Lens corrections

Learning goal

You will be able to remove in an editing program the traces a lens leaves on a photo, like corner darkening, bent lines and colored edge lines, and recognize the limits of these corrections. You will explain the profile correction the program applies with a single checkbox by reading the lens information the camera recorded, and know how to correct by hand when there is no profile; you will remove purple and green lines on high-contrast edges with the defringe tool; you will take into account why geometric correction crops the edges of the image and the cost of the edge crop; you will tell apart that sometimes leaving corner darkening on purpose is a choice; and you will understand that a wide angle stretching faces near the edge is not a lens flaw and can't be removed with profile correction.

The question

The highland frame is sharp and clean; but when you look at it whole on screen, its four corners seem shaded, and the distant horizon draws a slight curve toward both sides. When you enlarge the cyclist in the red jacket, you notice a thin purple line on the edge of the jacket where it meets the sky. In the four-person group photo you took with a wide angle the same day, on the other hand, the face of your friend at the very edge looks wider and more squashed than the others.

Chasing each of them separately with sliders could take a whole evening. Can lens flaws be fixed with one click? Which ones go with a checkbox, and which ones don't go even with a correction? In this lesson we will talk about the lens corrections in an editing program.

Context

No lens gathers light flawlessly. In Lesson 12.7 we met the vignetting of lenses that darkens the corners, the barrel and pincushion distortions that bend straight lines, and the chromatic aberration that separates colors from each other at the edges, and we mentioned the lens profile prepared to remove these flaws. Lens designers give ground on weight, size and price to make these flaws smaller; but no design brings them all to zero at once.

Editing programs take on part of this load. The flaws of most lenses have been measured in advance and introduced to the program; as long as the program knows which lens a frame was shot with, it can calculate these flaws and reverse them. This lesson tells how to turn these corrections on, when to step in by hand, and the cost of the corrections, that is, how much of the edges of the image you lose.

Simple explanation

How does profile correction work?

Profile correction is the program automatically reversing a lens's previously measured vignetting and distortion, using that lens's profile. From the EXIF information we saw in Lesson 15.5, the program reads which lens was used at which focal length and chooses the suitable profile. Usually ticking a single checkbox is enough: the corners brighten, bent lines straighten. Many programs also have sliders that set how strongly the profile will be applied; for example removing corner darkening not completely, but halfway.

A profile isn't always available. With very old lenses, lenses mounted with an adapter, or lenses that don't pass on EXIF information, the program can't know which lens was used. In this case the manual correction sliders come into play: straightening bent lines with the distortion slider, brightening the corners with the vignetting slider. While doing this, looking at a straight line in the frame, a horizon, the edge of a building or a door frame, makes it easier to find the right amount.

What does defringe do?

Chromatic aberration shows up in two forms. Toward the edges of the frame, on one side of high-contrast borders a reddish line, and on the other side a cyan one, can appear; this kind is most often removed on its own by the profile or a single checkbox. The other kind, on the other hand, appears especially in frames shot at a wide aperture, as a purple or green halo around dark edges standing against a bright sky or a window, and can be seen in the middle of the frame too.

Defringe is the tool that selects these purple and green edge lines and mutes their color. In many programs it is possible to pick the line's color with an eyedropper and remove it. But this tool can touch everything in the selected color range: it can also fade a purple sweater, the color of a flower, or the natural tones at the edge of lips and skin. After removing the fringe, you need to enlarge and check the other parts of the frame, especially faces.

What is the cost of geometric correction?

Geometric correction is the correction that stretches the shape of the image to remove distortion, or the converging verticals we saw in Lesson 16.4. When correcting barrel distortion, the program pulls the middle of the image slightly in and its corners slightly out; when correcting converging verticals, it widens the top of the image. After this stretching, empty, gray or white areas form at the edges of the frame.

An edge crop is the part cut from the edges of the image to remove these empty areas after geometric correction. Most programs can make this crop on their own; but the result can be something that stood in the frame during the shoot, for example the tip of a tree or a person's hand, being cut off at the edge. That is why, in frames you know you will correct, leaving a little space at the edges during the shoot helps. Corner darkening, on the other hand, isn't always a flaw that must be removed: some photographers leave or add it on purpose to gather the gaze toward the center; this isn't a mistake, it is a choice.

Why doesn't edge stretching go away with a profile?

Edge stretching is round objects and faces standing near the edge, in a frame shot with a wide-angle lens, looking stretched sideways and squashed. This isn't a flaw of the lens, but a geometric result of projecting a wide angle onto a flat plane; a lens that keeps straight lines straight stretches the volumes at the edge. That is why profile correction doesn't remove this effect; sometimes, because it straightens bent lines, it can even make it a little more noticeable.

