Exposure Correction Using Levels
Using Photsohop's Levels tool you can easily correct commom exposure problems. The photo in the following figure exhibits some underexposure and some deep shadows, caused by photographing the subject in the bright midday sun. Take a look at the tutorial below to see how easy it can be to correct exposure problems in your digital photographs.
|Exposure correction using Photoshop Levels|
This Photoshop tutorial builds on the two previous ones (Advanced Masking Techniques made simple & More Advanced Masking Techniques made simple) in that I'll be using a mask towards the end of this lesson to isolate a portion of the photograph that needs additional correction. If you haven't read those two tutorials you can still follow along to learn how Photoshop Levels can help you correct exposure problems, but it would be helpful if you start out with an image that has a selection saved as a channel to get the full effect of the techniques covered in this tutorial. If you're not sure what that's all about head back to the previous two tutorials and take a quick look... for the rest of you, let's get started.
NOTE: This Photoshop lesson was created using Adobe Photoshop CS2, but the same results can be achieved in just about any version of Adobe Photoshop.
Before we get started correcting the exposure problem that we'll tackle in this tutorial, we need to make sure that we understand what tools are available and how they work. One of the tools (new from about Photoshop 7 or Photoshop CS) is the Histogram. You can access the Histogram palette for any open image by choosing Window, Histogram. By default the Histogram palette is grouped with the Navigator and the Info palettes at the top right of the Photoshop screen.
A histogram is a graphical representation of the luminance values in any given image. The graph shows, from the left, the shadows, the mid tones and the highlights. To get an idea of how a histogram works with different images, take a look at figure 22.1.
NOTE: You may have noticed the small triangular icon in the low-key histogram in figure 22.1. If you hover over that icon, when it appears in the Histogram palette, you'll see a tip describing what that icon will do if clicked (see figure 22.2). This icon appears in an image's histogram when the luminance data has changed. Clicking the icon will dump the cached data and replace it with live realtime data corresponding to any recent changes made to the image. There are more features available in the Histogram palette, but I'll leave those for a more advanced Levels tutorial (in the near future). For now we'll stick to what we need to know to correct a digital photograph's exposure problems.
Figure 22.1 shows two portraits, both properly exposed, that exhibit two different graphs in their respective histograms. The high-key (many light tones) portrait on the left shows a much denser gathering towards the middle-bright tones and the highlights while the low-key portrait on the right shows a denser gathering towards the middle-dark tones and the shadows. This makes sense of course, because the low-key image is darker and the high-key image is brighter. You can see, though, that both graphs have a spread over the entire range, meaning that the bright image has some dark areas (the shadows under the model's chin, for example) and the darker image has some light areas (whites of the eyes, for example). The fact that the graph is filled to both sides (the shadows on the left and the highlights on the right) for both images is a good indication that both exposures are acceptable in terms of their respective exposures. If the exposure was off, say one of the photographs was underexposed, there would be a gap in the graph... a gap on the right would indicate underexposure while a gap at the left would indicate possible overexposure.
Figure 22.3 shows an underexposed photograph along with its histogram. Note the gap on the right side of the graph, indicating that there are no real highlights or bright areas in the photograph (strange, of course since this image was obviously taken on a bright sunny day).
The small spike at the very right end of the graph is due to the specular highlights, in this case the reflection off of the chrome on the old truck.
Exposure Correction Using Levels
Great, we now have an image that's underexposed according to the histogram. The quickest, and easiest solution is to apply a correction using Photoshop's Levels. Because I prefer to be able to back out of all changes, though, the exposure correction should be done using an adjustment layer.
NOTE: Using an adjustment layer, rather than making the corrections to the image data itself means that it will be possible to re-edit the changes at a later date. This is a very handy feature in Photoshop.
Before we get started, though, I see a hand in the back... what's the question? Oh, I see, you'd like to use the Brightness/Contrast tool to correct the exposure instead of using Levels.
The trouble with using Brightness/Contrast, and I agree it is tempting to use this "easy fix", is that it compresses the existing image data, whereas the Levels tool expands the image data. Take a look at figure 22.4. The histogram shows a really large gap at the left of the graph. This gap is the result of applying the Brightness/Contrast tool. When we finish fixing the exposure problems using the Levels tool, the graph should instead fill the histogram display.
