Unfortunately, Photographic lenses are seldom perfect. They produce some optical errors due to refraction caused by different wavelengths of light, different types of glass, lens construction issues, and other factors. Lens errors are stronger in some lenses and weaker in others, and often depend on the focal length and aperture of the lens being used. Zoom lenses especially represent a compromise between versatility and optical quality, with the most obvious errors occurring at large apertures and at the extreme ends of the zoom scale. The three most common types of lens error are:
Distortion
Wide-angle lenses (with equivalent focal lengths of 35mm and less) are particularly susceptible to barrel
or pincushion 
distortion. High-quality lenses tend to produce distortion that is at least symmetrical relative to the center of the image. Photoshop’s Lens Correction tool (introduced with CS2) and a range of third-party tools and plug-ins are available for correcting distortion errors.Vignetting
This effect causes a falloff of image brightness toward the edges of the frame. With high-quality lenses, vignetting can usually be counteracted by closing the aperture down one or two stops. Most RAW editors include vignette correction functionality, as described in Split Toning Tab. This is usually the best way to correct vignetting in RAW images.
Chromatic Aberration
These types of errors have various causes, including refraction caused by the different wavelengths of different colored light sources, and occur primarily at the edges of the image area. Aberrations usually take the form of blurring or color smears, and they are more evident in consumer-level cameras than in cameras with professional-grade lenses. Some chromatic aberrations are difficult to see with the naked eye and are most obvious at high-contrast object edges.
All current RAW editors include functionality for reducing chromatic aberrations, and the Photoshop Lens Correction tool can also be used to eliminate these errors.
Lens error corrections (especially distortion correction) always involve rounding errors and therefore a loss of image quality. Always perform lens error corrections using a RAW editor or on 16-bit images if you can. There are two basic approaches to correcting lens errors:
Judging corrections visually
This a simpler, more universal approach. Zoom in to your image (50% or 100% is a good starting point) and adjust it until you are happy with the result. Most lens error software tools include a preview window for displaying a selected image detail, but we recommend that you activate the full-frame preview feature if your software has one. A large (or a second) monitor is useful for this type of work.
This is the principle used by most RAW editors, the Photoshop Lens Correction tool, Power Retouche’s Lens Corrector [83], Richard Rosenman’s Lens Corrector PRO [100], and Grasshopper’s more expensive ImageAlign [95].
Using camera and lens profiles
This approach uses profiles based on the recorded optical characteristics of specific cameras and lenses to correct image errors. This method makes it possible to correct lens errors automatically without having to experiment with each image individually. The profile creation process in itself is complex, as it involves measuring the characteristics of every camera/lens combination. Ideally, a camera/lens profile will include characteristics for different aperture settings,[122] and for zoom lenses, measurements for different focal length settings. If you purchase a new lens, you will either have to create your own profile or wait until the manufacturer of your correction software includes it in the range of supported lenses.
PTLens [92], LensFix [93], DxO Optics Pro [68], Canon Digital Photo Pro [41], Adobe Camera Raw (since 6.1) and Adobe Lightroom all work on this principle, as does Photoshop CS5. Nikon Capture NX includes profiles for correcting distortion caused by Nikkor fisheye lenses. Bibble introduced lens correction tools in version 4. We expect more software manufacturers to enter this market segment in the near future.[123]
Note
Always correct lens errors as early in the workflow as you can, and always before you crop your image or perform selective color or contrast corrections. You should also correct lens errors before correcting perspective distortion. These types of filters assume that any distortion present is symmetrical and covers the entire image.If your RAW editor includes lens correction functionality, use this in preference to third-party tools. Profile-based lens corrections are preferable to other types, although you still might have to fine-tune the results they produce manually.
Some lenses (especially wide-angles) produce pincushion or barrel distortion at the edges of the frame.
Before Photoshop CS2 was introduced, our standard lens correction tools were either the Power Retouche Lens Corrector [83] or PTLens.
We corrected the barrel distortion in Figure 8-35 using a value of 10 and cropped the image to eliminate the resulting white borders. The final result is shown in Figure 8-36.
As ever, our intention is not to produce a perfect image, but instead to simply remove elements that could distract the viewer.
Figure 8-36. The corrected version of the image from Figure 8-34. Note the absence of distortion at the edges of the frame.