This effect is felt most in group photos and shows a person's face differently than it is: the face of the person at the edge widens, their chin squashes. Every face standing in that spot, whatever the color of its skin, is affected the same way; but this person may not recognize themselves in the photo. The solution is most often during the shoot: gathering people near the middle of the frame, stepping back a little and using a narrower angle, or not putting the most important person at the edge. Even if a limited improvement is possible in editing, the cleanest result comes from the right placement.

Visual evidence

In the highland frame you open the lens corrections panel and tick profile correction. The program finds your wide-angle lens and its sixteen-millimeter focal length from the EXIF; the corners brighten, the horizon line flattens. The program crops the empty areas that appear at the edges on its own; luckily you left space on the right edge while shooting, and the stone at the side of the road isn't cut off. You bring back half of the corner darkening; the gaze gathers on the cyclist again. You pick the purple line on the edge of the jacket with the defringe eyedropper and remove it.

Then you move to the portrait of your dark-skinned friend you shot against the light, against the sky. On the edge of their curly hair where it meets the sky there is a purple halo. When you apply defringe the halo disappears, but when you enlarge, the natural dark plum tone at the edge of their lips has faded a little too. When you narrow the tool's color range, the lips return to their own color, while the halo in the hair stays gone. In the group photo, the face of your light-skinned, freckled friend at the very edge is still wide and squashed; profile correction doesn't change this. You note that in the next group photo you will gather the four people in the middle and step back one step.

Common mistakes

Summary

Profile correction reverses the previously measured vignetting and distortion of the lens the program reads from the EXIF with a single checkbox; if there is no profile, it is corrected by hand looking at a straight line, while defringe removes purple and green lines on high-contrast edges but can also fade clothes, lips and skin edges in the same color range. Because geometric correction stretches the image, it requires an edge crop; that is why leaving space at the edges during the shoot helps, while leaving corner darkening on purpose is a choice. A wide angle stretching faces at the edge is not a flaw but a geometric result and doesn't go away with profile correction; to show people of every skin tone with their own face, the solution is the right placement during the shoot.

Mini assignment

In this assignment you will try lens corrections on your own frames and see their limits.

1. Choose a frame shot with a wide angle that has a straight line or a horizon in it and turn on profile correction; look at how the corners and lines change.
2. If the program cropped the edges after profile correction, note what was cut off; in the next shoot, leave space at the edges.
3. In a frame with a dark-edged subject against a bright sky, apply defringe; then enlarge and check the faces and colored clothes.
4. Shoot a group photo with a wide angle: first with someone at the very edge, then gathering everyone in the middle; compare the stretching of the face at the edge.

Check yourself

• While profile correction fixed the corners and lines, how much of the edges of the frame did it take?
• Did defringe also fade one of the natural colors in faces or clothes?
• In the group photo, does everyone, including the person at the edge, appear with the proportions of their own face?

Glossary

Mini quiz

1. What information does profile correction work from?

• A) The photo's file name
• B) The lens information in the EXIF and that lens's previously measured profile
• C) The photo's colors
• D) The card's speed

2. What is the reason empty areas form at the edges after geometric correction?

• A) The shape of the image being stretched
• B) The card being full
• C) The white balance shifting
• D) The file being corrupted

3. Why do you need to enlarge and check faces after applying the defringe tool?

• A) The tool blurs faces
• B) The tool brightens faces
• C) The tool can also fade lips and skin edges in the selected color range
• D) The tool crops faces

4. In a group photo shot with a wide angle, the face of the person at the very edge looks wide and squashed. Which is the most effective solution?

• A) Turning on profile correction
• B) Increasing defringe
• C) Increasing vignetting
• D) Gathering people in the middle during the shoot, or stepping back a little and using a narrower angle

Answers

1. B — The program recognizes the lens from the EXIF and applies the measured profile.
2. A — Gaps remain in the corners of the stretched image, and a crop is needed.
3. C — Defringe works by color range; natural tones in the same range can be affected too.
4. D — Edge stretching is geometric; the profile doesn't fix it, the solution is in placement.

Summary card

• Profile correction: lens from the EXIF, vignetting and distortion with one checkbox.
• If there is no profile, correct by hand looking at a straight line.
• Defringe removes purple and green lines; check faces and clothes.
• Geometric correction needs an edge crop; leave space while shooting. Leaving vignetting can be a choice.
• Edge stretching doesn't go with a profile; gather people in the middle.

Bridge to the next lesson

You edited the first few frames of the twenty-frame selection one by one; each one took half an hour. Yet they were all shot on the same day, in the same light and with the same lens; the white balance, profile correction and color settings you made on one will largely suit the others too. Is there a way to save the edit on one frame and apply it to the others in one go? How do I make my own preset? The next lesson talks about presets, syncing edits and batch editing.