Perspective Control Lenses
When there is time, I like to go for a walk in the middle of the day. If I feel like shooting I’ll bring along a DSLR with a 14-24mm lens in a shoulder bag. I don’t bring a tripod or any other accessories. Since stakes are low and I’m just amusing myself on a walk, I’ll correct any perspective distortion later in software. The results are fine, but I wouldn’t do it for paid work.
There is a great advantage to carrying a single camera with only one lens and hand holding. Maybe one needs to be surreptitious so as not to be spotted, or maybe carrying around heavy gear isn’t physically possible. Whatever the reason, there are situations where hauling a bunch of gear just isn’t feasible. However, the tradeoff for this advantage when photographing architecture is decreased image quality, and reduced ability to accurately compose in frame.
Disadvantages of correcting perspective in software
Imagining how vertical perspective distortion will look after it is corrected requires some mental gymnastics. The removal of the keystone effect must be performed in the mind’s eye. There is an element of faith to it – I think it will turn out this way, I hope there will be enough room to crop, I probably have enough padding, and so on.
There’s no way around it. When perspective is corrected in software, sharpness will degrade and resolution will decrease. Both of these factors will increase in severity the greater the correction needs to be. There are an array of tools available to correct perspective in software, but every method will negatively impact sharpness and resolution. Files can be sharpened and upsampled, but this does not recover information lost in software perspective correction, it only mitigates the effect of the loss.
Example of perspective correction performed in software.
Giving up the ability to visualize the final image and unnecessarily sacrificing image quality are not professional best practices. If money is on the line, I want to know how the final image is going to look before I walk away. Using a large format camera is one way to solve this problem, but we’ll put that aside for now. The other way is to use a 35mm or medium format system equipped with perspective control lenses.
What are perspective control lenses?
Since the invention of perspective by Brunelleschi during the Renaissance, it has been the convention to render architectural subjects in such a way that vertical lines appear vertical. The primary function of perspective control (PC) lenses is to afford photographers the ability to control and correct for vertical perspective distortion. To do so, these lenses were designed to replicate large format camera movements within the constraints of SLR camera systems.
Example of maximum rise on a Nikon PC-NIKKOR 35mm F/2.8 lens.
Older PC lenses can only shift up and down, some of them not very far. Shift refers to the lens moving up or down relative to the sensor. Shifting the lens upward is called rise, shifting it downward is called fall. Shift allows the composition to be changed across a single axis, usually vertical, sometimes horizontal, without inducing perspective distortion. All PC lenses, without exception, are manual focus, with the focus ring located on the main barrel. On most designs, the aperture control ring sits behind the focus ring. It can also be located toward the front of the lens.
A tilt-shift, or TS lens, is a newer, more advanced variety of PC lens that can perform two types of movement simultaneously. The other movement, tilt, is more complicated and is worth its own discussion. In short, tilt allows the focal plane to be adjusted away from its default position parallel to the sensor plane. In other words, it can be used to bring a subject plane parallel to the sensor plane when the camera position would otherwise not allow it.
I should also note that the third type of movement available on a large format camera, swing, is not physically possible with any type of body mounted lens. Since the old style PC lenses are less commonly used, I’ll mostly be referring to TS lenses moving forward.
TS lenses consist of two main components, one of which stays fixed in place to the camera’s bayonet mount. The second component, the optical portion of the lens, sits in a grooved channel and can slide up or down relative to the fixed element. This allows for shift. Further up the barrel, there is an curved articulation across which the optical elements can be adjusted for tilt.
The lens can be rotated in eight different directions so that movements can be performed in a variety of configurations. All movements are performed on the lens itself while the camera body as well as the lens mount remains in place. The majority of TS lenses employ a design where two separate knobs control tilt and shift respectively. Each is paired with a second locking knob used to keep any movements in place.
Nikon PC-E NIKKOR 24mm F/3.5D ED, a typical wide angle TS lens.
Tilt-shift lenses and barrel distortion
Like every other piece of equipment, TS lenses have their advantages and drawbacks. One one hand, they integrate into wide-ranging professional camera systems. This way, if a photographer wants their functionality, they don’t need to invest in an entirely different camera system. TS lenses are actually more expensive than comparable large format lenses, but in the context of the camera system, they provide value. On the other hand, they replicate some of the movements of a large format camera, but in a reduced and limited way. More importantly though, the wide variants suffer from barrel distortion.
On a large format camera, focus is achieved by moving the entire lens nearer or farther to the sensor. The wider the lens, the closer it needs to be in order to focus to infinity. This distance is indicated by the focal number, so the rear element of a 28mm lens needs to be able to sit at that distance from the sensor.
On large format cameras there is no minimum flange focal distance (FFD), which is how close the rear optical element can physically get to the sensor. In theory, these can be micrometers away from each other so long as they aren’t touching. Because of this, lenses designed to be used in this way are much simpler and functionally distortion free. Any camera with a body mount, such as a DSLR, has a much longer minimum FFD.
A long FFD means that the rear element of a wide angle lens must remain relatively far away from the sensor. In order to compensate for the fact that the rear element can’t get close enough, all wide angle lenses for DSLR and DSLM cameras, including any wide PC or TS lens, employ a retrofocus design. On a retrofocus lens, the rear nodal point sits behind the aperture, and the rear optical element is farther from the sensor plane than indicated by its focal number.
