Tutorial: Fixing Underexposed Images

Some photos are dark -- and should be because this was the intention of the photographer. But some of our best shots are sometimes dark unintentionally. Do you have some great shots that could just use a bit of lighting improvement to make them great? Doing basic exposure adjustment in Photoshop Elements or Photoshop (or any other photo editor) is easy and the results can be amazing. It is well worth learning these basic techniques.

Let's look at a set of controls available in the digital world that we simply don't have with film (unless you happen to be experienced in the darkroom). With a full-featured photo editor like Photoshop, there are a million ways to do everything and a million users who will fight to convince you that their technique is the best. We won't do that here. The idea is to give you some basic tools with which you can use more easily later. The two tools that we will look at in this tutorial are the levels dialog and screen adjustment layers.

Let's start with the levels dialog. This sort of adjustment is easy to do with almost every photo editing package. We'll describe the steps to take if you have Photoshop Elements or Photoshop CS, but other packages should be similar.

1. Always work on copies of your images. If you save a file using the same file name after adjusting brightness as we do here or other editing, the original file that you opened is gone forever. Additionally, note that repeatedly opening, editing, and saving digital images in JPEG format results in degradation in image quality.
2. Open the copy of the underexposed image you want to adjust in your photo editor.
3. Examine the image carefully. Is the main subject lit well? Is the background too light or dark?
4. Open the levels dialog by clicking ctrl-l (on Mac, command-l), or selecting "Levels" in the Image->Adjustments menu.
5. A dialog box opens that might look something like the picture below (this is an example of an extremely underexposed image). This dialog shows a histogram of the pixel values present in the image. Dark pixels are represented at the left side of the histogram, bright pixels at the right. The right side of the histogram in this example is almost totally empty. This means that there are few pixels in the image that are bright, as is expected for an underexposed image. [An overexposed image would have few pixels at the left side of the histogram and a clumping at the right side; a well-exposed image would have pixels spread across the whole width of the histogram.]
   
6. Slide the rightmost triangle under the histogram to the left until it starts to make contact with the rightmost part of the curve. As you do this, watch the image in the rest of the window (the "Preview" box must be checked for you to see the result of changes you make). It is OK to push the triangle to the left of the tail of the curve; this will force some pixels to be totally white when they would have been less than white otherwise. In the picture shown below, you can see that we have done this. You should see the image lightening up and the contrast improving. When you have hit the rightmost part of the levels curve, release the mouse button. What you have done is to adjust the white point of the image - changing the way that the bits in the image are interpreted in order to make everything lighter by "spreading out" the dark pixels into the lighter areas of the spectrum. It is OK to experiment here to understand the effect of moving these points around.
   
   The levels dialog looks like this after sliding the white point to the left (compare this to the previous picture):
   
7. If the image still looks dark, you can try sliding the middle triangle to the left a bit. Again, watch the image on screen as you do so. By adjusting this slider, you are changing the gamma of the image - which dictates what values are interpreted as middle tones.
8. If you slide the white point triangle too far, the highlights in the image will become blown out -- losing all detail in the bright parts of the image -- and the shadows will lighten unacceptably and get blotchy. Other effects come into play with drastic changes in the black point and gamma setting. Experiment to see the effects for yourself.
9. When you are happy, click OK. Save off your new masterpiece (be sure to use a different file name if you don't want to lose the original image!).

This levels dialog-based technique can work wonders. However, badly exposed images can't be saved this way. With a bit of experimentation you will have a good feel for what is possible using this technique.

Changes using the levels dialog as we have described it here will change the entire image. A better technique (though more involved) uses a screen adjustment layer to lighten the whole image and then selectively remove the lightening from parts of the image that don't need it. [Note that this technique is not available in every photo editor; again, we discuss Photoshop or Photoshop Elements techniques here.]

1. Open a copy of the underexposed image you want to adjust in Photoshop or Photoshop Elements.
2. Display the layers pallet by selecting it in the "Window" menu (or by hitting F7).
3. Create a copy of the underexposed image in a new layer by pressing ctrl-j (command-j on the Mac, or using the appropriate commands in the layers pallet or "Layer" menu). The image you see on the screen should not change yet.
4. Change the mode of the new layer to "screen" mode after selecting the new layer in the layers pallet (click on it if it isn't already highlighted). You change the mode of the layer using the pull down menu at the top (it should say "Normal" prior to this step). You should see the whole image lighten after the layer is changed to "Screen" mode.
5. Select a reasonably large soft-edged paintbrush in the tool box, make the foreground color black (in Photoshop, press "x"), and paint onto the new layer in the areas that you do not want lightened. You should see them darken as you paint (but you won't see the black paint come through because of the "screen" mode of the layer on which you are painting).
   [Advanced tip: use a brush with less than 100% opacity to allow some of the lightening to come through on areas that you paint in this step.]
6. Is your image not lightened enough? Duplicate the new layer by dragging it onto the "new layer" icon in the layers pallet (or by pressing ctrl-j/command-j).
7. Is your image lightened too much? You can lower the opacity of the newest screen layer in the layers pallet.
8. Flatten the image by choosing "flatten image" in the layer pallet's fly out menu (click the little triangle in the circle at the top right of the pallet).
9. Save off your newest masterpiece (again, be sure to use a different file name if you don't want to lose the original image!).

Does this all sound rather complicated? Believe it or not, this is harder to explain than it is to do - and it won't seem hard to you after you have done it a few times. You will even learn to love layers! This is a very powerful tool to understand; there are dozens of fantastic techniques that use a similar basic procedure.

Questions on these procedures? Please let us know.