Lens flaws

Learning goal

You will be able to recognize curved lines, darkened corners and colored fringes in a photo, explain their causes, and remove them in shooting or in editing. You will tell apart barrel distortion, which bends straight lines outward, from pincushion distortion, which bends them inward, and you won't confuse these with a wide angle's stretching at the edges; you will see why vignetting, which darkens the corners, becomes noticeable at a wide aperture; you will recognize purple or green fringes appearing at sharp, contrasty edges as chromatic aberration; you will learn how most flaws are corrected with a lens profile; and on faces left in a corner or standing in backlight, on every skin, you will check how these flaws look.

The question

In the courtyard of an old inn, you are photographing two men drinking tea under stone arches with a wide-angle lens. When you look at the photo, the pillars of the arches bulge slightly outward, and the corners of the frame are noticeably darker than its middle. On the edge of the white hair of the man sitting in front of the window there is a thin purple line; the young man in the corner, on the other hand, looks as if buried in shadow.

Why are the edges of the photo curved and dark? No lens can gather light perfectly; every design accepts some errors while reducing others. In this lesson we will talk about the three most common lens flaws, distortion, vignetting and chromatic aberration, their causes, and how most of them are easily corrected today.

Context

In Lesson 12.6 we said that many lenses give a slightly soft and dark image at the edges at their widest opening. This is only one part of lens flaws. A lens tries to gather light of different wavelengths, coming from different angles, into a single sharp image on the flat surface of the sensor; within the limits of glass and physics, this job can never be done quite perfectly.

Lens designers reduce these flaws with the shape and number of the glass elements and with special kinds of glass. But every reduction has a cost: a heavier body, a higher price or a smaller maximum aperture. Today many lenses are designed to leave part of the remaining flaws for software to correct. That is why knowing the flaws means knowing which one to remove in shooting and which one in editing.

Simple explanation

What is distortion?

Distortion is lines that are really straight looking curved in the photo. In barrel distortion, lines bulge outward from the middle of the frame; the middle of the frame looks swollen and the edges pulled back. It is common in wide-angle lenses and at the wide end of zoom lenses. In pincushion distortion, on the other hand, lines bend inward; the corners of the frame look pulled outward. This too often appears at the telephoto end of zoom lenses.

Distortion shouldn't be confused with the edge stretching we saw in Lesson 12.2. In edge stretching, straight lines stay straight, but objects at the edges of the frame, for example a face, stretch outward; this is the result of a wide angle being projected onto a flat sensor, and it happens with a perfect lens too. In distortion, on the other hand, the lens can't keep straight lines straight. When the edge of a building, a door frame or the horizon stands near the edge of the frame, distortion is noticed at once; on a face in the middle of the frame, it usually doesn't catch the eye.

What is vignetting?

Vignetting is the corners of the frame staying darker than its middle. Part of the light heading for the corners is caught by the lens body and its inner edges, and part of it reaches the sensor at a slanted angle and so gives weaker illumination. The result is a ring that slowly darkens from the middle toward the corners.

Vignetting is most noticeable at the widest aperture; it usually decreases when you stop the aperture down a step or two. Some photographers leave vignetting on purpose or add it in editing, because darkening corners can gather the gaze in the middle of the frame. But if there is something important in a corner, for example a face at the edge in a group photo, vignetting shows it darker than it should be. Dark skin left in shadow can lose its detail sooner with this darkening; light skin can take on a gray, dull tone.

What is chromatic aberration?

Glass bends light of different colors at slightly different angles; that is why a lens can't always gather red, green and blue light at exactly the same point. This is called chromatic aberration. At sharp, contrasty edges, for example at the edge of branches in front of a bright sky, strands of hair in front of a window, or the frame of a pair of glasses, it shows as thin purple or green fringes.

There are two kinds of chromatic aberration. One leaves fringes of opposite colors on the two sides of contrasty lines, mostly toward the edges of the frame; this kind doesn't decrease much when the aperture is stopped down, but is usually corrected easily with software. The other leaves purple and green fringes on bright edges just in front of and just behind the plane of focus; it is noticeable at the widest aperture and decreases as the aperture is stopped down. A purple line on the edge of the hair and face in a backlit portrait catches the eye on every skin and blurs the face's outline.

How are the flaws corrected?

Today many cameras and editing programs use ready-made correction information called a lens profile. A profile describes a particular lens's distortion, vignetting and chromatic aberration in measured form; the program recognizes the lens and erases these flaws automatically. Many cameras apply this correction themselves when shooting JPEG; with RAW, on the other hand, you may need to turn on profile correction in the editing program.

