Perspective correction

Learning goal

You will be able to decide whether to correct converging verticals during the shot with a shift lens or in the edit with a perspective tool, by looking at the building's height, the width of the street and the equipment you have. You will calculate from the shift range how far a shift lens can look up, and see in advance where this range falls short. In a frame you corrected with software you will notice that the upper part softens and the building gets squat, and bring back the proportions with a vertical stretch. You will choose whether to correct the verticals fully or partly not as a rule but according to how you want the frame to be read. You will take into account that the correction also stretches the people standing in the frame, and separately correct the tone that light reflected from a colored wall leaves on a face.

The question

On Saturday morning your friend goes to the slope behind the tram line. There, in a narrow street eight meters wide, is an old four-story apartment building with a bay window; its facade is brick, the window frames white stone. The opposite side of the street is a low wall painted yellow; going behind it or climbing higher isn't possible. When they hold the camera level, only the entrance door and the first floor are in the frame. When they tilt it up the apartment building fits, but again it leans back. They have two options: a shift lens they can borrow from the club, or the editing program at home. Tilt-shift or software?

Context

In the special lenses lesson you met the shift lens, whose optical part slides up while staying parallel to the sensor and keeps the verticals parallel while the camera stands level (Lesson 12.9). In the lesson where you first photographed buildings we talked about leaving space at the edges in the shot if perspective correction will be done in the program, that is, a correction margin (Lesson 16.4). In the lens corrections section you also learned that geometric correction stretches the image and then calls for cropping the edges (Lesson 17.7).

This lesson brings those pieces together in one decision question. Correcting the verticals is now a known job; what is asked is where to do it. A correction made in the shot loses nothing from the image but is expensive, heavy and limited. A correction made in the edit works with any camera, even a phone, but has a price. Which one to choose depends on the building, the street and what the frame is shot for.

Simple explanation

How far up does a shift lens look?

Shift range is how many millimeters at most the optical part of a shift lens can slide from the center. Most shift lenses made for full-frame cameras slide around twelve millimeters. This number directly decides how far up a level camera can see.

Let's see with a calculation. A lens with a focal length of twenty-four millimeters, on a full-frame camera held horizontally, sees about twenty-seven degrees above the horizon while standing level. In a street eight meters wide this reaches about four meters above the lens; together with eye level, more or less the first floor. When the lens is shifted up twelve millimeters, the top edge of the frame rises to forty-five degrees above the horizon, and the camera standing level sees eight meters up: about three floors. Holding the camera vertically increases this a little more.

The calculation shows this: shifting can photograph a three- or four-story building in a narrow street without loss; but it can't fit a six-story apartment building in a single frame from eight meters. When the range runs out, you either turn back to software or look for another way, such as combining shifted frames. Most shift lenses are focused by hand, are heavy and are usually used with a tripod; that is why renting them for a particular job instead of carrying them every day is also a common choice.

What does software lose?

To correct with software, the building is shot at a wide angle with the camera tilted up; the program then makes the verticals parallel by widening the upper part of the frame to both sides. This widening spreads the few pixels at the top over a wider area; the stronger the tilt, the more the top of the building softens. For a small frame on a screen this may not be noticed; in a large print, the detail of the cornice and the eaves blurs.

The proportion changes too. In a tilted shot the building's upper floors are both narrowed and squeezed upward; when the program opens up only the narrowing, the building looks shorter and squatter than it is. Vertical stretch / aspect adjustment is stretching the corrected frame upward a little to remove this squatness; the perspective tool of many programs has a separate slider for this. The right amount is found by looking at the real width-to-height of the windows: a square window should look close to square in the frame too.

The stretching is applied to the people standing in the frame too. A face in the upper part of the frame, like someone on a balcony, widens to both sides; a face at the bottom middle barely changes at all. A shift lens stretches no one, because it doesn't have to correct anything.

Are the verticals corrected fully?

Partial correction is knowingly leaving a small convergence instead of making the converging verticals fully parallel. When seeing a very tall building up close, from below, the eye expects the verticals to converge a little; a tower made fully parallel can seem to some viewers as if its top is opening outward. Some photographers therefore leave part of the correction on tall buildings.

