A soft focus lens is a photographic lens that is deliberately designed with controlled, residual spherical aberration rather than being corrected for this aberration to the highest possible degree as conventional sharp lenses are, producing a characteristic soft focus rendering that combines an underlying core of genuine optical sharpness with a surrounding halo of diffused light that imparts the distinctive luminous, flattering, and romantically soft quality associated with this style of image making. The deliberate incorporation of spherical aberration as a creative optical tool rather than a defect to be eliminated is the defining design principle of the soft focus lens, and distinguishes it fundamentally from both standard sharp lenses and simple diffusion filters.
Spherical aberration occurs in a lens when light rays passing through the outer zones of the lens aperture are brought to a focus at a slightly different distance from the lens than rays passing through the central zone, rather than all rays converging to a single precise focal point as an ideal aberration-free lens would achieve. In a conventional photographic lens, optical designers invest considerable effort and complexity in correcting spherical aberration to minimise or eliminate its effects, as uncorrected spherical aberration degrades image sharpness and reduces the crisp edge contrast that photographers generally seek. In a soft focus lens, this correction is deliberately withheld or partially reversed, allowing the outer zone rays to form a slightly defocused halo of light around the sharp central image formed by the paraxial rays passing through the central zone of the aperture, creating the characteristic soft focus glow that gives these lenses their distinctive rendering.
The optical mechanism of spherical aberration based soft focus produces a quality of diffusion quite different from that of simple defocus or filter based diffusion, and this difference is the reason that photographers who value soft focus effects often prefer a dedicated soft focus lens over alternative approaches. Because the soft focus halo is formed by light that has actually passed through the lens and contributed to the image formation process - rather than being scattered by a separate filter element placed in front of the lens - it interacts with the image in a way that is optically integrated and natural, with the diffusion most pronounced in bright highlight areas where the halo contribution is greatest relative to the underlying image density, and least pronounced in shadow and mid-tone areas where the halo contribution is proportionally smaller. This highlight-biased diffusion produces the characteristic bloom of bright areas into surrounding darker tones that is the most sought-after and aesthetically pleasing quality of true soft focus rendering.
Many soft focus lenses incorporate a variable soft focus control - a separate adjustment ring or setting that varies the degree of spherical aberration independently of the aperture setting - allowing the photographer to dial in any desired degree of softness from barely perceptible to heavily diffused while maintaining independent control over depth of field through the aperture. This combination of adjustable soft focus and independent aperture control provides the most flexible and precise means of managing the soft focus effect, allowing the balance between softness, depth of field, and exposure to be optimised independently for each specific subject and shooting situation. Without a dedicated soft focus control, the degree of diffusion in a soft focus lens is typically managed through aperture selection - wider apertures allow more of the outer, aberrated zones of the lens to contribute to the image and produce stronger diffusion, while stopping down progressively restricts the aperture to the better corrected central zone and reduces the soft focus effect, eventually producing an almost conventionally sharp image at very small apertures.
Soft focus lenses have been manufactured by several major lens makers for portrait and pictorial photography applications, with celebrated examples including the Rodenstock Imagon - a distinctive soft focus lens used primarily in large format photography whose adjustable aperture discs allow precise and repeatable control of the soft focus effect - the Nikon 105mm f/2.8 Soft Focus and 135mm f/2 DC lenses, the Canon 135mm f/2.8 Soft Focus, and various soft focus portrait lenses produced for medium format camera systems. These lenses are valued by portrait, fashion, glamour, and fine art photographers who seek the distinctive optical quality that genuine spherical aberration based soft focus produces and who find that neither digital post-processing nor filter based diffusion can fully replicate the naturally integrated, optically produced soft focus rendering of a dedicated lens.