A lens that maintains focus throughout the zoom range — only the framing changes, not the focal point.
Technical Details
Parfocal lenses achieve their property through complex lens constructions with 15-25 elements in 10-16 groups, which are moved relative to each other in precisely calculated curves. Typical cine zooms like the Angenieux 25-250mm T3.5 or Fujinon Premier 18-85mm T2.0 maintain focus across the entire focal length range with a deviation of a maximum of ±0.02mm at the sensor. The mechanical precision requires manufacturing tolerances in the micrometer range, which is why these lenses achieve weights between 2-8kg. Modern parfocal zooms use aspherical lenses and low-dispersion glass to minimize aberrations while maintaining constant sharpness.
History & Development
Canon developed the first parfocal zoom lens for television cameras in 1961, the TV Zoom 17-68mm. Angenieux perfected the technology in 1964 with the 12-120mm for 16mm cameras, which was used in Kubrick's "2001: A Space Odyssey." In the 1980s, parfocal zooms established themselves as the standard for documentary and ENG through lenses like the Zeiss 11-110mm. Since 2010, computer-aided designs and precise CNC manufacturing have enabled parfocal full-frame zooms like the Fujinon 20-120mm Cabrio.
Practical Use in Film
Roger Deakins used the Angenieux Optimo 24-290mm for the famous hotel sequence in "No Country for Old Men" to seamlessly switch between wide shots and details. In live broadcasts, parfocal lenses allow operators to make spontaneous framing changes without refocusing. Documentary filmmakers appreciate the flexibility in unpredictable situations. The disadvantage lies in the higher weight and reduced light transmission compared to prime lenses - typical cine zooms are 2-3 stops slower than comparable primes.
Comparison & Alternatives
Varifocal lenses lose focus when changing focal length and require refocusing – an advantage for precise focus control in a controlled studio environment. Prime lenses offer higher light transmission and better image quality, but require lens changes for different focal lengths. Modern servo zooms combine parfocal properties with motorized control for remote applications. In high-end cinema production, manually focused parfocal zooms like the Cooke Varotal series continue to dominate.