Since the days when the view camera declined in popularity and the 35mm camera came to the fore, photographers have been plagued by the distortion phenomenon known as “keystoning.” Keystoning occurs when vertical lines converge as the camera and lens are tilted above or below the horizontal plane. Today, digital images can magically generate geometric corrections with post-processing software, like Photoshop and Lightroom, instead of the aforementioned view camera or an expensive tilt-shift or perspective control lens. So, with all the “advancements” in the tech world, we are left with this question: Which method is better—correcting geometric distortion through optical means or by digital correction?
Non-product photographs © Todd Vorenkamp
What is Keystoning?
Keystoning is a type of perspective distortion also known as converging verticals. The term keystoning comes from the name and form of the wedge-shaped stone at the top of a masonry arch.
To understand this distortion, imagine standing in the middle of the proverbial two-lane highway that extends across the desert to the horizon. As the you look down the road, the farther away you look, the narrower the road appears. The road does not physically narrow, but, because distant but equally sized objects appear smaller than closer ones, the road appears to narrow.
Now, stand at the base of a tall rectangular building and look up. The same thing happens. The top floors appear to be narrower than the bottom floor—the same distortion effect.
Tilting a camera and lens vertically emphasizes this distortion. The same distortion occurs in the human eye, but we do some mental distortion correction in our brains that helps keep it from looking like all buildings are going to fall over backwards when we look up at them. And, of course, to our eyes, this effect is more noticeable in tall skyscrapers than in shorter buildings.
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Is Keystoning Bad?
Most architectural photographers remove keystoning from their images. If you pay attention, this is one thing that can separate the pros from the amateurs in real estate and architectural photography. However, because the human eye experiences this distortion (not to the extremes of photographic lenses) leaving a bit of keystoning can help an image of a building or structure look more normal to the viewer’s eye. How much keystoning is in the image really boils down to what looks good to your eye as the photographer, and your photographic intent. Extreme keystoning can work in an image if you are trying to emphasize height or distortion, and zeroing all keystoning on a tall structure can make it look a bit unnatural.
Fun Fact: If you are looking at a painting or drawing of a building and you see keystoning, it is likely that the artist based the rendering on a non-geometrically corrected photograph and not the actual building.
How Do We Avoid or Reduce Keystoning?
Not long ago, you had to employ a “perspective-control (PC)” or “shift lens” to avoid keystoning/converging lines in 35mm photographs of architectural subjects. Large format photographers could often shift (and tilt) their bellowed cameras to achieve the same effect. And, if you are working in the darkroom, there is a way to correct for keystoning during processing by tilting the film plane on an enlarger.
In today’s digital post-processing world, it is becoming ever easier and more seamless to perform geometric corrections in Lightroom and Photoshop. Even Snapseed, on my iPhone, allows photographers to bend lines. In Lightroom, the corrections traditionally fell under the “Lens Corrections” section. Currently, the Lens Corrections tab is used for correcting lens distortion, and the “Transform” tab does vertical and horizontal corrections, among other things. In Photoshop, under the “Filters” menu, is “Lens Corrections”—a one-stop destination for the recently split Lightroom options. And, for smartphone shooters, Snapseed’s geometric corrections can be found under “Perspective.”
Is it Better to Correct Optically or Electronically?
To analyze the different methods of correcting for keystoning, I set out with a pair of lenses—the Nikon PC NIKKOR 19mm f/4E ED tilt-shift lens, and my long-time workhorse, the discontinued Leica/Schneider PC-Super-Angulon-R 28mm f/2.8 perspective control lens (adapted with a Nikon F mount). My plan for capture was to shoot the same building with the lens shifted and then tilt the camera above the horizon to capture an image that would be corrected geometrically, later, in Lightroom. The post-processing plan involves adding the geometric corrections to the “tilted” shots and then doing some pixel peeping to see which method resulted in the cleanest images—the optically perspective shifted, or electronically corrected in Lightroom.
Here are the results:
Camera: Nikon D600
Lens: Nikon PC NIKKOR 19mm f/4E ED
Aperture: f/8
Subject: Federal Courthouse, in Brooklyn, New York
Analysis: The tilt-shift lens-shifted image and digitally corrected image look very similar. Does the lens-shifted image look more natural? Also, note that the digitally corrected shot has smaller dimensions than the lens-shifted image due to the post-correction crop, and some real estate is lost on the edges. The corner crops show no real effect on either image, other than the extreme distortion in the tilted shot and the fact that the lens-shifted image is noticeably wider than the digitally corrected image (note the position of the window with the flare and the surveillance camera). The crop of the eagle at the top of the building shows a much cleaner image from the lens-shifted lens than the digitally corrected photograph.
Conclusion: Without pixel peeping, you could make an argument for the overall look of either the lens-shifted image or the digitally corrected shot. You certainly lose field of view when cropping, after applying the geometric correction in post-processing, so if you need to shoot wide (most architectural shooters do), you might be better off with a shift or tilt-shift lens. Bolstering that argument is the cleanliness of the eagle in the optically shifted 100% crops.
Camera: Nikon D600
Lens: Leica/Schneider PC-Super-Angulon-R 28mm f/2.8
Aperture: f/4
Subject: Federal Courthouse, in Brooklyn, New York
Analysis: Again, the perspective control lens-shifted image and digitally corrected image look very similar, but we have lost the wider field of view. The door crops show that the digitally corrected image loses a bit of sharpness when compared to the lens-shifted images, but at the top of the frame, it looks like the digitally corrected image held up better than the shifted and fully shifted images. Likely, shooting at f/8 would have yielded better results here for both sets of images.
Conclusion: Here, the Super-Angulon lens appears to do better when pixel-peeped farther from the edge when shifted. At normal size, the digitally corrected image and lens-shifted images match well, except for the loss of the wider field of view following the post-processing crops.
Camera: Nikon D750
Lens: Nikon PC NIKKOR 19mm f/4E ED
Aperture: f/8
Subject: Borough Hall, in Brooklyn, New York
Analysis: Once again, the lens-shifted image and digitally corrected image look very similar, but we have lost the wider field of view following the geometric correction and crop—not a big deal if you can move farther away, but, at Borough Hall, I was backed up against a fence. The center crops show that the digitally corrected image loses sharpness when compared to the lens-shifted images.
Conclusion: Here, the tilt-shift lens appears shows the same advantage as before—an advantage in pixel-peeped sharpness and the true, wider field of view.
Final Thoughts
Digital geometric corrections are easy to create and handy for all types of photographers. If you are a casual shooter—photographing architectural landmarks while traveling, for instance—the digital geometric corrections available in Lightroom, Photoshop, or on your phone, are superb tools for making images of buildings and other structures look more natural to those viewing your images. Also, for real estate photographers and those sharing lower-resolution files, post-processing software for geometric corrections is really all you need.
However, for the professional or student architectural photographer, the true wide-angle perspective of a tilt-shift or perspective control lens, coupled with the higher overall image quality gained from optical lens corrections will give an advantage over digitally corrected files.
Have the digital corrections available to today’s photographers eliminated the need for a perspective control or tilt-shift lens? What do you think? Let us know in the Comments section, below.
