Mar 14 2010

photoshop techniques: correcting lens distortion

Some of my best clients are realtors and architects but if they present me with their own photography, I can count on the need to spend a little more time in Photoshop than usual. Why? Because invariably photography of architectural elements appears distorted. The effect, known as "keystoning" or "parallax distortion", is when the camera lens is tilted with respect to the subject. For instance, if the subject is a tall building, the tendency is to look up. Tilting the lens up results in the top of the building appearing farther away. Similarly, when using a wide-angle lens to shoot close-in interiors results in the walls appearing tilted toward the focal center.

This is a very quick tutorial to demonstrate how to recognize and correct this type of distortion.

The white lines drawn on these examples indicate the degree of distortion. These are extreme but it helps to view architectural images using a grid to check for distortion.

In Photoshop, the grid color and size are set in Preferences (Preferences > Guides, Grid, & Slices) Display the grid with View > Show > Grid (which also toggles the grid off).

Or, use guides to indicate true vertical or horizontal angles. Click on the vertical (y-axis) ruler and hold down the mouse button to drag a vertical guide. If you need the horizontal guides, click and drag from the horizontal ruler. The guides are shown in red:

The choice now is whether to use the Lens Correction filter (Filter > Distort > Lens Correction…) or Free Transform (Edit > Free Transform). I prefer Free Transform mainly because it’s a little faster, especially for simple distortion.

Command-click (or Control-click for PC) and drag a corner handle outward until the the distorted edge lines up to the grid or guide.

Repeat for the opposite corner.

This may affect the first alignment, so repeat the first corner, if necessary.

When the alignment appears to be correct, double-click the image or press the Return (Enter) key.

Simply correcting the vertical alignment may result in horizontal distortion where the aspect ratio is no longer correct (objects appear wider than they should). Once again, choose Free Transform and increase the height of the image by clicking (without Command this time) the center handle and pulling it up. To avoid tilting the image, hold down the Shift key as you pull.

Double-click the image or press the Return (Enter) key.

Compare before and after in these 3 images: