Menu: Adjust -> Barrel Distortion
A very useful tool for correcting Lens distortions you can find in menu Image-Adjust-Barrel Distortion.
Barrel distortion is associated with wide angle (or minimal zoom) lenses and it causes the images to appear spherical (curved outward). You can notice this when you have straight edge near the side of the image.
On the image above the left side is original image suffering from barrel distortion, the right image is corrected.
As opposite the Pincushion distortion is associated with Telephoto lenses (or maximum zoom) and the images appear pinched (bent inward) toward the center. The Pincushion is often less noticeable than barrel.
Most digital and compact film cameras suffer from this type of distortions especially if it has a zoom lens. The consumer digital cameras have been criticized by many professionals mostly because of their unacceptable lens distortions - effect which shows disturbingly on architectural and similar shots.
Photo-Brush can correct these distortions. Because it can use high quality Bi-Cubic interpolation, the result images are both clean and sharp without any trace that the image was manipulated. This is truly professional quality function and only very few software packages can produce similar results.
Type of Lens Distortion
For wide angle shots select Barrel, for telephoto shots you can use pincushion (may not be needed - depending on the lens)
For Barrel distortion most digital cameras should be in the range of 10 to 20 permille (or 1 to 2%). In our example we used 11 permille (1.1 %) to correct the image. That means our lens (Canon S300) has quite low (but still noticeable) barrel distortion and it shows quite serious quality of the lens (despite its tiny size).
The Bi-Cubic produces the best results, but is also the slowest. However it is strongly recommended to use Bi-Cubic.
The Bi-Linear interpolation has more soft-like effect, which may be some times desirable.
Find out once, use all the time
Once you find out the correction amount which works best, you can simply use it for all other wide angle shots from the same camera. You can even create simple Action to do it. Note that this works only for minimal (wide angle) zoom. As soon as you use zoom the correction amount would change to be less and then may go toward pincushion.
Important Note: The barrel/pincushion correction must be done before any Crop or size changes. (Including Perspective Correction). In fact the Barrel/Pincushion should be the very first step on the full image. If you crop image and then use barrel correction the effect would be obviously wrong.
When using pincushion correction the result image will have a black border in the corner. You will need to crop out this with crop tool.
On most images using the Barrel correction is enough, however in shots such as our example (front pictures, frames, paintings) the next logical step is to use Perspective Correction to make all the angles 90 degrees. (When you hold your camera by hand on front of a picture you almost always introduce some kind of slight perspective distortion)
Some more ideas for toying around
You may notice that you can enter quite large numbers into the correction which will produce drastically bent images. One way of using it is to play with Fish-Eye lenses (if you have one) - you can unroll the fish eye sphere into full frame image:
First we cropped the image so the sphere cut a bit into the 4 sides of the now rectangular image (to get the black frame out of the image), then we used the Barrel correction 420 permille (42%)
Here is a Pincushion used with extreme value (500 permille) to create a funny face. Sure your friends will thank you.