Today’s Question: I have a photo that is brighter on top than bottom. How would one fix that in Photoshop?
Tim’s Quick Answer: This type of correction calls for a targeted adjustment using an adjustment layer for the tonal adjustment (such as Levels or Curves) combined with a layer mask with a white-to-black gradient so that the adjustment itself will transition across the image.
More Detail: The concept involved with this type of correction is relatively simple, but the implementation can sometimes be tricky.
Basically, you want to apply an adjustment using an adjustment layer with a layer mask that is a gradient. You could start by adding an adjustment layer, such as using Levels in this case to apply a tonal adjustment. So, click on the Add Adjustment Layer button (the half-black and half-white circle icon) at the bottom of the Layers panel, and choose Levels from the popup that appears.
I then recommend applying an exaggerated adjustment so you can easily see the effect in the image. With the Levels adjustment, for example, you can drag the middle slider directly below the histogram on the Properties panel left or right, depending on whether you need to lighten or darken a portion of the image, respectively.
Next, select the Gradient tool from the toolbar (or by pressing “G” on the keyboard). On the Options bar click the gradient popup that shows the gradient preview, and in the Basic section choose the first gradient, which is a foreground color to background color gradient. Press “D” on the keyboard to set the default colors of white and black.
To the right of the gradient popup on the options bar select the first of the set of five buttons, which define the overall shape of the gradient. This will establish a simple linear gradient. Make sure the Mode is set to Normal and the Opacity is at 100%.
Because the adjustment layer is active you can now click-and-drag across the image to define a gradient. You want black to be on the side of the image that doesn’t need to be adjusted, and white on the side that does need to be adjusted. So, with white as the foreground color and black as the background color you can drag from the side of the image that needs to be adjusted toward the side that doesn’t. The distance you drag determines the size of the transition for the gradient.
Once you have the gradient in place you can refine the adjustment setting on the Properties panel. The tricky part is getting a gradient with just the right shape (distance and direction) combined with just the right adjustment to get a nice smooth correction for the photo. How easy (or difficult) this is will depend on the nature of the photo itself.
Note that while I’ve used the Levels adjustment as an example here, you could also use a Curves adjustment layer. Curves would be helpful if you needed to apply a more sophisticated tonal adjustment beyond simply lightening or darkening one side of the photo.