Fix Lens Distortions in HDR Images

Tone mapped high dynamic range (HDR) images don’t always come out of your favorite HDR application looking perfect. Many images have lens distortion, which wide angle lenses tend to accentuate.

Look at something with straight lines. It could be tiles on your bathroom wall, bricks, a fence, or graph paper. Are the lines straight? They should be. Now take a photograph of the same scene and examine whether the lines are straight in the photo or not. Most likely, they won’t be — even if you’re using a very expensive lens and camera. You’re seeing distortion in action.

Distortion happens when something that should be straight appears curved in your photographs. There are three types of lens distortion, as well as two types of distortion caused by how you hold the camera:

  • Lens

    • Barrel: The center of the image bulges out, as in this figure.

      image0.jpg
    • Pincushion: The center of the image is pinched inward.

    • Combination: Uneven distortion of either type (barrel or pincushion) that generally looks worse in the center but tapers off to almost straight by the edges of the image.

  • Camera position

    • Vertical: When vertical lines aren’t straight, you have vertical distortion. This happens when you point the camera up or down, which causes vertical lines to either fall away or toward you, respectively. See examples of vertical distortion in this figure.

      image1.jpg
    • Horizontal: When horizontal lines aren’t parallel to the ground, you have horizontal distortion. This happens when you point the camera left or right of the lines, which causes horizontal lines to cant one way or the other.

To fix lens distortion, in Photoshop Elements choose Filter→Correct Camera Distortion. In the Correct Camera Distortion dialog box that opens, as shown in this figure, there are several convenient options to choose:

image2.jpg
  • Remove Distortion: Repairs barrel (positive values correct bulging by pushing the center in) or pincushion (negative values correct pinching by pulling the center out) distortion. Notice the fairly large positive value here: This helps straighten out the lamppost.

    Use the grid lines (select the Show Grid check box at the bottom of the dialog box) to line things up, or turn the grid off to see the image better. You can change the color of the grid, plus zoom in and out.

  • Vignette: Lightens (positive values) or darkens (negative values) the corners of the image. Use to correct or cause vignetting. Adjust the Midpoint slider to change how little or how much the vignetting extends into the image.

  • Vertical Perspective: Adjust to make vertical lines vertical. Notice the large value in this example. This was necessary to make the lamppost vertical.

  • Horizontal Perspective: Adjust to make horizontal lines horizontal.

  • Angle: Straightens the image.

  • Scale: Enlarges the image. The one caveat here is that unlike transforming the image in the main window, you lose whatever extends beyond the dialog box preview window. In other words, it crops after it enlarges.

To illustrate the effect, this figure shows what happens when you add vignetting. If your images come out of tone mapping like this, correct it here. Not all vignetting is bad — some can look artistic. Notice, too, barrel and vertical distortion are corrected, the grid lines are off, and the image was scaled upward a bit to make it appear full-screen.

image3.jpg
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