Chromatic aberration is color fringing along high-contrast edges, caused by a lens that cannot bring every color of light to exactly the same point. It is why dark branches against a bright sky grow thin purple, green, red or cyan outlines, and why a photo that looks sharp on screen can look slightly cheap in a large print. Once you know which of its two kinds you are looking at, you can prevent most of it when you shoot and remove most of the rest in a few seconds of editing.
Why glass splits light into colors
A lens forms an image by bending (refracting) light. Glass does not bend every color by the same amount: its refractive index is a little higher for short wavelengths (violet and blue) than for long ones (red), so blue light is bent more sharply. This is dispersion, the same property that lets a prism fan white light into a rainbow, which you can put to creative use in prism photography.
Glassmakers rate dispersion with a figure called the Abbe number. The lower it is, the more strongly a glass spreads colors apart. A lens that used only one kind of glass would focus blue nearest to the lens, green a little farther back and red farthest of all, as the top half of the diagram below shows.
Two everyday clues show this is real. Many older manual-focus lenses carry a small red mark beside the focus index: infrared light focuses even farther back than red, so infrared shooters shift focus to that mark. And a lens that formed its image with curved mirrors alone would have no chromatic aberration at all, because a mirror reflects every color at the same angle. The mirror lenses photographers use still contain a little glass, so they are nearly, not completely, free of it.
Why the fringes are purple and green
Lens designers fight dispersion by pairing elements made of different glasses, one spreading colors strongly and one weakly, shaped so that one element’s color error cancels much of the other’s. A lens corrected this way is called an achromat, and it brings two colors, usually red and blue, to the same focus. Green still lands a little apart from them. That leftover error is called secondary spectrum, and it is the key to the colors you actually see.
Now follow an edge that is slightly out of focus. Each color spreads into a small blur disc on the sensor, and the discs are not the same size, because the colors focus at different distances. On one side of the plane of focus the red and blue discs are bigger than the green one, so the outer rim of the blur contains red and blue light only. Red plus blue is magenta, which reads as purple. On the other side of focus the green disc is the bigger one, and the rim turns green.
That is why so many lenses show purple fringes on edges in front of the focus point and green fringes on edges behind it. It is typical, not a law: which side gets which color depends on the design. Lenses that bring three colors to one focus are called apochromatic, and they shrink the leftover error much further, though they do not remove it entirely.
Curious what makes a lens apochromatic, and whether the label is worth it? Achromatic and apochromatic lenses explains the special glass and what each correction buys you.
Lateral and axial chromatic aberration
The color error shows up in two distinct ways. Telling them apart matters, because they respond to different fixes.
Axial chromatic aberration (also called longitudinal CA) is the one just described: each color focuses at a slightly different distance from the lens. It can appear anywhere in the frame, including the dead center, wherever an edge sits slightly in front of or behind the plane of focus. You see it most in wide-aperture portraits and close-ups, where hair or twigs in front of the subject pick up a magenta rim and bright background highlights pick up a green one.
Lateral chromatic aberration (also called transverse CA) happens when the lens projects each color at a slightly different size. Every color can be perfectly in focus, yet the red image is a touch larger or smaller than the blue one. At the center of the frame the difference is zero, and it grows with distance from the center. The telltale sign is two different colors on opposite sides of the same object, red on one side of a lamppost and cyan on the other, or blue on one side and yellow on the other, lined up along lines that run out from the center.
- Cause: Axial (longitudinal): Colors focus at different distances. Lateral (transverse): Colors are projected at different sizes.
- Where it appears: Axial (longitudinal): Anywhere, including the center. Lateral (transverse): Grows toward the edges, none at the center.
- Typical colors: Axial (longitudinal): Purple on one side of focus, green on the other. Lateral (transverse): Red and cyan, or blue and yellow, on opposite sides of an edge.
- Stopping down helps?: Axial (longitudinal): Yes, often a lot. Lateral (transverse): Hardly at all.
- Software fix: Axial (longitudinal): Partial: the fringe can only be desaturated. Lateral (transverse): Easy and nearly complete.
Lateral CA is easy for software to remove because it is purely geometric: the editor enlarges or shrinks the red and blue channels until they line up with the green. Resampling the channels costs a sliver of corner detail, but far less than the fringe did. Axial CA is harder, because each color is blurred by a different amount, and no scaling can make a blurred channel sharp again. Software can only find the fringe and remove its color, leaving a grey edge.
