Free Transform In Photoshop

It's possible in Photoshop to select the entire image with command-A (control-A PC) and then go to the edit menu and down to free transform. If you then hol down the control (alt PC) button while dragging one of the corners of the image, you can distort the image. The area that you drag outside of the original will be clipped so you are throwing it away (save the image first) but this can make simple corrections in perspective or it can be used to line up corners perfectly.

In my recent image of the leaves in the clay pot, I hadn't in fact got the camera exactly directly overhead and the pot showed the pot as leaning away from the camera - one edge was shorter than the others and two edges leaned in to each other. It was an easy correction to drag each corner until I got exactly what I wanted. I did this before cropping so I would have a little bit of image to play with.

I sometimes use this technique to remove an object from near the edge of the print. I have to be careful that the result doesn't look distorted but for many landscape images it works just fine - 'subtlety in all things'

It works a lot faster than lens correction in the filters menu and I had worried that it would result in significant image quality loss but I've not seen it and there have been a few times I have even done it more than once to get the final correction.

Once you get the image the way you want it, press return to actually do the transform. Should you not like what you see after dragging the corners and before hitting return, you can cancel with command period (control period PC). If you don't like it after, then you do the usual undo in Photoshop.