A look at Photoshop Layers, Part II
Layers is one of the most powerful features of Photoshop and I use them in every image. Here's one reason why.
I gave some background on layers in my last article “Introduction to Layers, Part 1.” I covered pixel layers in that article and I’ll show you the basics of adjustment layers here. (I have omitted fill layers. That’s for another tutorial.)
I’m using Photoshop CS3 but most image editing programs support layers. Of course, menus will be different in different programs and versions. And I’ve switched from a PC to a Mac so things will look a bit different, but I’ll try to cover both platforms.
What are adjustment layers?
Adjustment layers sit on top of pixel layers and alter colors and tonalities like a photo filter on steroids. But they don’t actually change anything in the pixel layers. They just change the way they are interpreted. By turning off the adjustment layers you can always revert to the original image without having to save multiple versions. That flexibility is a very powerful feature.
You can do most of the same things with adjustment layers that you can do with pixel layers. Some of the most useful are:
The Layers palette is where you work with layers. Remember that the active layer (the one your actions will affect) is the one highlighted. To highlight a different one just click it.
Let’s look at some of the basic adjustments using an image which was shot on a foggy day and is very flat. I might have done a little more to pump it up in RAW conversion but my main goal in RAW is to get a good histogram. You can see in the Histogram palette below that its ends just taper to the edges of the “well.”
The easy way to make a new adjustment layer is to click the icon at the bottom of the Layers palette that is a half-black half-white circle, circled in red in the figure below. You can also go to the menu bar at the top of the screen and click Layer > New Adjustment Layer and choose from the list.
Don’t use Image > Adjustments. That gives you the same choices as those from the Layer menu (and a few more) but those adjustments are made directly on the pixel layer itself. This is not good because if you want to modify an adjustment by doing an opposite one later, you will lose tonal values. You lose some of the 256 levels in the 8-bit histogram if a range of tonal values is darkened and then later lightened, or the reverse. This compresses a range of values into a smaller number of them and then stretches them out again (or the opposite), leaving gaps which result in a comb or picket fence look to the histogram. If this goes far enough, it causes posterization of the tonal values. This can be especially evident in an area with a limited tonal range such as a clear blue sky with a slight light-to-dark gradient that contains only a few tonal values.
Keeping things in order: first, black and white points
There is an order in which the basic adjustments should be made. The first thing to do with most images is set the black and white points. That allows you to precisely maximize the contrast (and then back it off to any lesser value you want), and to remove color casts from the shadows and highlights. You can then remove any remaining undesirable midtone casts. Going through a black and white point adjustment is a good way to evaluate an image for color casts, which can be elusive to your eye.
This is an important adjustment for many images, but many others will prove to be OK without it. It did little to improve this image so we’ll press on for now without using it. If you are familiar with it, by all means make it the first step in your adjustments. If you are not, going into the necessary detail is too much to put here. It will be the subject of the next tutorial. It is accomplished with one or more Levels layers after the darkest and lightest areas are identified.
This article is an overview of adjustment layers and if I wrote about each possible adjustment in detail it would be a book! Don’t worry; I’ll cover more aspects of adjustments another time.
Next, Levels again
After establishing the black and white point adjustments you can use another Levels layer to do more tweaking. You can also use Curves to do the same things but if you only do the basic adjustments you can do in Levels, there is no advantage in using Curves. I prefer to do adjustments in orderly steps, each different one represented by a new adjustment layer on top of the previous ones. Therefore I prefer doing the basic adjustments in Levels and then using a Curves layer on top of Levels when I need the additional control it offers for setting contrast.
The Levels adjustment lets you adjust the lightest, middle and darkest tones in the composite RGB channel (where you control overall lightness/darkness) and in the individual R, G and B channels (where you control color balance.) That gives you a lot of control over an image.
