Correcting skewed perspectives
Another thing to consider is that, when shooting architecture, we rarely stand 100% in front of the subject. Invariably, you'll find that all your pictures are shot slightly off-center, from the side, from the corner—in fact, at almost any angle but dead-center. What this means when you transform the perspective is that the left-hand transformation might not need quite as much distorting as the right-hand corner of the picture. The edited version will always look a little bit better, but if you want to be more accurate in your architectural reconstruction, you can always use other transformation modes, such as Distort and Skew. Look at the following image:
Here's a very typical example of a wide-angle lens distorting a tall building. I was standing as far back as I could get in St Mark's Square in Venice to get this shot (the further you stand back from the subject, the less the optical distortion will...