Let’s take a look at the picture in question to get an idea how horizontally converging lines manifest themselves. I used a 35mm lens, but the effect applies to any focal length, although it becomes more apparent with wide angle lenses.
This is a picture of Neuhaus Castle in Paderborn, Germany. There are many boring images showing this building, but I didn’t want to add to that pile of pictures, so I tried very hard to come up with a composition that is more engaging. Leading lines never fail (if done well), so I opted for this curve starting in the bottom right corner and leading the eye towards the castle. I moved the castle itself off to the right to break the total symmetry of the background and the resulting composition looked great – at least on the display of my camera.
Back home and with the picture imported in Adobe Lightroom there was no denying that something was off.
The green lines are perfectly horizontal, the red lines show the converging lines. As the bottom line is closer to the vertical center of the frame the effect is hardly noticeable at the base of the castle, but the roof shows significant left tilt as it is quite far away from the center line.
Let’s fix this
Adobe Photoshop offers a couple of tools for dealing with perspective, all sorts of transforms (free, skew, scale among others), perspective warp, puppet warp and so on. I tried them all, but none of these tools produced a satisfactory result. Research then pointed towards the Adaptive Wide Angle Filter and it turned out that this is a fantastic tool which yields great results.
I used the Contraint Tool from the tools menu, which lets you straighten curved lines as well as make lines perfectly horizontal or vertical. In order to straighten lines, click the start point of the line and then the end point. Photoshop will then warp the image such that the former curved line will appear straight. If you press the shift key while clicking the start and end point, the tool will act differently and help you make perfectly horizontal or vertical lines.
Applying this tool to both, horizontal and vertical structures, I could effectively fix the perspective. Here’s the final result.