Robert D Feinman

Photoshop Overlays

There are lots of ways to do the same thing in Photoshop. This allows "experts" to produce a seemingly unending supply of tips. So many, in fact, that one gets confused as to what to use.

Here is a step chart with equal steps on the top and the same chart with a duplicate layer set to screen mode and 100% opacity below it. This technique is frequently offered as a way to fix under exposed images.

The number over the steps show the RGB values for the original and the screened version. Notice that there is a considerable lightening of the dark values and a slightly lesser amount for the lighter ones.

Original Image
What the overlay has done is to alter the tonality gradient. So we should be able to achieve the same effect using a simple adjustment curve.

I created an adjustment curve layer and while it was opened I control-clicked on a series of steps in the original step chart to create control points. For each control point I edited the output value so that it matched the one in the screened curve.

Since this is a fairly smooth curve, it was not necessary to put a control point for each step. As a matter of fact Photoshop limits the number of control points so this would not be possible in any case.

This is what the curve dialog looked like after making all the adjustments.
Curve Dialog
This is the image after the curve adjustment layer has been applied to the upper step chart. Notice that the two step charts are now the same.

Is there any advantage to using one technique over the other? Well using an overlay layer can be faster. In addition you can easily try different blend modes and opacity percentages.

Using curves, however, allows you to make more precise changes. For example instead of having step three go from a value of 25 to 48 I could make it 47 or 49. This would not be possible with an overlay.

Another reason to use curves is that you can see what is happening to the image gradient explicitly. This will improve your understanding of tonal editing and enable you to make the right choices for other images. You can figure out what is happening by making your own step chart and measuring the changes.
after curve
After Curve
These types of tonal adjustments also apply to color changes. The new Photo Filter color balance tool falls into this class. It is just necessary to adjust each of the color curves individually.

Many of the third-party filters also fall into this class. What you gain is ease of use, not a unique result.

A big exception to this is the class of adjustments that use the surrounding pixels to make tonal adjustments.

One of the most highly publicized is the new Highlight/Shadow tool.

Notice in this dialog box that there is a radius parameter for both the shadows and the highlights. This means that a normal tone control won't work.
shadow highlight
Shadow Highlight Dialog
Here is the result of using the above values on the lower step chart.

The thing to note here is that in addition to changes in the tonality gradation there are changes to tonal level at the edges of each step.

The only way to achieve this type of effect with standard curve tools would be to define a series of unsharp masks for different brightness levels and then apply curves to each masked area. This is obviously much more labor intensive.

So it is possible to produce most of your edits using just the standard tools. This may make the "experts" less necessary, but should help the average user maintain his sanity!
shadow highlight
Shadow Highlight
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© 2004 Robert D Feinman