Chromatic Aberration and Color Fringing
Look closely at a photo's high-contrast edges - a branch against a bright sky, the rim of a backlit glass - and you'll often catch a thin purple or green fringe that isn't really there. It's chromatic aberration: the lens can't focus every color to the same spot, because glass bends blue light more than red. The same dispersion that paints rainbows is a defect inside every lens. This is the interactive guide to why colors split, and how optics and software put them back together.
One focus per color
A perfect lens would bring every ray from a point of light back to a single point on the sensor. A real lens can't - and one reason is color. The lens bends (refracts) light, and the amount of bending depends on wavelength: blue is bent a little more than red. So the "focal length" of the lens is subtly different for each color, and the colors land in slightly different places. Where they miss, you see fringes.
This is chromatic aberration (CA), and it comes in two flavors that behave very differently. Longitudinal CA spreads the colors along the axis - they focus at different depths - and shows up as halos even in the center of the frame. Lateral CA spreads them sideways - the colors are focused at slightly different sizes - and grows from zero at the center to worst at the corners. Understanding both is the key to spotting and fixing them.
Dispersion: why glass splits light
The root cause is dispersion: a material's refractive index - how much it slows and bends light - falls gently as wavelength rises. Blue (short) sees a higher index than red (long), so it bends more. Send white light through a prism and this fans it into a spectrum, blue deviated most. Glasses are rated by their Abbe number: high Abbe means low dispersion (colors stay together), low Abbe means high dispersion (they spread). Widen the dispersion and watch the fan open.
White light fanned into a spectrum
A beam of white light enters a prism and exits as a fan of colors, blue bent the most. The dispersion control stands in for the glass: low dispersion (a high-Abbe glass) keeps the colors nearly together; high dispersion (a low-Abbe glass) throws them wide apart. This same wavelength-dependent bending, inside a lens, is exactly what causes chromatic aberration.
Longitudinal CA
Because each color has its own focal length, when you focus one color sharply the others are slightly out of focus - focused a hair in front of or behind the sensor. A point of light therefore grows a soft colored halo: often magenta/purple on the near side of focus and green on the far side. This longitudinal (axial) CA appears everywhere, including dead center, and is worst on fast lenses wide open. Rack the focus and watch the halo flip color.
A point of light, focused one color at a time
A bright point through a fast lens. Each color focuses at a different depth, so as you shift focus the sharp color changes and the others bloom into a halo - purple-ish on one side of focus, green-ish on the other. Stopping the lens down (a smaller aperture) shrinks every color's blur and tames the halo.
Lateral CA and the fringe
The fringing you notice most lives at the edges. Lateral (transverse) CA happens because the lens magnifies each color slightly differently - the red image is a touch bigger or smaller than the blue one. The channels are perfectly aligned at the optical center and drift apart the farther out you go, so a crisp edge near the corner splits into a colored seam - red/cyan on one side, blue/yellow on the other. Push the CA and watch the edges bleed.
Channels that drift apart toward the corners
A high-contrast test pattern. Lateral CA scales the red and blue channels radially relative to green, so the center stays clean and the fringes grow toward the edges - exactly the pattern real lenses show. Notice the colored seams appear on the radial edges and get wider the farther they sit from the middle.
Correcting it
Lateral CA has a clean cure. Since it's a per-color magnification difference, you can undo it by scaling the red and blue channels back into register with green - which is precisely what a lens-profile correction does in the RAW converter. Optically, designers pair a converging low-dispersion glass with a diverging high-dispersion one (an achromatic doublet) so two wavelengths refocus together; apochromats correct three. Turn correction on and watch the seams close.
Re-registering the color channels
The same fringed pattern, with a correction control. Software correction scales the drifted red and blue channels back toward green by the opposite of the lens's CA; dial it to match and the fringes vanish. Overshoot and they reappear in the opposite colors - which is why real correction reads the exact amount from a lens profile.
Where you meet it
Chromatic aberration isn't only a lens flaw to fight - it shows up, and gets used, all over.
Best practices and pitfalls
Test your understanding
Six questions on dispersion, longitudinal and lateral CA, and correction. Instant feedback, no scores recorded - a wrong answer comes with a short explanation.
Quick check
Continue your journey
Chromatic aberration is dispersion inside a lens - here's the physics around it and where it lands in imaging.
The Physics of Light, Wavelength, and Spectrum
Refraction and dispersion - the mechanism behind CA.
Physics · 40Color in Nature
The rainbow - the same dispersion, but in raindrops.
Physics · 62Diffraction and Diffraction Gratings: The CD Rainbow
A different way light fans into color - and how it differs from dispersion.
Digital · 42Camera Color: From Photons to Pixels
Where lens CA sits in the camera's imaging chain.
Physics · 54Thin-Film Interference and Iridescence
Another optical route to color - interference, not refraction.
Digital · 39Color in Film and Video
Where a touch of CA gets added back as a stylistic look.