Photoshop CS2 introduced the powerful Lens Correction filter (Filter ▸ Distort). This tool combats vignetting, lens distortion, chromatic aberrations, and even perspective distortion. The fact that all these functions are pulled together in one dialog indicates that they use a single resampling algorithm. This means that performing multiple corrections with the tool shouldn’t cause exponential rounding errors. As of CS5, you will find the function under Filter ▸ Lens Correction.
Here, we will use the tool to correct the barrel distortion in Figure 8-37. The image was shot using a zoom lens set to its widest focal length, causing obvious barrel and perspective distortion. Using Photoshop’s built-in grid helps when judging results.
Figure 8-38. The Lens Correction filter combines several correction functions in one dialog. Preview updates with this filter are slow, even on a fast computer. This screenshot shows the Photoshop CS5 version of the filter.
After some tweaking, the image is still not perfect, but it looks much better in the preview shown in Figure 8-38. The tools at top left of the filter window include the Straighten tool 
that enables you to align the image to a horizontal or vertical line drawn along an edge within the image (e.g., the horizon). While the option Show Grid provides a grid for visual alignment help, the hand 
allows to reposition this grid. 
is the distortion correction tool. Once activated, it adjusts the image according to the position of your mouse relative to the center of the frame.
Note
These operations are quite complex, so the preview update can take some time. There is a preview update progress bar at the bottom of the window.
It is quite difficult to achieve satisfactory distortion correction using just the tool’s sliders, so we prefer to use the dialog boxes to enter numerical values that we then adjust incrementally using the 
and 
keys.
Photoshop CS5 introduced profile-based lens corrections. A number of profiles are included with the program, and more are available on the Internet. The number of profiles available is sure to increase in time, and you can even create your own using the Adobe Lens Profile Creator at http://labs.adobe.com/downloads/lensprofile_creator.html.
Figure 8-39. If you haven’t yet cropped your image and you have a profile that matches your specific camera/lens combination, Photoshop CS5 can automatically correct geometric distortion, chromatic aberration, and vignetting.
Here, we applied the filter’s Auto Corrections feature to an image shot with a zoom set to its widest angle (Figure 8-39).
Photoshop extracts the camera brand, camera model, lens model, and focal length settings from the EXIF data embedded in the image. If this information cannot be extracted or is simply not available, you can enter it manually in the Search Criteria pane. Photoshop will then automatically correct all the error types you have checked at A. Additionally, you can straighten the image using the tool and manually correct distortion using the tool as described in Correcting Lens Errors Using Photoshop.
If Photoshop doesn’t have a suitable built-in profile[124], you can select a profile for a different camera with an identical image sensor from the same manufacturer. If there are still no profiles available, you will have to switch to the Custom tab and make your corrections manually, as described in Correcting Lens Errors Using Photoshop.
If possible, we prefer to correct vignetting and chromatic aberration early in the workflow using a RAW editor (e.g., see Chapter 5).
The Auto Scale Image option provides a number of ways to fill image areas that are left empty once distortion has been corrected. We usually use the Edge Extension variant.
Figure 8-41. Shot with a zoom lens at its widest angle, this photo suffers from barrel distortion and vignetting.
Figure 8-42. The same photo, corrected using the Photoshop profile-based Lens Correction tool in Auto mode.
Along with Photoshop and the Lens Corrector program that we have already mentioned, we recommend the following alternative programs for correcting lens errors:
ImageAlign by Grasshopper [95]. This is a powerful (but expensive) Photoshop plug-in for correcting perspective and lens distortion.
PTLens by Thomas Niemann [92]. This is a value lens error correction plug-in. It offers an enormous number of camera/lens profiles. We recommend it, if your RAW converter doesn’t offer profile-based lens corrections or if you are shooting JPEGs and did not yet upgrade to Photoshop CS5.
Some RAW editors also support profile-based lens corrections. These include DxO Optics [68], Adobe Camera Raw starting with version 6.1, Lightroom 3, and (with some restrictions) Nikon Capture NX [47] and Canon’s Digital Photo Pro. (CNX and DPP only support Nikon and Canon lenses, respectively.) Bibble includes a plug-in to perform profile-based lens corrections.
Silkypix can also be used to correct perspective and lens distortion manually. We expect Apple Aperture to have profile-based lens distortion, vignetting, and chromatic aberration correction tools in the near future.
[122] Especially when correcting vignetting effects
[123] There is a correction example using PTLens in Correcting Perspective and Other Optical Anomalies.
[124] This is not the same as an ICC color profile. Rather, this type of profile describes the known geometric distortion, chromatic aberration, and vignetting caused by a specific camera/lens combination.