped to (120, 142), and pixels at (110, 110) might be remapped to (110, 128). When you look at the graphical representation of this remapping, it resembles a curve or arc, which comprises the tone curve.

    So in the typical S-curve that most people use to increase contrast in their images, you grab a point on the curve that represents the shadows or three-quarter tones in your image, let's say (64, 64), and drag it downward to (64, 50) to darken those areas. The curve now looks like a smile, showing you that Lightroom has also decreased the luminance values of the rest of the pixels in your image to varying degrees depending on their distance from the pixel you altered. You then grab a point on the curve representing the quarter tones and highlights, say (192, 192), and drag it upward to (192, 210) to brighten those areas. The curve resembles an "S" which shows that Lightroom has brightened the pixels with input values in the quarter tones and highlights, and darkened the pixels with input values in the three-quarter tones and shadows.

    The concept is the same for color images but applied to all three channels (RGB) or to the individual R, G, or B channels, if you choose to adjust them individually.

    I'm not sure how clear I made this, so feel free to fire back with questions if you have any.
     
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  5. jloden

    jloden Mu-43 Hall of Famer

    May 15, 2012
    Hunterdon County, NJ
    Jay
    Thanks - I do have a copy of Photoshop (birthday present from my wife) but haven't used it much yet solely because I don't know what I'm doing with it, haha. I'll check out Evening's book too.
     
  6. jloden

    jloden Mu-43 Hall of Famer

    May 15, 2012
    Hunterdon County, NJ
    Jay
    I'll have to re-read your post later tonight for details but I think I understand the gist of how it works - good explanation, thanks! I'll definitely let you know if I encounter any questions after I give this a longer look :smile:

    EDIT: actually, given 5 minutes with Lightroom open this all makes perfect sense now. I was going to ask how you know which point on the graph corresponds to a given tone but then I figured out the "adjust tone curve directly in the photo" tool :biggrin:

    Thanks again - that was a super helpful explanation and I think I "get it" now... pretty painless really. Just a matter of playing with it from here to get the hang of using it effectively.
     
  7. jloden

    jloden Mu-43 Hall of Famer

    May 15, 2012
    Hunterdon County, NJ
    Jay
    I have his Digital Photography series and got a lot out of them... actually I tend to go back and re-read them periodically because I learn something new each time after more experience.

    I've been thinking about picking up one of his Photoshop books but I don't think I realized he has a Lightroom book too. Thanks for the tip!
     
  8. savvy

    savvy Mu-43 Top Veteran

    714
    Sep 28, 2012
    SE Essex, UK
    Les
    I don't know if LR3.5 has the Targeted Adjustment Tool? LR4 has it and it's pretty flexible, you can click just on the part of the image you want correcting, drag up or down directly on your image, and adjust the curve only for that part - very powerful.
     
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  9. clockwise

    clockwise Mu-43 Regular

    38
    Apr 23, 2012
    New York, NY
    Brian R
    Happy to help!

    Also, I use CS5 and Bridge instead of LR, but in ACR you can easily add points to the curve that correspond to particular areas of the image by using the eyedropper tool. Just open the Curves dialog, equip the eyedropper tool from the tools palette, and hold Command (Ctrl on PC) while you click on the desired image area. You'll see a circle hovering over the curve to show you where the point will be placed, and when you click the point will show up there. You can adjust it using the arrow keys, which is much more precise than the mouse. I'd be surprised if there wasn't a similar feature in LR.
     
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  10. David A

    David A Mu-43 All-Pro

    Sep 30, 2011
    Brisbane, Australia
    I guess in a lot of ways you can consider the Curves tool a contrast control but with a lot more flexibility than you get in the Basic panel. What may not be immediately apparent to you is that there are actually 3 separate Tone Curve tools available in the panel, the parametric one that displays first, a point curve one with a lot more flexibility if you really want to go to town, and one for adjusting the individual RGB channels which allows you to do things like making tint adjustments and even getting some split toning and other toning style effects. There's a lot of scope in that panel but you're probably best advised to start with the parametric curve panel and get a grip on it first.

    One way of looking at it is as a graphical version of the Exposure/Highlights/Shadows sliders with more flexibility and which also allows you to change the luminance range controlled by each region. When you raise the curve in one of the 4 regions above the original diagonal slope in that region, you brighten the tones in that region. When you lower the curve below the original slope you darken that region. You also need to consider the angle of the curve slope. Making the angle more steep increases contrast, making it less steep reduces contrast.

    Finally, the Tone Curve operates on the image after the Basic adjustments have been made so it's a refinement for them. Try and get the image as close as possible to what you want using the Basic panel adjustments first, then tweak in Tone Curves. In fact you probably won't be able to duplicate the range of adjustments available in the Basic panel with the controls in the parametric Tone Curves though you will be able to do so with the point curve.

    Resources:

    Martin Evening's "The Adobe Photoshop Lightroom 4 Book" is extremely good and has a good explanation of Curves with examples. When I looked up the book on Peachpit Press's site it offered the free download of a chapter and the chapter I was offered was the Develop chapter which included that section. A friend recently looked at the site and he was offered a different chapter to it looks like there may be a bit of pot luck on what chapter is available for download at any time.

    The Luminous Landscape video tutorial package on Lightroom 4 is also extremely good.

    There are on line video tutorials available from a number of sources and you can just do a Google search for them. Quality varies depending on who does them but there are some good ones around.
     
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  11. My simple rule is: never, ever use a control that is labelled "contrast", and particularly not to increase contrast. Adjusting the curves is far more precise and far less destructive. It's also a way to adjust the overall luminance of an image without clipping or blocking the extreme parts of the image that are already close to pure white or pure black.
     
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  12. twokatmew

    twokatmew Mu-43 All-Pro

    Jun 1, 2012
    Lansing, MI, US
    Margaret
    I'd like to learn about the tone curve too. I'm reading Evening's LR 4 book now and am finding it very helpful, though I've not yet reached the Develop section.

    --
    Sent from my phone. Please pardon my brevity!
     
  13. Rig Vader

    Rig Vader Mu-43 Regular

    39
    May 24, 2012