Lens Distortion and Perspective Correction

A review of PTLens, lens distortion correction software for Windows and Mac OS, with further notes on correction in Photoshop and Bridge CS4.

PTLens uses a database of lens characteristics for correcting barrel, pincushion, and complex distortion caused by lenses. It also provides tools for correcting perspective distortion and chromatic aberration. The lens distortion correction, in my brief test, proved markedly better than what I could achieve with Photoshop. I used the Photoshop plug-in (the software also provides a standalone application and a plug-in for shift lenses): it identified my lens and focal length and applied an automatic correction. Although I could have tweaked the correction, it turned out to be as close to spot-on as I could detect.

PTLens does have one drawback: it presents as a preview a scaled-down image in its dialog box. Photoshop does the same–neither tool lets you preview in the image itself–but Photoshop’s Lens Correction tool permits you to zoom in over a comparatively large image. PTLens can zoom, though only for chromatic aberration, apparently, and it provides a small area to preview. This makes PTLens decidedly awkward to work with if you need to see zoomed in views. PTLens does provide a grid, which facilitates distortion, perspective, and rotation corrections, but even for these more global corrections, a larger zoomable image than PTLens provides can be handy. For its modest price, PTLens provides some excellent functionality, from what I can tell on first impression, but its limitations mean that you will still use Photoshop’s tools for some corrections.

Comparing PTLens with Photoshop

Photoshop correction on the left, PTLens on right

The above images were corrected for lens distortion. The PS example used the Filter > Distort > Lens Correction tool, with distortion, perspective and rotation corrections. For the PTLens example, distortion was corrected with PTLens and perspective, rotation, and warping corrections were applied in Photoshop. The image was shot at 18mm with a Nikon 18-200mm f/3/5-5.6 zoom lens, Nikon D300 body.

In Photoshop, it proved very difficult to get the precise correction, and I think this shows in the comparison. Quite possibly, the distortion had both barrel and pincushion distortion–common in wide angle lenses, according to the PTLens web site. Photoshop can’t correct such “complex” distortion; PTLens can. The combination of corrections in Photoshop also warped the image considerably, more than occurred with PTLens. I cropped the Photoshop image to remove warped edges. The PTLens image, after minor perspective and rotation correction, was easy to correct with the Edit > Transform > Warp tool. The Perspective correction had scaled the image down in the lower portion. Applying the warp transformation only on the lower portion distorted only the garden. Because the garden has no straight lines, the distortion isn’t even noticeable.

By the way, for chromatic aberration correction, if you are using RAW images, the best tool in my pack is Adobe Bridge. Photoshop’s correction is less fine-tuned. It’s a good idea to correct chromatic aberration before you even begin working in Photoshop.

Update: Much of the functionality offered by PTLens is now built into Adobe Bridge CS5, and I assume by extension to other Adobe Products.

Emigre Type Foundry: Historia

I have in my hands the most recent specimen book from Emigre Type Foundry and I am delighted. Emigre’s type books become collector’s items, and its not hard to see why–in small format, they extend to the printed document the quirky, fun and righteously well-defined aesthetic that permeates their typeface design.

This current offering, Historia, is something else. It’s not just a “greeked” specimen book, but a photographer Rudy VanderLan‘s journal of battlefield locations in the Mexican-American war of 1846–1848, with a recounting of that history, panoramic photos, and stylized, typeset “labels” for each location.

You can order the next specimen book free from Emigre, or buy a small collection for the modest price of $3. I did not see the current one online, though I expect it’s still available. Just don’t ask to borrow mine ;^}.