Let's get rid of the fear and bring up the Levels dialog box. Remember that we'll use an Adjustment Layer so that our changes will remain editable (this will remain true as long as you save your image as a .psd (Photoshop) file). Choose Layer, New Adjustment Layer, Levels... give the layer an appropriate name (I named the new layer "Overall Exposure Adjustment") to help you keep track of the various layers, etc., and click "OK" in the New Layer dialog box. Doing so will bring up the Levels dialog box (see figure 22.5).
There are way more choices available here, as opposed to the Brightness/Contrast dialog box. We won't be bothered with most of these for now, though, and we'll concentrate only on those parts of the tool that we need to correct the image's exposure troubles... So no worries... take a deep breath and we'll begin.
Take a look around the bottom of the graph in the Levels dialog box and you'll see three small triangles. The one to the left is black, the one is the middle is gray and the one on the right is white. These three sliders, currently at their default positions (shadows at far left, mid tones in the middle and highlights at the far right), enable you to make adjustments to the exposure of your image. You can adjust the shadows, mid tones and highlights by simply clicking-and-dragging the sliders. With an underexposed image, such as the one I'm working with here, what you will want to do is drag the highlights slider to the left until it gets close to were you start to see info in the graph. Figure 22.6 shows where I moved the highlights slider to and the changes it has made to the image... Make sure that "Preview" is selected so that you can see the exposure changes in realtime as you move the sliders.
NOTE: If you're working on an overexposed image, you will need to move the shadows slider to the right. Be aware, though, that correcting an image with blown out highlights may not yield usable results. There are tons of articles on the Web about why this is so. A quick search at www.google.com will yield plenty of info. Try searching for "digital camera blooming problem". "Blooming" is a problem where very bright pixels (even just the brightness off a person's skin in some cases) cause the neighboring pixels to be overly bright, as well. This effect "blooms" outward blowing out highlights as it does so, hence the name.
You can see, in figure 22.6 that I've moved the highlights slider to the left so that it's under where the actual highlight information starts in the graph. Doing so has lightened the image considerably (except for the buildings behind the truck, but we'll get to that momentarily.)
TIP: When I'm correcting images in this manner, I like to add just a touch of sparkle to them... how much I add depends on the image (always trying to be as subtle as possible, though), each one being individual in its own way, right? How I add the sparkle is by sliding the mid tones slider just a little to the left. For this image I slid it just enough to change the mid tones text value (seen above the graph in the Levels dialog box) for the Input Levels: from its default 1.00 to 1.11. Give this a try with your own images and see what this small change can do by alternately turning the Preview on and off. It's quite amazing how much sparkle this small change can add to a photograph. Some photographs may actually seem to jump from the screen.
Selective Exposure Correction:
Okay, we're looking pretty good, but as with many images taken in the bright midday sun, this one suffers from some pretty deep shadows. The buildings behind the truck, in particular, look like a large dark blob. Here's where the selection techniques of the two previous Photoshop tutorials (Advanced Masking Techniques made simple & More Advanced Masking Techniques made simple) come in. Because I now need to add some additional exposure correction, but only to part of the photograph, that part of the photograph must me masked off, or selected. If you have followed along with the "Masking" tutorials, you'll have a channel that holds your selected area (in some cases group photos of friends taken with an on camera flash will suffer from a very similar problem, where the friends further away from the camera seem to fade to black :) ). You can load the selection you saved by choosing Select, Load Selection and choosing the appropriate channel in the Load Selection dialog box.
Click on the Create new fill or adjustment layer icon. It's along the bottom of the Layers palette and it is a small circle filled half-and-half with black and white. Doing so will present you with a menu... choose Levels. You'll now get the Levels dialog box again, but this time only for what's within the selected area. Amazing stuff! To correct the dark shadows within the selected area I simply moved the middle slider towards the left until it lined up with start of the brighter info (see figure 22.7).
I encourage you to gather up a couple of your digital photographs and apply the techniques outlined in this tutorial to them. As my Mom always said, "practice makes perfect", and hey who knows, you may find some real gems in that collection of photos, now that you can perfect their exposure values.
One final note... I know I mentioned it ealier, but make sure you save copies of your photographs as .psd files (native Photoshop files). That way you can re-open the files in Photoshop and re-edit the Levels changes by double-clicking the Layer thumbnail for the Levels adjustment layer you created (see figure 22.8). When you double-click the Layer thumbnail the Levels dialog box will re-open with the same settings as you entered. Powerful stuff :)
That's it for now... Be sure to check out some of our other Photoshop tutorials, including the next one that demonstrates how to get rid of Red Eye.
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