If your image has a barrel distortion, it looks pushed out at the center. Lines that should be straight are curved. You must correct lens distortions before cropping the image. For distortion corrections to work properly, the axis of the camera lens must coincide with the center of the image.
Important: This effect works on greyscale and 16 million color (24-bit) images only. To increase the color depth of an image, see Increasing the Color Depth of an Image.
To correct a barrel distortion:
Choose Adjust > Photo Fix > Barrel Distortion Correction.
Set the Strength of the correction. Adjust this value until the distortion disappears. Look for lines in the image that are curved and adjust the Strength Value until they become straight. The image is resized as you adjust this value.
Preserve central scale Whether the scale of the center of the picture remains the same or is adjusted. If you mark this check box, pixels are either added to, or removed from the image. Clear this check box if you do not want to change the size of the image or layer.
Note: Notice the Result size group box. If the Preserve central scale check box is marked, the changes to the original image in the width and height display for the adjusted image.
Click OK.
Correcting Fisheye Distortions
Correcting Pincushion Distortions