The Lens Correction filter (choose Filter > Distort > Lens Correction) lets you address barrel and pincushion distortion, chromatic aberration, vignetting, and perspective errors.
Lens Filter Controls
The Lens Filter controls are arranged in five groups: the tool panel, the zoom controls, the grid controls, the main control buttons and Settings menu, and the actual filter adjustment controls.
Tool Panel. The Tool panel, at the top left corner of the Lens Correction dialog, contains five tools. The Remove Distortion tool is a rather blunt instrument—we can make much finer adjustments using the slider control—but we use the Straighten tool to set horizontals or verticals to rotate and straighten the image because it’s often easier than typing in an angle. The Move Grid tool lets us adjust the position of the alignment grid, which is useful when adjusting distortion or perspective. We never choose the Zoom and Hand tools from the panel, preferring to use the usual keyboard shortcuts—Option (Mac OS X) or Alt (Windows) to zoom out, Command (Mac OS X) or Ctrl (Windows) to zoom in, and the spacebar to scroll.
Grid Controls. The grid controls let you show and hide the grid and control its size and color. Also in this cluster is the Preview check box, which lets you toggle between previewing the adjusted and unadjusted image.
Remove Distortion. The Remove Distortion slider lets you remove pincushion or barrel distortions, which bow straight lines inward and outward, respectively. Figure 11-15 shows an image before and after correction for moderate barrel distortion. The grid is useful for checking barrel distortion.
Figure 11-15 Removing distortion with the Lens Correction dialog
You can drag the Remove Distortion tool toward or away from the center of the image to correct barrel and pincushion distortion, respectively, but we find it’s easier to use the slider. The up and down arrow keys change the value by increments of 0.1; add Shift to change it in increments of 1.
Chromatic Aberration. The Chromatic Aberration sliders work by changing the size of the red (for red/cyan fringing) and blue (for blue/yellow fringing) channel relative to the green channel. We showed an example of chromatic aberration when we discussed the Camera Raw corrections for it in Chapter 5. You can sometimes go crazy trying to eliminate chromatic aberration entirely, but if you can render it unobjectionable at 200 percent magnification, it’s unlikely to be noticeable it in the final image.
With digital raw images, we prefer to fix chromatic aberration in Camera Raw because it’s earlier in the workflow. However, if you need to make different corrections for chromatic aberration in different parts of the image, you might want to make this correction in Photoshop instead—simply copy the layer, run Lens Distortion with different chromatic aberration settings on each, and use masks to reveal the appropriate corrections for each part of the image.
Vignette. Vignetting, in which the lens illuminates the sensor or film plane unevenly, causing darkening in the corners, is most commonly seen when shooting at wide apertures. The Vignette Amount slider controls the amount of lightening or darkening, while the Vignette Midpoint slider controls how far from the corners the correction extends; lower values affect more of the image, higher ones confine the correction closer to the corners.
Setting Lens Defaults. You can save the Remove Distortion, Chromatic Aberration, and Vignette corrections as defaults for a specific camera, lens, and focal length by clicking the Set Lens Default button. Then, when the filter detects other images shot with the same camera, lens, and focal length by reading the image metadata, the setting you’ve saved becomes available in the Settings menu, and choosing it applies those settings. When you save lens defaults, test the settings on more than one image—even then, you’ll probably have to fine-tune the results for each image.
The Transform Controls. The Transform controls are image specific. They can reduce the perspective errors caused by tilting the camera, and while they do an impressive job, they don’t turn an SLR into a view camera. But they do provide a reasonable substitute for 35 mm tilt/shift lenses. Figure 11-15 shows an image before and after a perspective correction.
Edge. When you make distortion and perspective corrections, you lose some of the image. The Edge menu lets you deal with the corrected edges, but the Edge Extension option rarely does anything useful, so we usually stick with Transparency. If we need to preserve the aspect ratio, we use the Scale slider to fill the image area, eliminating any empty areas resulting from the lens correction. When we want to keep as much of the image as possible, we crop in Photoshop instead.