Musings on Photography

Local Adjustment with Curves

Posted in photoshop, technique by Paul Butzi on April 4, 2008


From the comments on this post:

Suppose you want to lighten or darken an area of an image … with curves, you either make a precise selection using various methods and then make the curves adjustment, or you make a curves adjustment and then create a mask to apply the curves adjustment. Or … with Dodge and Burn, you make a curve/mask adjustment in one step, ie: the darkening or lightening applies only where you put it. What a simple way to add a vignette.

The key words here are ‘precise selection’. They’re key because I never (what, never? Harrrrrrdly ever!) use the lasso tool and the other ‘selection’ tools.

What I do is use quick mask mode to quickly ‘paint’ a rough selection. To do this, I hit ‘q’ or click on the button with the round circle on it (below the color swatches in the tool palette) to enter quick mask mode. Then I pick the right size brush, set the flow rate to whatever I want, and just paint on the image where I want to make the change. If I want to change a large area, I’ll select it and use the ‘fill’ paint bucket to paint it solid. Toggling the color between white and black lets me fix up mistakes easily. And when I have some area painted, I switch back to regular mode by hitting ‘q’ or that button again. The not painted area is now the selection. To make the painted area be the selection, I hit shift-cmd-i. That lets me make a rough selection in just seconds.

From here, I create the curves layer. The selection is now the mask for that layer, and I rough in the curve adjustment I want. It can be anything from a straight burn or dodge to a complicated curve or color adjustment.

Best of all, as I adjust the curve, I can watch the image change (I have ‘preview’ checked). This is important not because I can stop when it’s right, but because I can make sure I’ve gone too far. This is a trick I learned in the wet darkroom. It’s easy to make an adjustment, think it’s about right, and then discover later that you weren’t bold enough. The solution to this problem is to always go past the optimal change. Then use the opacity slider on the layer to ‘dial back’ the effect until it’s just right.

Now, I can look at the image with the edit in place. If there are places where I want the change applied but didn’t get in the rough selection at first, I just click on the mask, select black as the foreground color, and set the flow rate low (say, 5%) and then paint on the image. I’m actually painting on the mask, so I’m actually changing the area the mask controls. If I go too far, I switch to white and correct the mistake. It’s pretty easy to get it right.

Having the burn/dodge or whatever other adjustment be a layer is a big help, because I can toggle the layer on and off. That lets me make sure I’ve made a positive change, because I can easily test “Which do I like better – A or B?”

Hours (or days, or years) later, I might decide I want to adjust that burn or dodge. If it’s a layer, it’s childs play to see what I did before (I can SEE the layer mask) and to change it. If I’d just burned and dodged the bejeebers out of the image, I’d have no way of easily seeing what I’d done.

Sometimes I’m very confident I know what the curve shape I want will be. In those cases, I create the curve layer first. By default it applies to the entire image, so I set it with the preview turned off. Then I click on the mask icon on the layer palette, so that my subsequent actions paint on the mask. I then fill the mask with black so that it doesn’t apply anywhere. Then I select white as the foreground color, pick a brush, and start painting the effect onto the image. With a low flow rate, you can build up different levels of effect, so it’s easy to feather the effect.

A lot of the editing of images I do involves what I think of as a gradient mask. In the wet darkroom, you can do burns and dodges with a gradient by moving the burn/dodge card or tool. In Photoshop, I get the same effect by painting a gradient onto the mask for the curve that adjusts the image. You can get some of the same gradient effect by creating a soft light layer, filling it with neutral grey, and then painting a grey to white/black gradient onto it to do a burn/dodge. But again, the curves layer is usually just as convenient and a whole lot more powerful.

In Photoshop there are usually four or five ways to do the same thing. Dodge & Burn is just another tool and another way to apply a simple, quick adjustment. Like all tools, it works in some situations, but not others. This is a bit like debunking the Levels command, because you can do the same thing in Curves.

