Chromatic Aberration of Lens

Chromatic aberration in lenses

Chromatic aberration (CA) happens because a lens spreads light out. Each wavelength bends at a slightly different angle, so they don't all land in the same place: some focus at different depths (longitudinal CA), others end up at different sizes across the frame (lateral CA). The Abbe number V_d tells you how much a given optical glass disperses, and a higher V means less of it. For a rough estimate of axial CA you can use Δf ≈ f / V_d, so a 50 mm lens with V = 50 spreads red and blue focus by about 1.0 mm. Apochromatic designs use ED (extra-low dispersion) glass elements to pull three wavelengths back to one focus.

Applications

Premium lenses (e.g., Zeiss APO, Canon DO, Nikon ED), refractor telescopes for astronomy, portrait and product photography where clean colour edges matter, and post-processing CA correction in Lightroom, Capture One and DxO with lens profiles.

FAQ

What is the difference between lateral and longitudinal CA? Lateral (transverse) CA pushes colours sideways and gets worse toward the edges of the frame. Longitudinal (axial) CA shifts where things focus and turns up as magenta or green fringes on bokeh.

Can stopping down fix CA? It cuts longitudinal CA a lot, since focus depth tightens up, but it does almost nothing for lateral CA. That one you usually clean up in software.

What does "ED" mean on a lens? ED is extra-low dispersion glass with a high Abbe number, often 80 or more. Makers use it to keep CA down in long telephotos and apochromatic designs.