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Introduction to Curves

From: Photoshop CS6 Image Optimization Workshop

Video: Introduction to Curves

The Curves Adjustment in Photoshop has a reputation as being very difficult to use. But I think in large part, that's because it's not exactly intuitive and it's not always understood. But once you understand the basic concepts behind curves and how you can configure the curves interface to suit your preferences. I think you'll find that it's not quite as intimidating as you first thought. Let's take a look at some of those settings. I'll start by adding a Curves Adjustment Layer. So at the bottom of the Layers panel, I'll click the Create New Adjustment Layer button, and click on the Curves Option in the popup menu.

Introduction to Curves

The Curves Adjustment in Photoshop has a reputation as being very difficult to use. But I think in large part, that's because it's not exactly intuitive and it's not always understood. But once you understand the basic concepts behind curves and how you can configure the curves interface to suit your preferences. I think you'll find that it's not quite as intimidating as you first thought. Let's take a look at some of those settings. I'll start by adding a Curves Adjustment Layer. So at the bottom of the Layers panel, I'll click the Create New Adjustment Layer button, and click on the Curves Option in the popup menu.

That will add a Curves Adjustment Layer and also give us the curves controls in the Properties panel. Now, of course at first glance here, things might look a little bit intimidating. We've got a lot of buttons, a lot of controls, and it's not exactly clear where to get started. With many adjustments in Photoshop, you are able to simply move a slider left or right, or perhaps several sliders. And you'll see a very clear indication of the change within the image. Here, things are slightly more complicated. But once you understand the concepts involved, I think you'll agree that curves is both powerful and reasonably easy to use.

Let's first take a look at the overall concepts behind curves. Curves operates on the notion of before and after values. The gradient along the bottom of the curve display here is our before values. So for example, we've got black on the left and white on the right, with all the shades of gray in between. And those represents the brightness values within our image. The after values are shown as a gradient along the left edge, with black at the bottom and white at the top. And so, we can actually read the curve, you'll notice that the curve by default is not a curve at all. It's a straight line that goes from the bottom left corner up to the top right corner.

If we go to the bottom gradient, let's start at middle gray. If we follow straight up until we get to the curve, and then move straight to the left, then we can read the difference between the before value and the after values. So, in this case, a pixel in the image that had a middle grey luminance value still has a middle grey luminance value. And that's because that curve has not been modified. If I drag the curve upward, you'll see that the image gets brighter. If we follow the middle grey, once again we'll start at the bottom gradient, the before values, and move upward to middle grey until we intersect the curve. And then move directly to the left,and we'll see that we're ending up on a brighter shade of gray then we started with.

That's reflected in the image of course, and we can also just look at the overall curve. And see that all points, other than the black end point and the white end point, the curve is higher than it's original starting value. Notice by the way, that the change is greatest in the area where I grabbed that curve. That's where we're focusing our adjustment with it gradually tapering off to other tonal values within the image. Conversely, if we drag that curve downward, w'll see that the image gets darker. And that's because our before values are now aligned with darker after values.

And of course, we can simply see that the curve is below its original starting point. I'll go ahead and reset the curve to show you a couple of other concepts here. If I increase the steepness of the curve, In other words, the curve goes uphill at a steeper rate than the original curve. Then we've increased contrast in the image. That's because we're effectively compressing the tonal range for the image. Now, pixels that had been some shade of gray in terms of luminance have been made pure black. And some values that have been a lighter shade of grey in terms of luminance are now white.

And so, we're enhancing contrast by making that curve steeper. If we make the curve more shallow, then we have less contrast in the image. And that's because now, our black value has become some shade of grey that's lighter than black. And our white value has now become some shade of grey that's lighter than white. If I make the curve go downhill, in other words the opposite of what we started with, then we have an inverted image. I'll go ahead and reset the curve again, and I want to illustrate just one basic concept that relates to one of the greatest benefits of using curves.

Here, you can see a classic S curve. You'll notices that the curve looks a little bit like an S that's been stretched out a little bit. And with this curve, I'm increasing contrast for the mid tone range. You can see that that portion is steeper than it originally was. And I'm tapering off that adjustment out to the white point, and the black point. So, I've enhanced contrast within the image without sacrificing shadow or highlight detail. So that just gives you one small sense of the benefits of working with curves. So now that we've seen some of the basic concepts related to how curves affects the image and how you can sort of read the curve. I want to show you some configuration settings.

I'll go ahead and reset the Curve. And then, I'm actually going to adjust the individual channels here. Don't worry about what I'm doing. I just want to be able to set things up so that I can illustrate some of the other concepts related to curves. There we go, we'll call that good enough. Well, in fact, I'll just brighten this up. I just want this curve to be away from the others there. So again, don't worry about what I've accomplished. Obviously, in this case, it has just ruined the image. But I want you to be able to see some of the concepts as I present them to you. On the panel popup menu over to the right of the tab for the Properties panel, I'm going to click and choose Curves Display Options.

That will bring up a dialog where we can adjust the display options for Curves. The first option is to show the amount of light versus pigment. Well, as photographers, I think we're always thinking about light. And so, more of something means more light and therefore a brighter image. If we chose Pigment, we would be thinking about ink on paper. All that really means is that we're inverting the shape of the curve, so we would drag downward to brighten and upward to darken. I think that's backwards. And so, I'd rather leave that option set to light. We can also display channel overlays.

And that's why I adjusted the individual color channels. Don't worry about how I accomplished it or what it's done to the image. Just realize that with this option turned on, I can see the individual color channels. This can be a little distracting if you apply some complicated adjustments to your image. But it also enables you to see exactly what's going on even if you're not on an individual channel. If I turn that option off, you'll see that I only have my RGB channel at the moment. For example, I prefer to be able to see all of those. The Histogram Display appears behind the curve, and that the enables us to see at a glance, the initial tonal distribution for the image.

Bare in mind that this Histogram does not update based on changes you apply. The Histogram represents the image before you applied your adjustment. With the Baseline check box turned on, you can always reference that baseline to see is the curve higher or lower, is it steeper or less steep, etc. And the final check box is Intersection Line. And that just means, as you're adjusting anchor points on the curve, do you want to see a line connecting each of those gradients? Making it a little bit easier to read while you're working.

And finally, we can also adjust whether we want a grid to show in 10% increments or 25% increments. In theory, I would want to use the 10% increments because it will make me feel like I have more control. In actual fact, it does not change the behavior of curves at all. And I actually find the 10% grid gets a little bit distracting, so I leave that option set to the 25% increments. I'll go ahead and click OK. One last thing I want to show you related to the presentation of curves, relates to the Histogram. You'll see I have a little warning icon over here to the left. And that indicates that current Histogram is not based on the actual image data, but rather based on cache data.

If you'd like to update the Histogram, you can simply click that button. The vast majority of the time that's going to make very little difference at all, but I just wanted you to be aware of why that button exists. That takes care of the overall concepts related to how curves works and how you can configure the interface. With that information, I think you'll find that as you work with curves, you'll gain confidence. And start to better understand how you can use this tool to really optimize your photographic images.

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Photoshop CS6 Image Optimization Workshop

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