Correction has a cost too. When distortion is corrected, the edges of the image are pulled and a small piece is cropped from the edges of the frame; that is why, when shooting with a wide angle, not putting something important at the very edge of the frame is a good habit. Tools that remove color fringes fade purple and green tones, and if used too strongly, they can affect other colors at the edges too; after correcting, you need to look again at the edges of faces and the color of skin.

Visual evidence

You go back to the inn's courtyard. On the edge of the hair of the light-skinned, white-bearded old man sitting in backlight in front of the window, there is a thin purple line. In the corner of the frame, the face of a dark-skinned young apprentice sitting at the foot of the stone wall is buried in a dark shadow together with the vignetting; his eyes can barely be made out. The pillars of the arches bulge outward.

In the editing program, you turn on the lens profile. The pillars straighten; a thin strip is cropped from the edges, but nobody leaves the frame. The corners brighten, and on the apprentice's face the curve of his cheekbone, the line of his eyebrows and the hand holding the tea glass can be made out. With chromatic aberration correction, the purple line on the edge of the old man's hair disappears; the white of his beard and the pinkish color of his cheek stay as they are. The same frame; the flaws came from the lens, and a profile did the correcting.

Common mistakes

Summary

No lens is perfect: distortion bends straight lines outward in barrel and inward in pincushion, and is different from a wide angle's edge stretching; vignetting darkens the corners and becomes noticeable at the widest aperture; chromatic aberration leaves purple or green fringes at contrasty edges. Today most flaws are corrected with a lens profile, but distortion correction crops the edges a little and removing fringes can affect other colors too. On faces in a corner or in backlight, these flaws show on every skin; after correcting, you need to look again at the edges of faces and the color of skin.

Mini assignment

In this assignment you will find and correct your own lens's flaws.

1. Photograph a surface full of straight lines, a brick wall or a door, at both ends of your zoom, with the lines near the edge of the frame; note at which end you see barrel and at which pincushion distortion.
2. Photograph a plain sky or a white wall at the widest aperture and with the aperture stopped down two steps; compare the darkening of the corners.
3. Photograph a branch or strands of hair in front of a bright window at the widest aperture, and look for colored fringes at the edges by magnifying.
4. Compare the same frames in the editing program with the lens profile on and off.

Check yourself

• Were you able to find which kind of distortion becomes noticeable at which end of your lens?
• Did the vignetting decrease when you stopped the aperture down?
• After profile correction, were the eyes of the face in the corner and the edge of the backlit hair cleaned up?

Glossary

Mini quiz

1. In a photo taken with a wide-angle lens, if the straight edges of a building bulge outward, what is this?

• A) Pincushion distortion
• B) Vignetting
• C) Barrel distortion
• D) Chromatic aberration

2. If the corners of the frame are noticeably dark at the widest aperture, what can be done?

• A) Stop the aperture down a step or two, or correct the vignetting in editing
• B) Lower the ISO
• C) Move the focus point to the corner
• D) Lengthen the shutter speed

3. A thin purple line at the edge of strands of hair in front of a bright window is a sign of what?

• A) Barrel distortion
• B) Chromatic aberration
• C) Wrong white balance
• D) Camera shake

4. Why can't the stretching of a face at the edge of the frame with a wide angle be removed completely with profile correction?

• A) Because the profile only works on black-and-white photos
• B) Because it changes the face's skin
• C) Because the aperture is wrong
• D) Because edge stretching isn't a lens flaw, but the result of a wide angle being projected onto a flat sensor

Answers

1. C — Straight lines swelling outward from the middle are barrel distortion; it is common with a wide angle.
2. A — Vignetting is noticeable at the widest opening; it decreases when the aperture is stopped down, and can also be corrected with software.
3. B — Different colors failing to gather at the same point at contrasty edges leaves purple or green fringes.
4. D — Edge stretching happens with a perfect lens too; the solution is to bring the face in from the edges.

Summary card

• Barrel: straight lines outward; pincushion: inward. Edge stretching is something else.
• Vignetting: corners darken; noticeable at the widest aperture, decreases when stopped down.
• Chromatic aberration: purple or green fringes at contrasty edges.
• A lens profile corrects most of them automatically; distortion correction crops the edges.
• Look again at faces in corners and in backlight after correcting, on every skin.

Bridge to the next lesson

When choosing the lens profile, the editing program reads the lens's name and model from the information inside the file. The same information is written on the lens body too: focal length, maximum aperture, perhaps a stabilization abbreviation and the manufacturer's series codes. What do the markings on a lens tell you? The next lesson talks about reading these markings and abbreviations, and understanding what a lens can do by looking at its body.