RGB shift

Look through a cheap magnifier at the edge of a black frame, or at a branch against the sky in a photo from a budget camera, and coloured fringes run along every contrasty edge. That is chromatic aberration: a lens bends different colours by different amounts. This effect makes it on purpose — as an optical flaw, as misregistered printing or as the glitch of a worn videotape.

How it works

The refractive index of glass depends on wavelength; that is dispersion. Blue bends more than red, so a simple lens has a slightly different focal length — and hence image scale — for each colour. The resulting shift, lateral chromatic aberration, is zero at the centre and grows in proportion to the distance from the axis. The “radial” mode does exactly that: each colour samples the picture at its own slightly different scale, and “Shift” sets the spread at the frame edge. In “directional” mode the channels simply slide apart along the “Direction”, like printing plates out of register.

Without “Spectrum” you get three copies, red, green and blue. With it the effect takes seven wavelengths from blue to red and builds the channels from them through smooth sensitivity curves: a rainbow fringe, as with real glass, instead of three ghosts. “Distortion” adds a radial warp that grows with the cube of the distance from the centre: barrel for positive values, pincushion for negative ones.

A bit of history

Newton believed a lens telescope could never be rid of colour fringes, and in 1668 he built a reflecting one instead. He was wrong: around 1733 Chester Moor Hall paired lenses of crown and flint glass, whose dispersions differ, so their colour errors nearly cancelled, and in 1758 John Dollond patented the achromat. Ernst Abbe gave dispersion its measure: ordinary BK7 crown has an Abbe number of 64, F2 flint 36, and the lower the number, the brighter the rainbow at the edges. An achromat brings two colours to one focus, an apochromat three.

What to tweak

• A “Shift” of 0.003–0.008 is a barely visible fringe that makes a picture feel photographic; from 0.03 up it is an outright glitch.
• “Directional” without “Spectrum” is misregistration: three coloured outlines, as in cheap printing or an anaglyph.
• “Distortion” at 0.3–0.6 is the barrel of an action camera or a cheap wide-angle; negative values give a telephoto's pincushion.
• Aberration lives on edges and all but vanishes on smooth gradients. It looks best on Op art, Wave interference, Maze and Guilloché and spirograph — especially after Glow.