Focus

Technical Details

The depth of field is determined by three parameters: aperture, focal length, and object distance. With a 35mm lens at f/2.8 and a focus distance of 3 meters, the depth of field is approximately 1.2 meters. Modern cine lenses have focus rings with 270-300° rotation for precise adjustments, while photo lenses typically only offer 90-120°. Follow focus systems use standard gear modules (0.8 pitch) and enable focus pulling at speeds from 0.1 to 15 seconds between focus points.

Split focus describes the setting between two planes of focus, while rack focus (focus pull) refers to the dynamic change in sharpness during a shot. Deep focus keeps multiple planes of the image sharp simultaneously, typically achieved with apertures from f/8 or wide-angle lenses under 24mm.

History & Development

The first film cameras around 1895 used fixed-focus lenses without adjustment capabilities. In 1908, Zeiss introduced the first adjustable cinema lenses. Gregg Toland revolutionized the deep focus style in 1941 with "Citizen Kane" using extreme wide-angle optics and an f/16 aperture. The introduction of zoom lenses in 1950 necessitated new focus tracking systems.

Since the 1990s, electronic focus systems have enabled precise repeatability through encoder data. In 2010, wireless follow focus systems like the Preston FIZ system became established, offering sub-frame accuracy.

Practical Application in Film

Orson Welles used 24mm lenses at f/16 in "Citizen Kane" (1941) for consistently sharp images from 60cm to infinity. Sergio Leone strategically employed telephoto lenses at f/2.8 in his Westerns to isolate characters from the background. "The Social Network" (2010) systematically used f/1.4 settings with 85mm lenses for minimal depth of field.

Focus pullers work with measuring tapes and marks on the follow focus, with distance changes precisely predetermined to within 2-3cm. During Steadicam shots, focus is tracked wirelessly with a range of up to 500 meters.

Comparison & Alternatives

Autofocus systems in consumer cameras achieve focus times of 0.1-0.3 seconds, but are too slow and unpredictable for professional film production. Manual focus remains the standard, as it is the only way to achieve precise timing and artistic control.

Split diopter filters allow for different planes of focus without a focus pull, but create a characteristic dividing line in the image. Focus stacking from photography combines multiple focus planes digitally, but is rarely used in film due to the time involved.