The Levels dialog box is shown below. The histogram will vary with the contrast of the image. To increase the contrast, bring the right and left Input sliders (circles in the figure below) to, or partway to, the points where the information in the histogram starts. To lighten or darken mid-tones, move the middle slider left or right. You can do this in the RGB channel that comes up initially to control overall lightness/darkness. Or if you do it in the individual color channels you can also control color casts in shadows, highlights and mid-tones, removing them or creating them. (Removing them in shadows and highlights is done more easily and precisely using black and white point adjustments.
To access the color channels click on the Channel window at the top of the dialog box (also circled) and do the same kind of slider moves in the individual channels.
As you move the end sliders you can hold down the Option key (Alt for PCs) and you will see the areas that are being clipped, that is, pushed to pure black or white. You want to minimize these areas in most images.
I didn’t move the end sliders in for this image as the histogram was already touching the corners of the “well” and the mid-tone darkness was OK. But I left the null Levels adjustment in the stack in case I decided I wanted to make any modifications later. (Conversely I could have deleted it and made another one later if I needed it.)
Further control over contrast
The next step I like to do is to use another adjustment layer to control contrast in ranges of tones. This image badly needs more contrast, but it needs to be confined to the mid-tones. The image can’t afford to have the darks and lights clipped, which would block up the very dark tones to full black and blow out the very bright tones to pure white.
The tool we need is Curves. It will let you leave the very important tonalities at the ends of the histogram untouched and let you work on ranges of mid-tones. Until you gain some experience, a Curves adjustment should only have two points, one for mid-dark tones and one for mid-lights. (Dark tones are at the bottom of the diagonal line, lights at the top.) A very gentle S-curve such as the one in the figure below will increase contrast in the mid-tones. In the tonal range where the curve is steeper the contrast is higher. Making it higher in one tonal range will make it lower in the others. (We can also adjust contrast in certain areas of the image, such as the hydrant, without affecting others. I’ll show you that below.)
Increasing contrast in an image usually punches up color, but it doesn’t do much for this one. If it causes too much saturation you can go to the top of the Layers palette and (making sure the Curves layer is highlighted) change the mode to Luminosity.
Conversely, you can lower contrast with an inverted S-curve, where the lower point is pulled up above the diagonal line and the upper point is pulled below it.
You can also lighten or darken mid-tones by moving both points up or down the same amount so the mid-point of the curve falls above or below the reference straight line. Above or left is lighter, below or right is darker.
The tones where the steepness of the curve is highest have the most contrast. In the example above the contrast is centered in the mid-tones. A curve like the one shown below would increase contrast in the lighter tones.
You can hold the mouse button down (or touch your Wacom pen to the tablet) and move the cursor around in the image and you will get a moving point on the curve showing where a given area lies in tonality. I did this to find the range of tonalities the top of the hydrant lay in, so I could try to increase its contrast.
This correction helps a little, but we still have some way to go.
If I want to adjust one of the colors without affecting others, such as tweaking the color of a blue sky, I usually do a Selective Color layer next. But in this image at this point I don’t think I want to do that. I can do it after the next step if I change my mind.
The last adjustment layer I usually do is Hue and Saturation. I do it last because saturation has been improved by the previous steps in which contrast was optimized. Since over-saturation will cause loss of detail in brightly colored areas, you don’t want to go too far with it. It can easily blow out one channel (or two), pushing their lightest values against the right wall of the histogram. Always watch the Histogram palette as you work. You can set it to show the individual color channels as well as the composite RGB values by clicking the options list in the upper right of the Histogram palette.
Here is the first attempt to increase saturation for this image. The changes are not easy to see looking from one figure to another, but as you are making adjustments, click the Preview check box off and on and you will see the changes clearly. Or after the fact, click the eyeball thumbnail of an adjustment layer to see a before and after view.
The leaves are starting to look a bit garish and I couldn’t go much further with saturation without starting to overdo yellows. (Things that look green are often actually yellows.) So I left the Master slider at +28 and clicked the Edit field at the top left of the dialog box and chose each color channel except yellow, one by one, and further increased the saturation in each as far as looked good. Below you can see I brought up the blues a lot. To better see what I was doing I exaggerated the effect, pushing the slider far to the right, then backed off.
