To solve this during the film era, photographers used large format camera movements to tilt the lens plane and keep it vertical—perfectly parallel to the building for straight and true lines. With a perspective control or tilt/shift lens on a digital SLR, the same effect can generally be accomplished, although there’s some limit to the amount of movement these lenses can produce.
Whether you’re pushing a perspective control lens beyond its limits or simply relying on a plain old wide-angle lens to photograph architecture, chances are high that you’ll end up with pictures of distorted buildings. This distortion is known as keystoning, where the lines of the building all appear to point toward the middle of the top of the frame. Luckily there’s a simple solution for correcting this distortion in Photoshop: the Lens Correction filter.
Open the distorted image as usual in Photoshop. Then, under the Filter menu, choose the Lens Correction option. The Lens Correction window then opens up with the Auto Correction tab active. This is the place where you might go to correct other distortions caused by lenses—say, the chromatic aberration or vignetting of a lesser quality wide-angle lens. You can also create lens-specific profiles based on your own lenses. But we’re interested in the second tab, for custom corrections.
In the Custom tab you can manually fix many of the things that can automatically be controlled in the Auto Correction tab—problems like color fringe, barrel distortion, vignetting and more. But if you scroll toward the bottom of the window you’ll see the set of sliders that were made for architectural photography: under the Transform heading, they are the Vertical Perspective and Horizontal Perspective controls.
To correct for keystoning distortion, simply click and drag the Vertical Perspective slider to the left, into the negative values. Keep an eye on the preview window to the left, and simply stop once the lines are again parallel.
One word of warning: if the top of the building is too close to the top of the frame, you may run out of room before you’re able to make the lines truly parallel. In these cases, use the Scale slider toward the bottom of the panel to shrink the image on the canvas and make as much room as possible with which to work. If you still can’t get the straight lines perfectly straight, simply correct as much as you can before cropping the top of the building. In most cases, a little bit of distortion is preferable to deleting the pinnacle of the structure.
If the perspective on the building is angled at all, you can use the Rotation Angle control to move the perspective change in accordance with the angle of the structure.
While you’re here, you may consider spending a moment with the Horizontal Perspective slider. I find it particularly helpful with architectural exteriors that don’t have the ideal left-to-right perspective. If, for instance, I want to emphasize the front façade more than the side of the building, I can drag the Horizontal Perspective slider to effectively change camera positions and make the better façade more prominent in the frame. It’s another great tool for subtle refinements to architectural images, all within the Lens Correction filter.