This isn't a rule. A frame shot for an architecture firm often wants fully parallel verticals, because it is expected to show the building measurably, like a drawing. A personal frame can keep a little convergence to make the height felt. What matters is that the remaining convergence looks like the result of a decision: a tilt left at half a degree is often read as carelessness, while a distinct but measured convergence is read as a choice.

Visual evidence

Your friend tries both ways. They fit the twenty-four-millimeter shift lens borrowed from the club to the tripod, hold the camera vertically and shift the lens all the way up. Three of the four floors enter the frame; the stone carvings of the window frames are sharp, the verticals bolt upright. The top floor and the roof, though, stay outside.

Then they fit the sixteen-millimeter lens, tilt the camera up and photograph the apartment building together with its roof, leaving a wide margin at the edges. When they correct the verticals in the program, the roof too is bolt upright; but the building has gone squat, the windows have turned into horizontal rectangles. With a vertical stretch they bring the windows to their real proportion. The window frames on the upper floors are clearly softer than in the frame shot with the shift lens.

On the balcony beside the bay window a dark-skinned old woman is watering her pots. Your friend calls up from below to ask permission, and the woman waves with a smile. In the frame corrected with software the woman's face has widened to both sides; in the shifted frame it is natural. Light reflected from the yellow wall opposite has left a greenish-yellow tone on the woman's brown skin; in the edit your friend selects only the face and adjusts the warmth and tint, and the redness in her cheek comes back. The apartment building's brick, meanwhile, stays as it is with the yellow light.

Common mistakes

Summary

Verticals are corrected without loss in the shot with a shift lens, but the shift range limits how far up a level camera can look. Software works with any camera, but softens the top of the building, makes it squat and stretches the people in the frame; vertical stretch brings back the proportions, while partial correction is not a rule but a decision made according to how the frame will be read.

Mini assignment

In this assignment you will correct the same building in two ways and compare the results.

1. Choose a building in a narrow street no more than four floors high; note how much of it enters the frame with the camera standing level, and if you can find a shift lens, shoot with it.
2. Photograph the same building at your widest angle, tilting the camera up and leaving a generous margin around it; correct the verticals in the program and bring back the windows' proportion with a vertical stretch.
3. In one frame, give a place to someone standing in front of or at a window of the building; after the correction, look at whether their face stretched and whether a nearby colored surface left a tone on their skin.

Check yourself

• In the frame you corrected with software, how much did the detail at the top of the building soften?
• Are the windows in their real proportions?
• If you left a convergence in the verticals, does it read like a decision, or like carelessness?

Glossary

Mini quiz

1. What does the shift range decide?

• A) The lens's widest aperture
• B) How far up a level camera can see
• C) How far the plane of focus can be tilted
• D) The lens's closest focusing distance

2. In a frame with a strong perspective correction made in software, what happens to the top of the building?

• A) It looks sharper
• B) Its color changes
• C) It softens because a few pixels are spread over a wide area
• D) Nothing changes

3. How is a corrected building's squat look removed?

• A) With a vertical stretch, by looking at the real proportion of the windows
• B) By narrowing the frame horizontally
• C) By raising the contrast
• D) By rotating the frame

4. Why does the face of a person standing on a balcony in the upper part of the frame widen after a strong software correction?

• A) Because of the lens's color aberration
• B) Because the person moved
• C) Because the exposure fell short
• D) Because the program widened the upper part of the frame to both sides

Answers

1. B — The farther the optical part can slide, the farther up the level camera's frame reaches.
2. C — Because the widening stretches the pixels at the top, the cornice and eaves detail blurs.
3. A — Stretching the frame upward brings the windows and the building back to their real proportion.
4. D — The correction widens the top; the face there stretches sideways together with the building.

Summary card

• Shifting is lossless; the shift range draws its limit.
• Software works with any camera; it softens the top and makes the building squat.
• Remove the squatness with a vertical stretch, by looking at the windows' proportion.
• A strong correction stretches the faces at the top of the frame too.
• Partial correction is a decision; a half tilt reads as carelessness.

Bridge to the next lesson

They can now correct the verticals, but the brick apartment building in front of the yellow wall had stayed in shade in the morning, dull and flat. Passing the same street in the late afternoon, your friend sees that the sun is striking the facade from the side and the bricks suddenly stand out and redden. At what time of day does a building look its best? The next lesson explains how the direction a facade faces decides when it will catch the sun, and how a building's lights and the sky are balanced at blue hour after sunset.