PTLens vs. Adobe Lens Correction Filter
Prior to CS5 the Lens Correction Filter used one parameter (a single slider) to correct distortion while PTLens can utilize up to 3 parameters. One parameter often does a fine job on standard and telephoto lenses. Wide-angle lenses typically require more than one parameter for accurate correction.
Starting with CS5 the Lens Correction filter can automatically correct distortion based on several pre-calibrated parameters. This document compares the Adobe's old single-slider Lens Correction filter, the new automated Lens Correction filter, and PTLens.
Complex distortion can be a challenge to correct. We'll examine the distortion in Canon's 24-105mm f/4L lens mounted on a Canon 5D full frame camera. This is one of Canon's L-Series lenses designed to meet professional requirements and retails for approximately $1000 US.
The distortion at full wide angle, or 24mm, consists of barrel distortion near the center and pincushion distortion at the corners. This type of complex distortion is often called moustache distortion as it resembles a handlebar moustache such as the one found on Sir Arthur Conan Doyle.
In our first attempt to fix distortion we adjusted the single slider in the Adobe's old Lens Correction filter to correct barrel distortion. This resulted in excessive pincushion distortion near the corners. We really need two sliders: one near the center to correct barrel distortion and one near the corners to correct pincushion distortion. Indeed this is done by both Adobe's new Lens Correction filter and PTLens. In both cases multiple parameters (aka sliders) are utilized. If you examine the red curb at the bottom of the image you'll see less cropping occurs with PTLens. But the difference is small.
Let's take a closer look, using 100% crops, to compare the accuracy of PTLens and the Adobe's new Lens Correction filter. A red straight-line feature was added to the image near the top of the frame after correction to measure the error. The left and right ends of the line were pegged to an image feature. As a consequence the amount of error is depicted in the center frame.
At 24mm PTLens is slightly better than Adobe's new Lens Correction filter. The image has marked moustache distortion and is tricky to correct. By hand-selecting calibration points I was able to achieve superior results. At 28mm PTLens fares even better although distortion is a bit less. Distortion changes when you zoom so it's important to calibrate at several different focal lengths. From this example it appears that the number of calibrations done for the Lens Correction filter were not sufficient to accurately characterize this lens.
For this particular lens 12 images, each at a different focal length, were utilized for calibration in PTLens. Documentation for the Adobe Lens Profile Creator, version 1.0, recommends using 6 different focal lengths for wide-angle lenses. Here is a link to the original image (2 MB) so you may reproduce these results on your computer.
At 105mm we have simple pincushion distortion that can be characterized with a single parameter. Both PTLens and the Lens Correction filter do an excellent job.
Automation in Photoshop
Care must be taken when using the new Lens Correction filter in Photoshop. To ensure correct results the Lens Correction dialog box must be displayed. There are two cases when the dialog may not display: when re-executing a filter (Control/Command-F) and invoking a filter from a Photoshop Action.
When the dialog box is not displayed the same settings used for the previous image are applied. For example if you just corrected a 24mm image and then ran the filter (with no dialog box) on a 105mm image it would apply the parameters previously determined for the 24mm image to the 105mm image. It gets even more interesting. Suppose you had two cameras: a Canon 5D and a little Canon G10. Correct distortion with the G10, re-execute the Lens Correction filter on an image taken with the 5D, and you'll find the 5D image corrected with settings based on the G10!
When re-executing the Lens Correction filter be sure the image has the same camera make, model, lens, and focal length as the previous image. In an action I suggest you enable the checkbox in the Action palette that will cause the Lens Correction dialog to display on invocation. This is not a problem with PTLens as it will automatically adjust for the correct camera make, model, lens, and focal length on each invocation whether or not the dialog is displayed.