One distortion of photographic perspective is known as the keystone effect or vertical perspective convergence. It is common to see this type of distortion in architectural, street, and travel photographs. The most common instance of this is when tall buildings appear to lean toward the vertical centerline in a photograph. The distortion has become so prevalent that most people don’t notice it when taking pictures or just assume it has something to do with their lens focal length.
Vertical convergence and distortion are unwelcome to many photographers since they contradict Linear Perspective cues. This alters the appearance of the scene from what we expect.
Why Distortion Occurs
The cause of vertical convergence is actually pretty simple. As mentioned, modern lenses are designed to show straight lines. However, this only works when the camera is pointed straight at or “in line”, with the object that is being photographed. This is due to the fact that the distance between the camera and the object does not change.
However, the distance changes once the camera is tilted. As an example, if you’re trying to photograph a tall building straight on with your camera, you can only see the bottom half. Vertical lines are straight, but you can’t see the rest of the structure.
When you angle the camera higher, the whole structure is in the frame, but now the vertical lines, the sides of the building, are converging. This is because the top of the building now recedes in the distance from the lens, just like a road receding in the distance.
If you want to take a picture of a tall building without this happening, there are a few ways to do it. Changing your viewpoint is the most obvious way to do this. A high vantage point is needed; you want to capture most of the building from front-on at the mid-point. It is possible to shoot out a window of a neighboring building or to hire a crane if money is no object and there are no buildings around.
You can change your viewpoint in two ways if you can’t physically get higher. The first method uses shift lenses and the second involves perspective correction software.
Shift Lens
Changing the position of the lens has been part of photography since the beginning. Originally, view cameras, created in the 1840s, had a bellows system that let photographers move the lens relative to the film.
Using the shifting mechanism, the photographer’s viewpoint moved up and down. In the past, shifting to correct perspective convergence was considered just as critical as focusing, shutter speed, and aperture in the capturing process.
Because the lens captured a much greater field of view (circular) than the film, shifting was possible. This is similar to how modern shift lenses work. Architecture photographers commonly use shift lenses to limit vertical convergence on large buildings. These lenses are, however, only compatible with DSLR cameras or medium format cameras, and they are also very expensive (up to $1,000).
Software Correction
Using digital technology, this shifting process has been replicated, changing the photographer’s viewpoint artificially.
The pixels in an image are reworked in software such as Photoshop, Lightroom (or Lightroom Classic), and GIMP (GNU Image Manipulation Program) in order to eliminate the distortion by straightening vertical lines. The majority of these programs allow you to drag a corner if you want the image to be placed in a ‘correct’ position. You can also use sliders to force the image back into place.
Using the software has the drawback of resampling the image after significant adjustments, which can reduce sharpness.