How much stopping down shrinks axial fringes
Stopping down does not change where each color focuses. What it changes is the size of every blur disc. For an edge a fixed distance out of focus, the blur disc’s diameter is proportional to the diameter of the aperture, so it shrinks in step with the f-number: going from f/1.8 to f/3.6 halves it. The fringe is the difference between those discs, so it shrinks by the same proportion until it is too narrow to see.
| Aperture | Stops smaller than f/1.8 | Fringe width compared with f/1.8 | An 8 pixel fringe becomes |
|---|---|---|---|
| f/1.8 | 0 | 100% | 8 pixels |
| f/2.5 | about 1 | 72% | about 6 pixels |
| f/3.5 | about 2 | 51% | about 4 pixels |
| f/5 | about 3 | 36% | about 3 pixels |
| f/7.1 | about 4 | 25% | 2 pixels |
Each row is simple geometry: the fringe width scales with 1.8 divided by the new f-number, and the number of stops is twice the base-2 logarithm of the f-number ratio. Real lenses do not follow the table exactly, because their corrections change a little with aperture, but the pattern holds well: two stops roughly halves an axial fringe, and many fast lenses look clean by f/4 to f/5.6. Lateral CA does not appear in this table at all, because it comes from image size, not blur. A corner fringe at f/2 is usually still there at f/11. For more on how f-numbers and stops relate, and why stopping down also deepens your depth of field, follow those links.
Purple fringing on high-contrast edges
Purple fringing is the violet or magenta outline around dark objects against a bright background: branches against an overcast sky, window frames seen from a dim room. Why it is so visible is a matter of brightness.
The bright side of the edge is often several stops brighter than anything else in the frame. Only a tiny fraction of its defocused red and blue light spills past the edge onto the dark side, but because the source is so bright, that tiny fraction is still bright enough to show as a colored glow against near-black. The same branch against a mid-toned hill gives off far less stray light, and the fringe disappears into the noise. When the sky is clipped to pure white, the camera also loses the true color balance of the bright side, which tends to make the fringe wider and more saturated.
Purple fringing and chromatic aberration overlap, but they are not identical. Some purple edges come partly from the sensor rather than the lens: light striking a pixel at a steep angle near a very bright area can spill into its neighbors, and the camera’s color reconstruction (demosaicing) can invent false color on tiny, very bright details such as sparkles on water or chrome. Whatever the mix of causes, the fixes in the field and in the editor are the same.
Three conditions make purple fringing worse:
- Extreme contrast. A dark subject against a sky several stops brighter is the classic case. Shooting into the light, as in backlighting and silhouettes, makes scenes that are both attractive and demanding.
- Clipped highlights. Once the bright side of the edge is pure white, the fringe has nothing to blend into. Keeping the sky just below clipping narrows it.
- Wide apertures. Larger blur discs mean wider fringes, so a lens that fringes badly wide open may be clean two stops down.
Which lenses fringe most
Every lens has some chromatic aberration, because all glass disperses light. Knowing which designs are prone to it tells you when to be careful.
- Fast lenses at wide apertures. A fast lens at f/1.4 or f/1.8 makes large blur discs, so axial fringes are wide. The same lens at f/5.6 hides them.
- Wide-angle and ultra-wide lenses. Lateral CA grows with distance from the center of the image, and a design that covers a very wide angle of view has to balance color over a huge field. Expect red and cyan edges near the corners of a wide-angle lens.
- Long telephoto lenses. The color focus error grows with focal length, which is why serious telephoto lenses use low-dispersion elements to control it.
- Zooms with very large ranges. Correcting color across a tenfold zoom range is very difficult, so all-in-one zooms often show lateral CA at one end of the range or both.
- Simpler, older designs. Fewer elements and fewer special glasses mean less correction. Older manual lenses designed for film can fringe noticeably on a high-resolution sensor.
Designers have three main remedies. They pair glasses of high and low dispersion, they use special low-dispersion and fluorite-type glass, and in some lenses they add a diffractive element, a finely ridged surface that spreads colors in the opposite order to glass and so cancels some dispersion with less weight. Each extra corrective lens element adds cost, size and weight, which is why the problem is most visible where those were kept down.
