Stop a lens down and the purple fringing around a backlit branch tightens up and mostly disappears. Stop the same lens down further and check the corners of the frame instead, and a completely different color fringe is sitting there, unmoved by every aperture change you just made. "Chromatic aberration" names both of these, and treating them as one phenomenon hides the fact that they come from different optical causes and behave nothing alike.
The One That Responds to Aperture
Longitudinal chromatic aberration is a focal-plane shift. Glass bends short wavelengths more than long ones — blue light refracts more sharply than red — so a simple lens doesn't bring all colors to focus at the same distance behind it. Blue focuses slightly closer to the lens, red slightly farther back, green somewhere between them. If the sensor sits at green's focal plane, red and blue both arrive slightly out of focus, in opposite directions along the axis.
That gap between focal planes is fixed by the glass — it doesn't move when you change the aperture. What does change is the width of the cone of light converging toward each focal point. Wide open, that cone is wide, so a wavelength focusing at the "wrong" plane produces a large blur circle by the time it reaches the sensor. Stop down and the cone narrows, so the same fixed focal-plane gap now produces a much smaller blur circle — small enough, at a narrow enough aperture, to read as effectively in focus. The dispersion in the glass never went away. The aperture just shrank the visible consequence of it.
The One That Doesn't Care About Aperture
Lateral chromatic aberration is a different failure: not a focus shift but a magnification difference. Different wavelengths get magnified by slightly different amounts by the same lens, and at the center of the frame that difference is negligible — but it grows toward the edges, where a red-channel image of an object ends up very slightly larger than the blue-channel image of the same object. The two colored copies don't overlap, and the mismatch reads as a fringe running along the edge of anything high-contrast near the frame's border.
This error tracks the angle at which a ray enters the lens, not the width of the aperture. A ray entering at a given angle from the axis gets magnified differently for blue versus red regardless of whether the aperture is wide open or stopped all the way down — the aperture controls which rays get through, not how much any surviving ray gets bent relative to its neighbors of a different wavelength. Stopping down removes some rays from the image entirely, but the rays that remain still carry the exact same lateral displacement they always had.
Why Both Trace Back to One Property of Glass
Both aberrations share a root cause: dispersion, the fact that a given piece of glass doesn't bend every wavelength by the same amount. This is quantified by a glass's Abbe number — low Abbe number means high dispersion, so blue and red separate more, both along the axis and in magnification; high Abbe number means the opposite. Achromatic lens designs pair a low-dispersion glass element with a high-dispersion one chosen so their errors largely cancel at two wavelengths; apochromatic designs extend that correction to a third wavelength, using combinations of specialty glasses (often including fluorite or extra-low-dispersion elements) that a simple two-glass achromat can't match. Neither design eliminates dispersion — no achievable glass combination bends every wavelength identically — they only push the residual error down to a level where it stops being visible under normal use.
Because the correction is never complete, every lens design leaves behind some specific, characteristic amount of longitudinal and lateral CA, in some particular color balance — some lenses lean toward a purple fringe, others toward green or magenta, depending on exactly which wavelengths their correction handles well and which it doesn't fully reach. That residual signature is stable for a given optical formula, which is why certain vintage and specialty lenses became recognizable by their uncorrected color fringing as much as by anything else about their rendering. For how a related property of the same glass elements — anti-reflective coating — became its own kind of signature, see our piece on Zeiss T coating. The same optical measurements that reveal an aberration's severity are what an MTF chart plots directly.
Neither aberration is a manufacturing accident. Both are the visible edge of a real physical limit — that no simple piece of glass bends every color exactly alike — showing up in two different places in the optical path, on two different schedules of correction.