If I decide later the green leaves are too overpowering, I’ll come back to this adjustment layer and back off the yellows. But first I’ll adjust the rest of the image. The leaves may look OK when I have done a little more work. That’s the beauty of adjustment layers; you can tweak them as an image progresses.
The hydrant looks drab compared to the background. I’ll select it and see if I can improve it. You can often do wonders with the Magic Wand or the Quick Selection Tool. The latter worked well in this case. For a natural look to the adjustment, I made sure to feather the edges slightly and then shrink it a little, since feathering expands the selection by half the feather amount both inside and outside the original edge. When these tools don’t do a good job I use the quick mask method, which I showed in my tutorials titled “Virtual Fill Flash”, “A Split Neutral Density Filter on Steroids” and “Refining Selections.” It is a very powerful technique with which you can make a complex selection with varying softness of the edges in different areas.
A wonderful thing about adjustment layers is that when you make one with a selection in place, the selection is incorporated as a mask. So in this case the adjustment will only apply to the selected area, the hydrant. So with the selection in effect I made a Curves layer and did an increase in contrast combined with darkening as shown below. You can see the mask for the Curves layer below, circled in red.
But this gave the bottom of the hydrant too much contrast. Making sure the masked layer was still active, I took a very large, soft brush, with the foreground color set to black, to paint over the bottom part of the white area in the mask (which is like a stencil.) But I reduced the opacity of the brush to 40 percent so I would subtly reduce the intensity of the effect, not block it completely. Then I painted over the bottom of the hydrant, as shown below. (The very large diameter brush is a bit hard to see against the colors in the image.)
Here is what the mask looks like after I brushed over it. It started with no gray areas. To see the mask like this, Option-click the mask thumbnail in the Layers palette. (PC: Alt-click.) Then do the same to toggle back to a normal view.
With the amount of contrast I have created the image now shows a slight red-magenta cast which I hadn’t noticed when its contrast was lower. I clicked on the adjustment thumbnail of the Levels adjustment layer, the first adjustment I did, to re-open the dialog box. Then I clicked on the gray eyedropper (circled in red) and clicked several areas I thought should be neutral and when I found one that gave a result I liked I clicked OK.
Here are the before and after views:
You can gain valuable flexibility by stacking adjustment layers of the same type that do different things, because you can then modify them independently. But be sensible about using too many adjustment layers of the same kind. For example, if you made a Levels adjustment that darkens the image and you later decide it is too dark, instead of making another layer to lighten it, just re-open the dialog box for the offending layer and back off the settings. Or you can reduce the opacity of the layer. It isn’t strictly necessary to do things this way but an orderly approach makes it easier to keep track of what you have done. Having two adjustment layers at cross-purposes doesn’t bother Photoshop, but to the human user, simplicity is going to give you the control to create the most elegant adjustments.
Excessive flattening will get you nowhere
Another caution: you sometimes see people advocating frequent flattening of images, or sprinkling merged layers in the Layer stack. A merged layer is a pixel layer that represents a flattening of all the layers beneath it. It is created immediately above the active (highlighted) layer by the holding Cmd - Opt - Shift then hitting N and then E (PC: Ctrl - Alt - Shift.) Such a layer increases the file size by the amount of the original Background-only image, and is rarely necessary. Each adjustment layer already affects the entire stack below it as though it were flattened.
There are many more things you can do with adjustment layers. I use them in virtually every tutorial, so check out some of the others and see what you can discover.
Diane Miller is a widely exhibited freelance photographer who lives north of San Francisco, in the Wine Country, and specializes in fine-art nature photography. Her work, which can be found on her web site, www.DianeDMiller.com, has been published and exhibited throughout the Pacific Northwest. Many of her images are represented for stock by www.MonsoonImages.com.