Correcting Distortion Caused by a Taking Lens

If you receive files that exhibit common flaws caused by cheap lenses, or even expensive lenses that were used on a sensor for which they weren't specifically designed or were used at an extreme angle that results in keystoning, you can use the Lens Correction filter (Filter ^ Distortion ^ Lens Correction) to counteract the distortions. The filter can correct the most common flaws: barrel, pincushion, chromatic aberration, vignetting, and vertical and horizontal perspective.

The Remove Distortion slider in the dialog box corrects for barrel distortion when moved to the left and pincushion distortion when moved to the right. Barrel distortion is most noticeable on images taken with retrofocus wide-angle lenses; the result is a bulging out of the straight lines, resulting in a barrel-like effect. Pincushion distortion is caused by a telephoto lens that has a negative rear group of elements. The effect is usually more pronounced at the tele end of a zoom lens, which might also exhibit barrel distortion at the wide end (see Figure 11-16 for an example of an exaggerated pincushion distortion).

Figure 11-16: Left—Exaggerated barrel distortion. Right—Exaggerated pincushion distortion.

Another lens flaw commonly encountered is chromatic aberration (Figure 11-17). Chromatic aberration is the result of a combination of factors caused by the light's registering incorrectly on a digital sensor (it's also present in film-originated photography but is not so apparent).

The effect is most pronounced when wide-angle lenses are used, and is noticeable on high-contrast edges, such as foliage or buildings shot against the sky or a steel chair against a dark background.

Previously, there were no commands for correcting chromatic aberration within Photoshop; you could do this only within Camera Raw. However, now you can correct chromatic aberration not only on your raw files but also if you output JPEGs directly from your digital camera.

The filter has two sliders for correcting chromatic aberration. One slider corrects red/cyan color fringing and the other blue/yellow color fringing.

Figure 11-17: Exaggerated chromatic aberration (left), most noticeable around the outline of the dragon and the plinth, and corrected version (right).

To correct color fringing, take the following steps:

1. Choose Filter ^ Distortion ^ Lens Correction.

2. Zoom into the image by using the Zoom tool from the toolbox in the dialog box, or use the usual keyboard shortcuts for changing the magnification.

3. Move the Fix Red/Cyan Fringe slider to adjust the red channel relative to the green channel. This reduces or fixes any red/cyan color fringing.

4. Move the Fix Blue/Yellow Fringe slider to adjust the blue channel relative to the green channel. This reduces or fixes any blue/yellow color fringing.

5. Click the OK button to apply settings and exit.

If the grid gets in the way, turn it off by deselecting the Show Grid option at the bottom of the dialog box.

Vignetting is caused by a number of factors that result in light falloff toward the edges of a photograph, manifesting as unacceptable dark corners. It might be caused by a lens aperture being used wide open, an incorrectly set matte box, or a lens hood. Though it's always best to avoid vignetting in the field, if confronted with an image that cannot be reshot, the Vignetting sliders of the Lens Correction filter can be used to minimize the falloff or even cure it in most cases (Figure 11-18).

Figure 11-18: Exaggerated vignetting effect (left), noticeable around the perimeter of the image, and corrected version (right).

If the lens type, focal length, and f-stop used to take the photo are included in the Camera Data (Exif), you can use the filter's Set Lens Default button to save any values in the Settings section of the panel. Next time you use the lens, you can apply the values by selecting Lens Default from the Settings pop-up menu. However, all three fields need to be present before the button becomes active. If the button is grayed, information in one or more of the Camera Data fields is missing. If your image lacks one of the required fields, you can still use the panel's menu to save any custom settings, including the Transform settings, which the Set Lens Default button does not save. To access the Lens Correction panel menu, click the right-pointing triangle by the Settings pop-up menu and select Save Settings.

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