The term stability refers to steadiness or unsteadiness in technical jargon of cinematography and is an amount of lateral and vertical motion in the screen image often caused by motion-picture cameras or projectors.
In cameras, only pull-down claw mechanisms are used for intermittent movement of film. This is explained by the fact that claw mechanisms are small in size and provide greater accuracy in the intermittent film transport systems than the other types of intermittent mechanisms such as Geneva (Maltese cross) mechanisms used in some 16mm and all other larger-gauge projectors, etc.
The accuracy of the frame (image) steadiness is the most important figure in the intermittent mechanisms of cameras among the other factors like the perforation pitch accuracy of various films (negatives, reversals and positives), print-process quality, projection stability, etc.
Measurement of unsteadiness:
The below must not be confused with the camera registration tests done at the rental facilities by assistant cameramen despite there are similarities.
There are two methods, double (overlapping) exposure or single exposure. The former is the most used method since the latter requires a specially designed optical device.
Filming should be done with normal focal length lenses.
Frame unsteadiness measurements of cameras are made by filming a Vernier test chart with a double-run exposure of film.
The Vernier test chart was so designed to detect clearly weave or jump (horizontal or vertical unsteadiness) of 0,1% frame height or width. It contains two sets of Vernier scales (bars), the horizontal set is used for weave measurement and the vertical set for jump measurement. Each Vernier set consists of two scales indicating relative movement of the two scales of 0,105% frame height or width per division. The camera mechanism is tested by exposing the chart with the EXPOSURE 2 scales obscured. The film is then re-wound and placed in the camera for a second exposure with the EXPOSURE 1 scales obscured.
Film, after processed, is seen through a microscope or a binocular magnifier with a magnification of at least 10x and by optical projection. Film should be processed by a proper ϒ (gamma: films’ contrast response to chemicals by time and temperature during processing).
Relative movement of the scales reproduced on the film negative is a measurement of lack of precision in location of the film in the gate.
The amount of displacement is measured on the film image. This should be checked on at least 25 different images at the beginning, middle and end of the film.
Rostrum and high-end cameras having the registration pin(s) are capable of locating a specific film frame in the same position repeatedly to an accuracy of better than ±0,05% frame height, so usually undetectable. Precision SFX cameras have an accuracy 0,007-0,008%.
Relative movement of the scales reproduced on the film is a measurement of lack of precision in location of the film in the camera gate and is independent of the perforation accuracy of the film.
A measurement is given in fractions, percentage (such as 1/1000 = 0,1% = 1‰) or length unit (mm). Values always change based on filming speed (fps).
Main causes of unsteadiness related to camera and magazine:
Poor engineering, machining and adjustment of claw mechanisms, film guides and pressure plates
Lack of registration-pin or extended retraction time of pull-down claw (The former is far better than the latter, if any)
In film gate (channel),
Improper lateral pressure (by side guide rail attached to aperture plate)
Improper vertical pressure (by camera or magazine plate)
Some camera examples:
AATON 35-II and Penelope steadiness: Lateral and vertical steadiness to 1/1000th of image dimension
AATON 35-III and XTRprod steadiness: Lateral and vertical steadiness to 1/2000th of image dimension
ARRI 765: Picture steadiness better than 1/1000 of frame height
ARRI 35III: Picture steadiness deviation is less than 1/1000 with reference to
ARRI 16SR: Picture steadiness better than 1/1250 of frame height
ARRI 16SRII: Picture steadiness better than 1/1000 of frame height
éclair NPR image stability: Vertical unsteadiness better than 1/1000th of frame height
éclair ACL image stability: Vertical unsteadiness better than 1/1000th of frame height
Cinema Products GSMO: Picture steadiness ±0,0127mm
KINOR-35P: Image unsteadiness, no more than 0,01mm
KINOR-35C and 35H: Image unsteadiness, no more than 0,015mm
RODINA: Image unsteadiness, no more than 0,01mm
KONVAS-AVTOMAT, all models: Image unsteadiness, no more than 0,02mm
KINOR 16CX, all models: Image unsteadiness, no more than 0,01mm