One problem is solved while another is created since retrofocus wide angle lenses all suffer from barrel distortion. On some lenses of this design, the distortion will be painfully obvious, on others it will be very well-corrected, yet still present on close inspection. Sometimes it can be corrected using lens profiles, however lens profiles are useless for TS lenses.
This is the fundamental trade off in the practical application of wide-angle TS lenses. They’re designed to be useful to photograph architecture, where the ability to correct vertical distortion is valuable. But they introduce barrel distortion, a problem which nearly cancels out the utility of being able to correct verticals in the first place. But it’s still a trade that’s worth making if TS lenses are used carefully. The other option, large format digital camera systems, present some serious technical challenges of their own.
Pro tips
Wide angle TS lenses require a bit of finesse to use in a way that maximizes their advantages and downplays their drawbacks. I have years of experience using them, so hopefully this can save a few headaches.
Leave buffer space. The most obvious barrel distortion will be around the edges, so it’s never a good idea to put any important information there. As with any lens, this is where optical performance is weakest. Plan to crop, and leave a bit of extra room around the edges to do so.
Avoid lines near edges. The closer one is positioned to a vertical line, and the closer it is to the edge, the more it will exhibit barrel distortion. The closer a horizontal line is to top or bottom of the frame, the more likely it will also exhibit barrel distortion, in addition to the next problem.
Watch for arc distortion. Imagine a one point perspective image where the horizontals are level and the verticals are plumb. In this scenario, the top horizontal, say the cornice of a building, will appear to bulge upward in the center. This is an unfortunate result of shifting the nodal point on a retrofocus lens. The barrel distortion is dragged along with the shift. This can be mitigated (but not truly corrected) in Photoshop using the “arc upper” option on the Warp tool.
Double check focus. When using any TS lens, always double, even triple check focus. Especially across the frame. Sometimes tilt can be jostled out of alignment, which will throw part of the image out of focus. Conversely, there are some instances where tilt can and should be used to adjust the position of the focal plane.
Zero the lens. This is part of large format best practice but it also applies here. When a shot has been completed, tilt and shift should always be returned to their default positions. There are two reasons for this. First, the lens is least vulnerable to damage in its default position. Second, when setting up for the next shot, it's easier to work forward from neutral, rather than working backwards from the last shot’s movements.
Knobs are delicate. With repeated use, the main shift and swing knobs will start to loosen, and should be routinely tightened with a small screwdriver. Worse, the locking knobs will gradually strip if they are consistently over-tightened.
Use guided upright correction. TS lenses will allow most perspective distortion to be corrected on-location and in-camera, but not all of it. They are frustratingly imprecise. Even if a TS lens was used, it is still advisable to correct perspective again in software. Only use guided upright correction, the other options won’t necessarily be accurate.
Watch for vignetting. Some lenses, not all of them, will allow the user to shift so far that the lens’ coverage area will be exceeded. This appears as a dark circular halo at the image edge.
Beware lens flare. The widest lenses available, the Nikon 17mm PC-E and the Canon 19mm TS-E, are exceptionally prone to flare. These have a huge concave front element bulging out beyond the barrel, and if there is any direct light entering through the edges, it will induce flare. This can be exceptionally difficult to remove in software. Flare can be avoided by being aware of the possibility, and shading the lens when possible.
Use a tripod. It’s best practice to use a tripod in tandem with TS lenses. I’ve handheld with them before, but the results are always substandard, and it’s not even easy to do.
f/11 and forget it. On digital cameras, the best aperture to use for optimum sharpness when focused at or near infinity is the middle value of the lens. For most TS lenses on the market, that’s f/11.
Availability of perspective control lenses
I keep a spreadsheet of every 35mm and medium format perspective control lens that I know about. There are about 50 PC lenses, give or take, that have ever been produced. In practice that number is much less, for example Nikon produced four versions of its 35mm PC lens and two of its 28mm PC lens over the years. From the 1970s through the 1990s, Schneider-Kreuznach produced variants of its PA-Curtagon 35mm and PC-Super-Angulon 28mm for a number of other manufacturers’ systems. And of course, there are probably some that I’m not yet aware of.
Vintage PC lenses are available on the secondary market, and are generally inexpensive unless they’re rare or collectible. Some are almost worthwhile. The 1980 Nikon PC-NIKKOR 35mm F/2.8 is overpowered considering how inexpensive it is. However, all older wide angle PC lenses will invariably have an unpleasant amount of barrel distortion.
There are only five companies that have PC lenses that are in production as of this writing. All are TS lenses and they are manufactured by Canon, Nikon, Fujifilm, Samyang, and Laowa. I’ve tested Laowa’s 15mm and 20mm TS lenses and they’re not suitable for professional use. Although I haven’t tested one yet, I’m skeptical of the Samyang 24mm TS.
That leaves three manufacturers. Fujifilm is the newest player in this area, they are producing a 30mm and 110mm for their G series mirrorless line. These are the first set of PC lenses ever produced for a mirrorless mount. This limits their utility as they can only be used on Fujifilm G cameras. Any other PC lens of any mount can be adapted to any mirrorless camera for which an adapter is made. So, in terms of PC lenses currently in production, the two most reliable options remain Nikon and Canon.
In my experience, Canon’s wide angle TS lenses are much better corrected for barrel distortion than those of Nikon. I’m curious to test the Samyang, and am cautiously optimistic about Fujifilm. In the grand scheme of things, whenever a manufacturer produces a medium format digital back with built-in flat field correction that costs less than a new luxury car, all of this will be moot. That said, I suspect we’re going to be stuck with TS lenses a little while longer.