Lens aberration where lines of different orientations focus at different planes, producing soft, blurred image edges.
Technical Details
Astigmatism is measured in diopters and arises from different radii of curvature of the lens surfaces in tangential and sagittal directions. In full-frame lenses, the aberration typically appears 15mm from the image center with values between 0.2 to 1.5 diopters. Modern aspheric lenses reduce the effect to below 0.3 diopters. A distinction is made between tangential astigmatism (focal line radial to the image center) and sagittal astigmatism (focal line concentric to the image center). The astigmatism coefficient A₃ mathematically describes the strength of the aberration.
History & Development
Astigmatism was first scientifically described in 1860 by the physicist Thomas Young. In photography, Ernst Abbe at Zeiss recognized its importance for lens design in 1881 and developed initial correction methods. The Cooke Triplet from 1893 was one of the first lenses with reduced astigmatism. Modern ED (Extra-low Dispersion) glasses since the 1970s and aspheric elements since the 1980s now enable correction to below 0.1 diopters in top-tier lenses.
Practical Use in Film
In wide-angle shots in "Blade Runner 2049," Roger Deakins deliberately used slightly astigmatic vintage lenses for more organic image edges. Documentarians often use more affordable lenses with higher astigmatism, creating characteristic "soft" corners. In studio photography, astigmatism charts with radial line patterns are used for quality control. The aberration intensifies at wide apertures and significantly weakens at f/5.6, which is why landscape shots are usually filmed at f/8-f/11.
Comparison & Alternatives
In contrast to spherical aberration, which occurs symmetrically, astigmatism is direction-dependent. Coma aberration creates comet-shaped blur, while astigmatism forms linear focal planes. Modern floating element designs dynamically correct astigmatism during focusing. Digital corrections in post-production can partially compensate for the effect, but only for static values, not for focus-dependent variations.