Preventing fringing when you shoot
You cannot remove chromatic aberration entirely in camera, but you can often reduce it enough that editing becomes a quick touch-up rather than a rescue.
- Stop down one or two stops for axial fringes. If a portrait at f/1.8 shows magenta hair edges, f/2.8 or f/4 usually cleans them up, as the table above shows. This does nothing for lateral CA. Your lens’s sweet spot is usually a good compromise between fringing, sharpness and diffraction.
- Keep critical edges away from the corners. Lateral CA is zero at the center and worst at the edges. If a dark spire against a white sky is the point of the picture, placing it nearer the middle reduces fringing.
- Protect the highlights. Expose so the bright sky behind your subject is not clipped. A histogram that just touches the right edge gives a narrower fringe than one with a tall spike of pure white. See dynamic range for why backlit scenes push this limit.
- Change the angle to the light. A few steps to one side can put a branch against cloud rather than the sun, or a window frame against a wall instead of a blown-out view, which removes the contrast that makes fringes visible.
- Focus precisely. Axial fringes live just in front of and just behind the plane of focus. A subject that is exactly in focus shows much less fringing on its own edges.
- Use in-camera lens correction for JPEGs. Many cameras correct lateral CA for known lenses when they make a JPEG. If you shoot raw, the same correction happens in your editor instead.
Removing chromatic aberration in an editor
Almost every raw editor offers the same two controls, whatever it calls them. Use them in this order, and do it before sharpening, because sharpening raises edge contrast and makes any remaining fringe brighter and harder to remove.
Step 1: remove lateral CA automatically
Turn on the lens profile or the “remove chromatic aberration” option. For lateral CA this is close to perfect: the editor knows or measures the misalignment and rescales the red and blue channels to match. Check a corner at 100 percent before and after; red and cyan edges near the corners should vanish. The same profile can also correct lens distortion and vignetting.
Step 2: defringe what remains
Whatever is left is usually axial CA: purple in front of focus, green behind. Defringe controls find edges that carry a chosen hue and remove that color from the edge only. They usually give you a purple amount with a hue range, a green amount with a hue range, and often an eyedropper you click on a fringe to set the range for you. Raise the amount until the fringe turns grey, then narrow the hue range so only the fringe color is touched. Work at 200 percent or more.
Step 3: fix stubborn areas locally
Global defringing can grey out things that are genuinely purple or green: violet flowers, green leaves against the sky, a purple jacket. When that happens, lower the global amount and paint the correction only where the fringe is, with a local brush and masking. A brush that lowers saturation works as a fallback in editors without a local defringe control.
In a layer-based editor, or on a JPEG with no raw controls, the same two ideas work by hand: a lens correction filter with red/cyan and blue/yellow sliders realigns the channels for lateral CA, and a hue and saturation layer targeted at the fringe color, hidden with a black mask and painted back only over the fringed edges, greys out what remains.
Keep the correction subtle and judge it at 100 percent or more. Greying the fringe is usually enough; trying to paint it away completely tends to leave a visible smudge along the edge.
Fixing distortion and vignetting at the same time, or wondering when a lens profile does more harm than good? Lens corrections covers what each profile correction does, what it costs, and when to leave it off.
Which fringe is it? A four-question check
Before you reach for a slider, find out what you are looking at. Zoom to 200 percent on the colored edge and answer four questions.
- Is it at the center of the frame too? Lateral CA is zero at the center, so a fringe there is axial CA or something else.
- Are the colors different on opposite sides of the same object? Red on one side and cyan on the other, lined up along a line from the center, is lateral CA.
- Does it change with aperture? A fringe that shrinks as you stop down is axial. One that stays the same at every aperture is lateral, or not CA at all.
- Does it flip color across the focus plane? Purple on edges in front of your subject and green on edges behind it is the signature of axial CA.
- Two complementary colors on opposite sides of edges, growing toward the corners: What it probably is: Lateral CA. What fixes it: Lens profile or CA checkbox in the editor.
- Purple or green rims on out-of-focus edges, anywhere in the frame: What it probably is: Axial CA. What fixes it: Stop down when shooting; defringe in the editor.
- Purple glow on dark edges against a clipped sky: What it probably is: Axial CA made worse by clipping, sometimes with sensor spill. What fixes it: Protect the highlights; defringe.