Sure. There are more ways to skin a cat than there are cats. My goal here isn’t to get the rest of the world to always tackle things the way I do. It’s to argue that if we’re only to have ONE tool, at least let it be the one which is vastly more powerful, instead of the one which can handle only a small number of simple cases.

And, for what it’s worth, when I’m teaching students to use Photoshop, I tell them to ignore the levels command. Better by far, I tell them, to pick the one tool which does it all, and master that tool. The curves interface is vastly better designed that the ‘levels’ interface, and does everything that levels does easily. By using the more powerful tool, students learn to think of the image contrast and tonal range in the context of the tool they’re going to use to adjust it both globally and locally. It just doesn’t make sense to learn a number of tools that each handle a special case (levels, brightness/contrast, burn/dodge) when they can learn one tool that gives them all those sub-tools with a uniform, easily mastered interface.

In my experience, the problem most people face with Photoshop is not that the task of editing images is hard. It’s that Photoshop offers such a complex and overwhelming set of tools that people can’t get a toe-hold on the thing. The trick is to explain that there’s a very simple subset of tools that offers them ALL of the editing power, and all they need to do is learn those tools and ignore all the rest. The primary editing technique I teach is curves layers with masks.

6 Responses

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  1. Martin Doonan said, on April 4, 2008 at 2:14 pm

    I agree with all that. This sort of arguing was one of the key reasons I went to Lightzone. Why can’t Adobe (or others) provide a stripped-down version with only the key tools at a price that I’d like?
    Maybe the reason I like to master a limited number of tools is laziness – less to learn for more overall gain.

  2. Mike Mundy said, on April 4, 2008 at 3:29 pm

    Hmmm . . . what if the subject is, say, palm fronds against a sky? A LOT of palm fronds, thin and lacy. And you just wanted to select the palm fronds? Wouldn’t the painting method be too tedious?

    Or maybe I’m missing something . . .

  3. Paul Butzi said, on April 4, 2008 at 3:58 pm

    Hmmm . . . what if the subject is, say, palm fronds against a sky? A LOT of palm fronds, thin and lacy. And you just wanted to select the palm fronds? Wouldn’t the painting method be too tedious?

    Why would I select just the palm fronds?

    Let’s suppose (just for argument) that I wanted to make the palm fronds lighter and leave the sky alone. I’d select the whole region that included palm fronds. Then I’d add a curve layer, and I’d pin down the part of the curve that mapped the tones of the sky (but presumably not the palm fronds), and I’d adjust the part of the curve that mapped the tones of the fronds (but not the sky).

  4. Dan L said, on April 7, 2008 at 7:00 am

    Paul … thanks for your reply … I agree that Curves combined with a mask is the most powerful tool.

    Also, for selecting the palm fronds in preparation for a Curves adjustment, there is a wonderful tutorial over at the Luminous Landscape called, “Tough Selections Made Easy”. The link is here:

  5. Mike Mundy said, on April 7, 2008 at 3:58 pm

    Thanks, guys!

  6. Rosie Perera said, on April 8, 2008 at 1:51 pm

    I often use the magic wand tool or Select Color Range for stuff like this, and then fix up the selection if necessary with the lasso tool. I have encountered situations where the tone of the part I wanted to darken or lighten up was so close to the background tone (precisely the reason why I wanted to do some dodging/burning, to make it stand out better from its background) that there seems to be no way around doing a fine-tuned selection.

    Julianne Kost, in her Photoshop CS2 Fundamental Techniques training videos (she’s got a CS3 version out now too, but I’ve been working with the CS2 ones), makes selection look like a piece of cake. And with practice I’ve gotten to the point where it is pretty much a piece of cake.

    I guess I’m also the type of person who delights in the fact that there are multiple ways to accomplish the same thing in Photoshop, and I often mix and match between the different ways. I was the kind of kid who in elementary school math loved to figure out 6×9=54 both by memorizing it and also by realizing that it was 6 less than 6×10. I’m still that way.

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