Adjustment Layers
This tutorial arises out of a conversation with M'lyn about my use of Adjustment Layers in collaging. And my realizing that a lot of people who dabble with layers may be making a whole lot of unnecessary work for themselves by not knowing what an Adjustment Layer is and how to use it... and so here, we are, a quick tutorial. (AD images are from Lush, thanks for the link Jenni Lou!)
7.1 - What's an Adjustment Layer?
An adjustment layer in Photoshop is a special sort of layer that -- surprise! -- allows you to adjust things, specifically, it allows you to use any of the color-altering affects under the "image->adjust" menu.
These are commands most people use (or should use!) quite a bit like: levels, curves, selective color, hue/saturation, posterize, etc. Usually, when you want to use one of these commands, you activate the layer you want to effect, call up the dialogue box for the one you want, enter your values and then the change you made becomes part of the image, exactly the same way as if you'd applied a filter.
But when you use the adjustment layer version instead, you get several key advantages:
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7.2 - I want an adjustment layer!
In the layers palette an adjustment layer shows up as a white layer with the name of the adjustment effect in it. In this example, it's the top layer, labelled "Hue/Saturation".
To create it, I simply control-clicked on the "new Layer" icon, marked in red above. You can also select "new Adjustment layer" from the Layer or Layer Palette fly-out menus. A dialogue pops up. Select the type of adjustment you'd like to make, play with the controls for that adjustment as usual, and click "okay".
7.3 - Unlimited Undo using Adjustment Layers
Any color adjustment you've made via adjustment layer can always be undone by simply deleting the layer. Or you can temporarily hide the effects by hiding the layer (click on the eyeball symbol on the adjustment layer).
If you decide you don't like the effect, you can double-click on the adjustment layer to bring back its dialogue box. Now you can tweak the effect to your heart's content. You can do that at *any* time, and as many times as you want, and it won't mess with any steps you've done since you first added the effect like using the history palette will.
You can also adjust the opacity of the adjustment layer just like you would any other layer. This works just like using the "filter->fade" command, except that again, you can do it anytime you want, and you can go back and tweak it whenever you'd like too.
You can stack different kinds of adjustment layers right on top of each other, and they'll interact just as if you'd done one adjustment the regular way, and then did the next adjustment, and so on.
7.4 - Mass Adjustment
I think all the details mentioned in 7.3 are reason enough to use Adjustment Layers like crazy, but there's one other big reason that's going to delight anyone who likes to work with many different images on different layers, and who has ever gone through applying a level change or color-balance to each one of those layers individually:
Adjustment Layers effect all the layers beneath them.
Let's take for example this image:
It's made up of 2 images plus a text layer, each on its own separate layer:
Let's say I wanted to tone down the saturation. Without adjustment layers, I've got 2 options: (1) I merge the layers into one and apply Hue/Saturation to desaturate or (2) I apply Hue/Saturation individually to each of the layers. But I'm lazy. I don't want to do it twice, or to use the load/save commands to ensure I'm applying exactly the same settings to each image. And I want to be able to reposition my layers later on, I don't want to merge them now. So I just stick an adjustment layer over both those 2 layers (see the illustration of the layers palette in 7.2) and viola! In one fell swoop, I get:
Notice how it effects the text layer too. And unlike the merge option, I have complete freedom to move my layers around and rearrange them in new ways. *And* if I decide I want to add a 3rd image to the composition? Just stick it in there, and the same adjustment will be automatically made to it as well. Not to mention all the other "undo" advantages mentioned in the last section.
With 2 images, redoing the saturation change on each manually isn't such a big deal, but take a moment to contemplate how much time and boredom you'll save yourself on an image with a dozen layers all of which need to have their levels adjusted, or to be tinted to another color, etc, and I think you'll join me in adjustment layer worship.
7.5 - Selective Adjustment
Remember at the very beginning I mentioned about the ability to adjust just a certain portion of an image? See, it turns out an adjustment layer is just like a mask. Paint black on adjustment layer, and you can hide the effect of the adjustment wherever you've painted. So for example, in this rather unattractive but hopefully illustrative example:
I've added a Hue/Saturation Adjustment Layer over the top of everything, adjusting the settings to pale everything out by dropping the "saturation" and boosting the "lightness". But then, I went back and painted with a black airbrush over the right-most Wes' face, bring back the original color in that area. Here's what my adjustment layer looked like:
Use your imagination, there's a lot you can do with that technique.
7.6 - Clipping Adjustment Layers
Sometimes, you may want an adjustment layer to affect a single layer, rather than all layers beneath it. In this case, you "clip" or "group" it with the target layer. When you clip an adjustment layer to a layer, it only effects that one layer, so you can make localized changes.
In PS5 (and probably later versions too), you can have the adjustment layer active, and then control-G to group it with the previous image. Or alt click on the line between any two layers, and they'll be grouped or ungrouped. Or when the "new adjustment layer" dialogue comes up, check the "group with previous layer" box.
A grouped adjustment layer looks like this in layers palette:
Note the little dotted line between the "Hue/Saturation" and "Scowling Wes" layers. I'm told there's an arrow indicator on later versions of PS. With the layers grouped like that, the desaturate effect I applied using the adjustment layer only effects the layer its been clipped to (scowly Wes):
Btw, this is similar to the technique I talk about in section 16 of the masks, masks, masks tutorial.
7.7 - Image Quality Issues
One last point, though I'm not sure if I've ever noticed the effects firsthand. I was reading my new "industrial production techniques" ps book, and they were talking about how every time you do a regular color adjustment, you're damaging the image data just a little, so that doing one big color adjustment is better than doing a million different little tweaks. They were talking about print, which is probably a lot more sensitive to that, but still it makes sense. They said that using adjustment layers however is the equivalent of doing just one big adjustment, even if you stack all sorts of different ones on top of each other (which is half the fun!) because the calculations are just all merged together.
7.8 - Conclusion
Go forth & Adjust!