How to Avoid Converging Verticals
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If you have ever photographed buildings, especially tall ones, I bet you have encountered this problem at least a couple of times: the buildings seem to be leaning backward, with their originally vertical lines converging towards the top. Those in the habit of reading photography forums on the Internet have likely also noticed that many people associate this phenomenon with wide-angle lenses and treat it as a form of lens distortion. This is wrong.
The technical term for converging verticals in a two-dimensional image is keystoning. The word originates in architecture and bridge building, where the trapezoid shaped middle stone in the top of an arch that holds all the other stones in position is called a keystone. In photography, keystoning occurs whenever you tilt your camera up or down. When photographing a tall building, you'll often tilt the camera upward both in order for the building to fit into the frame and in order for you to get rid of any unwanted foreground. This is when the vertical lines of the building converge towards the top in the photo. Tilting your camera downward may happen when shooting from a vantage point – in this case, the vertical lines of the buildings underneath will converge towards the bottom. This will happen irrespective of the type of lens used, although the effect will indeed be more pronounced with wide-angle optics, simply by virtue of their being more powerful at creating a sense of perspective – keystoning is, after all, a vertical perspective effect. But if you refrain from tilting your camera and manage to keep it perfectly level, you won't have to deal with keystoning, even if you use an ultra-wide lens.
So the short answer to avoiding a convergence of verticals in your photos is to hold the camera perfectly level. This you can achieve simply by making sure all verticals in the frame are parallel to the left and right edges of your viewfinder, but if that's too difficult – e.g. because the finder is too small –, you can get assistance in the form of a hotshoe-mounted spirit level. In addition, some cameras can display grid lines in the viewfinder or in Live View, originally meant to help with composition, but usable for lining up your verticals as well. Finally, a number of high-end Ricoh compacts (all reviewed here on PhotographyBLOG) as well as a couple of Nikon and Olympus DSLRs have 'electronic level gauges' or 'virtual horizons' built in to help you keep your camera straight.
Problem is, if you do this, the building(s) might not fit in the frame! What can you do then? Below you can find a couple of solutions.
Solution 1: Shoot from Mid Height
Obviously, if you can set up your camera at exactly the middle of the height of the building you would like to photograph, the problem will be solved because in this case, you will have to tilt it neither up nor down, just select the right focal length and shoot. That's very convenient when you own a crane wagon, but otherwise your chances of finding a spot like this are, unfortunately, slim. If there is another building opposite to the one you want to take a photo of then you might be lucky enough to take a shot from a window on the middle floor, but this seldom is the case. Thus, for the most part, this solution is theoretical only.
Solution 2: Use A Very Wide Lens
This may come as a surprise, especially if you thought that keystoning was caused by the use of a wide-angle lens – but as discussed above, this is not the case. So if you are forced to shoot from ground level, consider using ultra-wide optics, perhaps an UWA zoom (or a wide-angle converter if your camera has a fixed-mount lens). In this case, you may be able to get the whole building in the frame without having to tilt your camera upward. Of course this entails having a lot of foreground in the frame, but this is not necessarily a bad thing. As long as the foreground is interesting, not too cluttered and either leads up nicely to the main subject or counterpoints it in an intriguing way, there's no problem.
Unfortunately, foregrounds aren't always cooperative – sometimes they are dull and boring, other times confusing or competing too much with the main subject. In these cases you need to crop the foreground off and keep only the top half of the image. Naturally, this practice is wasteful of megapixels – but if you have a high-resolution camera and/or do not need to make a big print from the photo, this solution can work very well.
Solution 3: Use A Shift Lens
This is the professional solution. 'Shift' or perspective control lenses are the ultimate tools for those shooting architecture from ground level on a regular basis. Shift lenses project an imaging circle that is oversized relative to the size of the sensor or film frame, and can be shifted a few millimetres up or down (usually even to the left and right, which again translates into up and down when the camera is in portrait orientation). How can a movement of just a few millimetres make such a difference in framing? The following illustration, where the circle represents the usable imaging circle and the red rectangle denotes the sensor or film frame, hopefully makes it clear.
The reason the house is depicted upside down is that this is how the lens projects its image onto the sensor. Notice that the red rectangle, which represents the sensor or film frame is at the same height in both illustrations – just as in reality, where it stays put while the lens is shifted. The projection of the scene shifts along with the lens and its imaging circle, so a different part of it falls onto the sensor – in this case, the entire house from top to bottom rather than the bottom part of the building and some foreground, as is the case with the lens un-shifted.
Shift lenses are great tools, but they come with a hefty price tag attached – not to mention that they are only present in a couple of manufacturers' line-up. So if you want to use them, you will have to dig deep into your pocket and maybe even switch systems. Getting a used view camera may actually be cheaper and even better, as most view cameras allow highly sophisticated movements – but you need to learn how to master them, and of course find a way to get scans, enlargements or both from your exposed sheet film.
Solution 4: Correct in Post-processing
If none of these solutions work for you – which is likely to be the case for most compact camera owners – there is still something you can try. Take a photo of the building with the camera tilted, and put up with the converging verticals for the time being. Don't go for a very tight composition unless you have to. Later, when you have downloaded your photo to the computer, open it in an image editor that has a perspective correction feature (in Photoshop, for instance, you first need to select the entire image, then go to Edit -> Transform -> Perspective). The vertical perspective effect can be fully eliminated this way, though you need to know that this solution involves a lot of interpolation, which means the maximum print size is reduced. Also, you lose some of the peripheral parts of the photo in the process (hence my suggestion to compose loosely). Finally, you may have to stretch the image vertically, as it can look as if it was compressed in that direction once the verticals are sorted out. That said, while correcting the effect in post-processing may not yield as perfect results as the use of a shift lens, it works reasonably well.
When Keystoning Is Not Bad
Keystoning is not like the plague, so you don't have to avoid it at all costs. A very mild amount of it might even be preferable over the “sterile” results obtained via the use of a shift lens, as we often expect to see some of it in a photo. At the other extreme of the scale, a very strong vertical perspective effect can yield dramatic results, which you can exploit for a powerful effect, see example below.
All images in this article © Zoltan Arva-Toth