Keystoning, also known as converging verticals, is a common issue that affects images, but one that’s easy to fix. Whether you’re using a computer, a smartphone or a tablet, we show you how to fix converging verticals.
The keystone effect is a type of distortion that affects many images. In photography, it’s usually found when capturing architecture – and it’s often referred to by the term converging verticals, as this is how linear details appear.
So what does this do to images? The effect makes buildings and other structures appear as though they’re falling backwards. You don’t tend to notice this in reality, and even in some images it might not be something you object to, that’s if you realize it at all. But it’s less than ideal when buildings are the main focus in an image – and once you spot it, it’s hard not to notice it.
So why does it happen? Can you avoid it? And how do you fix converging verticals when this affects your images?
Why it happens
Normally, in order to fit an entire building into the frame, we need to angle our cameras or smartphones upwards. This angling makes the bottom of the building closer to the camera’s sensor than the top, and this has the effect of changing the magnification between the two areas, relative to the camera’s sensor.
As the change is gradual, this has the effect of making a building appear increasingly smaller as its height increases – and it’s this that makes it look like it’s tilting backward. This is more pronounced as you get closer to the subject as you’ll be considerably closer to the bottom of the building than you will be to the top. As you get farther away, the difference diminishes, so the effect becomes less of an issue.
How to avoid converging verticals
If possible, you should try to minimize converging verticals from forming at the time of capture, rather than leaving it to post-production. The benefits extend beyond simple timesaving; the process of rectifying this distorts the image, as you’re essentially stretching part of it, which compromises fine details. Unless you’re planning on enlarging and printing your images, however, you can generally get away with this when using a modern camera with a high-resolution sensor.
Ideally, you would capture a building from an elevated position, with the sensor parallel to the building itself, but being able to get to such a position is rarely possible. One solution is a tilt-shift lens, which is sometimes referred to as a perspective-control lens. This kind of lens allows you to shift the lens’ optical axis relative to the camera’s sensor, which helps you to get more of the building in without the same kind of angling, although these lenses are expensive and somewhat fiddly in use, and really only intended for professional photographers who capture these kinds of subjects with some frequency.
If you have a standard wide-angle lens, you can try to position your camera parallel to the building and zoom out so that you capture it in its entirety, before cropping away the unwanted parts of the scene using software or your phone, or even your camera. Incidentally, some cameras offer perspective correction as part of their post-capture correction options.
If you do end up capturing an image with significant keystoning and you’d like to remove this, the process is fairly simple.
How to fix the keystone effect using Photoshop
Software programs have long offered tools for fixing issues with perspective, and these can be used to correct images in a matter of seconds.
Adobe Photoshop users can do this by clicking on Edit > Transform > Perspective, creating a new layer beforehand if the option is initially not available. Then it’s simply a case of dragging one of the top corners outwards until the image appears to be sufficiently corrected, as in the example below.
How to fix the keystone effect with your phone or tablet
There may be a specific app that you like to use to process your photos on your smartphone or tablet, although the default suite of editing options in the native camera apps of many modern smartphones typically give you the control you need to make the necessary changes.
Here, for example, we’ve not needed to go beyond the standard Gallery app on a Samsung Galaxy S10+. We just found the image we wanted to process, clicked on the pencil icon that allows you to start editing your image, and found the tool to correct perspective, which is the second one down next to the photo.
Now it’s just a case of selecting vertical rather than horizontal correction and shifting the slider until the scene looks sufficiently corrected.
How much is too much?
It’s tempting to apply this correction up to the point where the building in the scene appears to be completely parallel, but this can result in the image looking very obviously corrected. Ask a general rule, it’s best to apply this correction until you’re almost at this point, as this will look more natural.