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More on Photoshop Levels

Last week I covered the basic 1-2-3 of adjusting Levels in Photoshop. For many images, this will be all you'll need. If you're looking for more advanced Levels techniques though, allow me to offer a couple here.

Adjustment Layers

What happens if you change your mind after finishing a Levels adjustment? The changes you've made are now part of the image. Yes, you can use Edit >> Undo, or click on a previous state in the History palette, but anything else you had done in the mean time will be lost at the same time. Once you save the image and re-open it, you can't undo your Levels adjustment no matter what.

What would be nice is if you could keep your Levels settings as "live" adjustments that you could return to any time you wanted. Imagine if your choices were still there, just as you left them and could be tweaked later, at any time in the future. Wouldn't it be great if they actually got saved with the image, so you could even tweak them in a later Photoshop session? Well, guess what? That's precisely what Adjustment Layers are all about.

Photoshop CS Layers palette with Levels adjustment layerAs with many things in Photoshop, creating an Adjustment Layer can be done in more than one way. From the menu, you can do so via Layer >> New Adjustment Layer. Or, from the bottom of the Layers palette (top of the Layers palette in Elements), click on the small, black and white circle. In either case, you will find a number of different types of Adjustment Layers listed in the pop up menu. For the purposes of the task at hand here, select Levels and you will get the same Levels dialog we looked at last week.

Photoshop Elements 3.0 Layers palette with Levels adjustment layerMake your adjustments the same way you normally would, but keep in mind that you can return to them later if need be so you needn't obsess over them as much as you might when done the traditional way.

Once you click on "OK," you'll get a new entry in the Layers palette for the Adjustment Layer. In it will be two linked rectangles. The one on the left is your Levels adjustment. If you double click on it, it will open back to exactly how you left it and you can tweak the settings as often as we want without lessening image quality since Photoshop dynamically applies the adjustment on the fly rather than committing your changes every time you close the dialog. The rectangle on the right of the Layers palette entry is a Layer Mask, something we'll defer discussing until a future article.

You can attenuate the overall effect of an Adjustment Layer by changing its opacity at the top right of the Layers palette. Opacity defaults to 100% but can be lowered to any desired level. With the Adjustment Layer selected, click on the control and move the slider as needed or simply overtype the number to whatever you want. If you prefer, you can also change the number by clicking on the Opacity control and using the up and down arrow keys on the keyboard. If you lower opacity all the way to zero, it will have no effect at all on your image. As with a regular layer, you can also turn off an Adjustment Layer by clicking on the "eyeball" icon to its left.

If you have multiple stacked Adjustment Layers, Photoshop will logically combine them all together, applying them all at once rather than one at a time as would be the case if you did the equivalent edits directly. This minimizes data loss from rounding which is normally one of the main causes of image degradation. While working in 16-bit mode makes images more forgiving of data loss, Adjustment Layers actually help prevent loss in the first place. Used together, you can keep your images looking their best.

This added capability doesn't come for free though. An Adjustment Layer will increase the size of an image file somewhat, but not as much as an additional image layer. The added size is usually an equitable cost to get the most out of your images.

The Relationship Between Levels and Saturation

An image using the RGB color model doesn't store luminance (brightness) and hue (color) information separately. Any time we change the red, green and blue values, we affect both brightness and color saturation. Sometimes, as when correcting for the reduced contrast that results from shooting on a hazy day, this can be desirable. But we don't always want to affect both. Many times we only want to correct exposure issues and messing with saturation at the same time will change the appearance of our image in ways we don't intend. Granted, we can decrease saturation after we adjust Levels, but this seems a rather clumsy solution. We need a better way.

Some sources advise converting your image to Lab color mode before adjusting Levels. Lab mode is a way of describing color entirely differently than RGB does. Developed by the Commission Internationale de l'Eclairage (International Commission on Illumination), it still uses three color channels, but not for red, green and blue. Instead, the "L" channel isolates all the luminance (brightness or darkness) information in the image while the "a" and "b" channels contain the color information. The "a" dimension describes where a color is along the red-green axis while "b" describes the blue-yellow color balance. If we convert an image to Lab color via Image >> Mode >> Lab Color we can then adjust Levels on only the "L" channel and avoid color change issues. Unlike the full version of Photoshop, Photoshop Elements doesn't support changing to Lab color; neither does it support operations on individual channels. Levels done with Normal blending modeThis isn't a problem though since this whole method of avoiding color changes isn't really necessary.

There's a better way that goes by the name of Blending Modes.

At the top left of the Layers palette is a drop down list that normally says, well... "Normal." This control is used to change the Blending Mode for each layer in an image. Blending Modes determine how each layer is combined (blended) with the one underneath it. There are quite a few values that I'll go into in more detail in a future Phototip, but for now, look at the very bottom of the list where you will find "Luminosity." If we change the Blending Mode on an Adjustment Layer to Luminosity, the choices in it will only affect how bright the layer underneath it is, leaving its hue and saturation unchanged.

Levels doen with Luminosity blending modeIf you want to save the space an Adjustment Layer adds to your file and prefer to adjust levels directly from the Image menu, you can still do it in Luminosity mode provided you do so immediately after finishing with Levels. Under the Edit menu will be an option for "Fade Levels" that will let you select the Blending Mode. As its name implies, you can also fade the effect of your Levels change by altering the Opacity here as well, the same as can be done for an Adjustment Layer. You still only get this one shot at Levels when you do it this way. Once you do any other adjustments, your change is committed and can only be undone by reverting to a prior History state.

Next week start to look at Curves adjustments in Photoshop. Many people are somewhat intimidated by Curves, but they're really just fancy but powerful version of Levels, so I hope you'll stick with me.


Date posted: December 19, 2004

 

Copyright © 2004 Bob Johnson, Earthbound Light - all rights reserved.
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