Swirly bokeh lenses explained

Swirly bokeh is the effect where out-of-focus highlights toward the edges of the frame stretch into ellipses that all point around the centre, so the background appears to rotate around the subject. The Helios 44-2 made it famous and the Carl Zeiss Jena Biotar 58mm f/2 it copied did it first. The cause is not a secret formula. It is mechanical vignetting in a fast, compact Double Gauss lens, and every variable that controls it is one you can change.

What actually happens

A point of light behind the plane of focus renders as a disc the shape of the aperture. In the centre of the frame that disc is round. Toward the edges, light from an off-axis point has to pass through the lens at an angle, and in a lens with a front or rear element only just large enough for the on-axis beam, the barrel and element edges clip part of the beam. The clipped disc becomes a lemon or cat's-eye shape whose long axis runs tangentially, at right angles to a line drawn to the frame centre. Because every cat's eye in the field is oriented the same way relative to the centre, the eye reads the pattern as rotation. That is the swirl. Optically it is a fault: the lens is not delivering its full aperture to the corners.

Why Biotar-type lenses do it

The Biotar and its Soviet copies are six-element, four-group Double Gauss designs from the 1930s to 1950s, built fast (f/2 at 58mm, f/1.5 at 75mm and 85mm) in small barrels. Their designers accepted heavy mechanical vignetting at full aperture as the price of size and speed, since film users stopped down anyway. Modern fast primes are built with oversized front and rear groups precisely to avoid cat's-eye highlights, which is why a current 50mm f/1.4 renders round discs to the corners and no swirl. The vintage look is the absence of that correction.

The four variables that control swirl

1. Aperture. Swirl is strongest wide open and gone by about f/4. Stopping down makes the iris, not the element edges, the smallest opening in the path, so off-axis beams stop being clipped and highlights become round again.
2. Background texture. Swirl needs many small highlights. Backlit foliage, a gravel path in sun, dappled light through trees, or distant fairy lights work; a smooth wall does not, because there is nothing to stretch.
3. Distances. Put the subject about one to three metres away and the background several metres further back so the highlights are far enough out of focus to form discs. Highlights just behind the subject render too small to show shape.
4. Sensor size. The stretched discs live in the outer third of the 35mm frame. A full-frame sensor captures them; an APS-C sensor crops half of that zone away and a Micro Four Thirds sensor almost all of it. A focal reducer restores the full-frame view on a crop body and brings the swirl back.

Swirl, soap bubbles, and busy bokeh are different faults

Three vintage rendering effects are often lumped together. Swirl, described above, comes from mechanical vignetting. Soap-bubble bokeh, where each highlight has a bright rim and a dimmer centre, comes from undercorrected spherical aberration in simple designs such as the three-element Trioplan 100mm f/2.8; the bubbles stay round to the edges because the Trioplan does not clip its beams heavily. Busy or nervous bokeh, where backgrounds break into hard-edged doubled lines, comes from overcorrected spherical aberration and is a feature of some sharp modern designs. A lens can show more than one at once: the Meyer Primoplan 58mm f/1.9 swirls and bubbles together. The Jupiter-9 85mm f/2, a Sonnar with a 15-blade iris, is often expected to swirl and does not; its highlights stay round and smooth, which is a different vintage look.

Vintage lenses known for swirl

Getting the most swirl from a Helios

Shoot the 44-2 at f/2 on a full-frame body, put the subject two metres away with backlit leaves five to ten metres behind, and frame so the subject sits in the central third. The centre stays reasonably sharp while the edges rotate. Use the preset ring's smooth travel to stop down a third of a stop if the wide-open glow is too much; the swirl survives to about f/2.8 on the 44-2 and to about f/2.8 on the 40-2 as well. Focus with magnification, because the plane of focus at f/2 on a 58mm is shallow and the subject needs to be genuinely sharp for the rotating background to read as intentional.

Some owners reverse the front element of a Helios 44-2 to exaggerate the effect. It works, because a reversed front group vignettes even more heavily, but it also destroys sharpness across the whole frame and cannot be undone without reassembly. A Helios 40-2 gives more swirl than a modified 44-2 with no loss of centre sharpness, and a Biotar gives the same drawing with better build. The Helios 44-2 variant guide explains which version of the 44 swirls most (the 8-blade single-coated 44-2) and how to be sure you are buying one.