It’s not uncommon when photographing man-made objects, to encounter various types of geometric distortions (like keystone distortions) in the final photo. This can result from a difference in height between photographer and subject, tilting or shifting the camera with respect to the subject, or both. One of the best ways to handle these types of distortions -perspective cropping - has been made even better in Photoshop CS6.
The new Perspective Crop tool is actually an expanded variation of the “Perspective” option we used to have with the standard Crop tool. Adobe created a new tool from that function, along with a new grid marquee that makes it much easier to align edges and apply an accurate crop.
Find a photograph where the subject appears to be slanted or falling away from the viewer. In this case a shot of an old cathedral, with the camera looking upward at a collection of stained glass windows and stonework.
Press Shift-C until you see the new Perspective Crop icon.
Find a corner of the subject and then drag out a crop marquee to the other side. Rectangular subjects work best but you can use a partially rectangular subject as a starting point and then extend the marquee, as you’ll see. Make sure the Show Grid option is turned on in the Options bar. Notice the gridlines inside the marquee.
Click and drag the top or bottom edge of the marquee so that it extends to the top of the subject. In this case I used the corners of the stonework around the lower stained glass windows because I knew it approximated a true rectangular shape, despite its age.
Go to the end of the marquee that is over-extended beyond the shape of your subject, click the corner handles in those areas and move them inward so that all four corners of the marquee match up with the four corners of your subject.
Once the marquee accurately covers the space of the object you’re using as a reference, you can begin to extend all sides of the marquee to the edges of the frame. Note that this process will result in areas that need to be cropped away in the final step, and that you may need to zoom out to see the entire marquee.
When you’re satisfied the shape of the marquee accurately reflects the angles created by the keystone effect or similar distortions, click the check box in the Options bar to crop the image. The initial result is shown below, with final (standard) crop marquee applied.
If you get a result that feels closer to the original, crop away any empty pixels to get your final result. Afterward you may need to Free Transform (Command/Ctrl-T) the cropped image to stretch it out slightly.