Whether you shoot with an expensive digital SLR or an inexpensive point-and-shoot camera, if you shoot at wide angles, you’ve probably noticed lens distortion in at least some of your images. If your image editor or management app doesn’t have a built-in facility for correcting this distortion—neither iPhoto nor Aperture do—TweakerSoft’s LensTweaker ( ; $30) may be the ideal solution.
LensTweaker addresses two types of geometric distortion: barrel distortion, which causes vertical and horizontal lines to bow outward, and pincushion distortion, which causes vertical and horizontal lines to bow inward. These distortions are not necessarily a sign of a bad lens; even the priciest lenses can exhibit geometric distortion when used at very wide angles. For example, the image below, shot with a 16mm lens on a Canon EOS 5D, exhibits barrel distortion.
LensTweaker lets you correct images manually, through a simple slider interface, or you can build special calibration profiles for specific lenses at specific focal lengths; LensTweaker can then use these profiles to automatically correct images.
After opening an image in LensTweaker, the program’s palette displays a simple set of distortion-correction controls. LensTweaker automatically reads both the lens and focal length of the shot from the EXIF information stored in the image and displays that information in the palette.
Two Distortion sliders are provided which let you bow the image in or out. The idea is that to correct distortion, you warp the image in the opposite direction that your lens warped it. The upper slider provides a broad-brush correction, offering a fairly large amount of warping. The lower slider applies less distortion, allowing you to fine-tune your corrections. The distortion controls also include a straightening slider.
Here’s the same image shown above, but after being corrected using LensTweaker:
If you routinely shoot images at a particular distortion-prone focal length, then it may be worth your time to calibrate your camera so that LensTweaker can automatically correct your images. When you press the Calibrate button, LensTweaker will fire up an assistant that walks you through the process. During calibration, you print out a special test target, shoot it with your camera at the appropriate focal length, and then open the resulting image back in LensTweaker. The program analyzes the image to determine how much distortion is present; it can then calculate and save the appropriate correction.
This calibration process works very well, but there are a couple small details to be aware of. First off, LensTweaker recommends that you print at the largest size provided by your printer. However, the application’s Print dialog doesn’t provide access to any type of Page Setup functionality, so there’s no way to specify paper size. Though I have a 13- by 19-inch printer, I had to use a letter-size test target; this worked fine, but was not as easy to use as a larger target would have been. Second, and this isn’t really a LensTweaker limitation, you need to be very careful when shooting the target. You’ll want to use a tripod, and possibly a level, to ensure that the front of your lens is parallel to the plane of the printed target. At wide angles—especially at the close distance you need to use to shoot a small target—even a slight bit of tilt can introduce distortion unrelated to your lens.
Once you’ve built a calibration profile, LensTweaker will remember it and can automatically employ it whenever you open an image taken with the profiled lens at the profiled focal length.
The quality of the corrections performed by LensTweaker is very good; however if you push those corrections too far, you will see softening in the corners of your images. LensTweaker isn’t designed for huge corrections—for example, you can’t use it to turn a fisheye image into a corrected, rectilinear image—and if you’re used to the quality of the Lens Correction filter in Photoshop, you’ll probably find LensTweaker to be a bit soft.
LensTweaker also includes some basic image-adjustment tools—brightness, contrast, saturation, tint, and exposure—that work well enough, although most people will probably perform such edits in another part of their workflow. In fact, I would have preferred having a crop tool instead, since rotation and distortion correction almost always requires a crop afterwards. Also, since almost any lens that suffers from distortion problems probably also has some vignetting issues, it would have been nice to have vignette correction, as well. Finally, some kind of reference grid overlay would be nice for determining exactly how much correction you need.
Despite these complaints, LensTweaker is a good little program. At $30, it’s reasonably priced, and a downloadable demo version, which watermarks your final images, is available so you can give it a try before buying.
LensTweaker 1.0.1 requires Mac OS X 10.4 or later; it’s a Universal Binary.