- Haze, colored blobs or a line of shapes running from a light source: What it probably is: Lens flare. What fixes it: A hood, shading the lens, or moving the light out of frame.
- A glow centered on a bright light rather than lining a dark edge: What it probably is: Flare or veiling glare; true sensor blooming is rare on modern cameras. What fixes it: Expose less or accept it; CA tools will not help.
- Light and dark outlines that are neutral in color and equal on every side: What it probably is: Sharpening halos. What fixes it: Lower the sharpening amount or radius.
- False color across a fine repeating pattern, not along one edge: What it probably is: Moiré. What fixes it: Change distance or angle; moiré reduction in the editor.
- Random colored speckles in flat areas and shadows: What it probably is: Color noise. What fixes it: Lower ISO; color noise reduction.
- Color that stays when you stop down and center the subject: What it probably is: Real color in the scene, such as a colored rim light. What fixes it: Nothing: it is real.
Seeing glow and colored shapes around the sun rather than along dark edges? Lens flare explains where that stray light comes from and how a hood or a hand stops it.
Common mistakes with chromatic aberration
- Expecting stopping down to fix corner fringes. That only helps axial CA. Lateral fringes near the corners need the lens correction in your editor.
- Treating “purple fringing” and “chromatic aberration” as the same thing. Most purple fringing is axial CA, but clipping and the sensor contribute too, and lateral CA is rarely purple at all.
- Assuming a fringe at the center is lateral CA. Lateral CA is zero at the center. A fringe there is axial, or not CA.
- Pushing defringe to the maximum everywhere. It drains real purple and green from the whole frame. Use the lowest amount that works and a local brush for the rest.
- Sharpening before removing fringes. Sharpening makes the fringe wider and more intense. Remove CA first, then sharpen.
- Judging at fit-to-screen size. Small fringes hide on screen and show up in prints. Check edges at 100 percent or more before you export.
- Blowing out the sky behind silhouettes. Clipped highlights widen the fringe. Expose to keep the sky just short of clipping; a highlight warning shows you when you have crossed the line.
- Expecting an expensive or apochromatic lens to have none. Better correction means smaller fringes, not zero. Even very good lenses show a trace on extreme edges wide open.
Want to check you can tell axial from lateral CA, and distortion from both? The lens aberrations quiz explains every answer.
Try this: find your lens’s fringes in 10 minutes
Find dark, fine detail against a bright sky: bare branches, a power line, a railing. Set the camera on something steady and take four frames with the key edge in the center: one at your widest aperture, then one, two and three stops smaller (for an f/1.8 lens, f/1.8, f/2.5, f/3.5 and f/5). Then take two more with the same edge in a corner, one wide open and one at f/8.
Open the files at 200 percent. In the centered frames, the fringe should narrow as you stop down; that is axial CA, and the frame where it disappears is the widest aperture you can use for backlit scenes with that lens. In the corner frames, look for red and cyan edges that look the same at both apertures; that is lateral CA. Turn on your editor’s lens correction and those corner fringes should vanish. If you want a fuller workout for the same lens, how to test a lens adds sharpness, distortion and decentering checks to the same session.
Frequently asked questions about chromatic aberration
Is chromatic aberration a sign of a bad or damaged lens?
No. Every lens has some chromatic aberration, because all glass disperses light, and better designs only reduce it. Fringing that is much worse on one side of the frame than the other can point to a decentered element, which is a different fault worth checking.
Why does an old lens fringe more on a new camera?
A higher-resolution sensor records detail a lower-resolution one blurred away, and that includes fringes. The lens has not changed; you are simply seeing its color error at a finer scale. Older lenses designed for film also had less color correction than recent designs.
Should I correct chromatic aberration on every photo?
Lateral CA correction is harmless on almost every image, so many photographers apply it by default on import. Defringing can remove real color, so use it only when you can see fringes at 100 percent.
Does chromatic aberration matter in black and white?
Yes, though it looks different. After conversion, a fringe becomes a light or dark outline along the edge, which reads as softness. Removing CA before you convert to black and white gives crisper edges.
Can a filter reduce chromatic aberration?
No. The color error comes from the lens’s own glass, so nothing placed in front of it can undo it. A poor-quality UV or protective filter can add flare and softness of its own.