Circular light projection from a lens that must fully cover the sensor; full-frame requires 43.3 mm, Super35 requires 31.11 mm diameter.
Technical Details
Full-frame lenses produce an image circle of at least 43.3 mm in diameter, Super35 lenses 31.11 mm, and MFT lenses 21.6 mm. The usable image circle is typically 10-15% larger than the sensor size to compensate for manufacturing tolerances and mechanical adjustments. Cinema lenses like the Zeiss Master Primes offer an image circle of 46.3 mm for Super35 coverage with reserve. Illumination follows a cos⁴ function, causing brightness to naturally decrease towards the edges.
With anamorphic lenses, the image circle is elliptically distorted – Panavision Anamorphics cover 31.11 mm horizontally and 23.76 mm vertically. Shift and tilt lenses require disproportionately large image circles: Canon TS-E lenses achieve diameters of up to 67 mm for full-frame coverage at maximum shift.
History & Development
The concept of the image circle originated with the first photographic lenses around 1840. It became crucial in 1895 with cinematography, as standardized film formats required precise coverage. Zeiss introduced the first systematic image circle calculations in 1926 with the Biotar lenses.
The transition to digital sensors from 2000 onwards intensified the requirements – CCDs and CMOS chips are more sensitive to fall-off at the edges than film. The RED One (2007) established Super35 image circles as the standard for digital cinema cameras. Today, computer-aided optical designs enable precise image circle optimization for specific sensor sizes.
Practical Application in Film
For "Dunkirk" (2017), Christopher Nolan deliberately used lenses with a tight image circle for 65mm IMAX sensors to create natural vignetting. Roger Deakins, on digital productions, combines full-frame lenses on Super35 sensors for maximum edge sharpness and uniform illumination.
For drone shots, small sensors (1/2.3" on DJI Mini) only require a 7.81 mm image circle, but this allows for compact, lightweight optics. Underwater housings shift the effective image circle due to refraction – marine productions account for a 1.33x larger requirement.
Comparison & Alternatives
The image circle differs from the circle of confusion, which defines the tolerance for blur. The crop factor describes the ratio between the lens's image circle and the actual sensor area used.
Modern Speed Boosters from Metabones optically reduce the image circle by a factor of 0.71x, allowing full-frame lenses to be used on Super35 sensors while simultaneously increasing the aperture. Digital extenders virtually enlarge the sensor area used but require correspondingly larger image circles from the lenses.
Variable sensor modes, as found in the ARRI Alexa LF (switchable between Full-Frame/Super35), use the same image circle for different image crops, enabling focal length simulation without changing lenses.