Distance from lens mount to sensor that must be precisely calibrated for proper focus. PL-Mount: 52mm, Canon EF: 44mm.
Technical Details
The back focus differs from the flange focal distance (FFD), which is measured from the lens mount to the image plane. Standard values are: PL mount 52mm, Canon EF 44mm, Sony E-mount 18mm, Micro Four Thirds 19.25mm. Precise adjustment is made via a knurled system on professional cameras with an accuracy of ±0.02mm. Deviations lead to systematic focusing errors, which are particularly noticeable at wide apertures and with telephoto lenses. The back focus is calibrated using a focus chart or collimator.
History & Development
The back focus system evolved in the 1960s with the introduction of interchangeable lens systems for 16mm cameras. Arriflex established the current industry standard with a 52mm back focus with the PL mount (Positive Lock) in 1982. The digital revolution from 2000 onwards brought shorter back focus distances, as electronic sensors require less space than film transport mechanisms. Modern mirrorless systems use back focus values below 20mm, thus enabling more compact lens designs.
Practical Use in Film
Every camera change or lens swap requires a back focus check, especially in multi-camera setups for continuous cutting. Christopher Nolan insists on daily back focus calibration of all cameras for his productions. On "Dunkirk" (2017), this practice prevented focusing errors during the elaborate IMAX sequences. Zoom lenses are particularly sensitive to back focus deviations, as the focus must remain constant across the entire focal length range. Adapters for third-party lens systems alter the effective back focus and require corresponding correction.
Comparison & Alternatives
Back focus refers exclusively to the mechanical mounting, while front focus describes optical focusing. Mirrorless cameras partially compensate for back focus deviations through electronic correction in live view. Traditional film and DSLR cameras require mechanically precise adjustment, as the optical viewfinder receives the focusing image directly from the lens. Modern cinema cameras like the Arri Alexa offer factory back focus adjustment via precision threads, while more affordable systems rely on shims.