Focus puller
A focus pulling device or focus pulling device (English follow focus ) is a device attached to a camera lens with which the distance setting ring of the lens can be moved and thus the focus of the image plane can be changed.
While the distance setting can simply be carried out on the distance setting ring for lenses of still cameras, this is more difficult with film cameras that continuously record image sequences, as one wants to avoid the viewer in the cinema or in front of the television seeing an uneven change or even interruption (if at the ring must be grasped) of the focus, or even by touching the lens the camera is shaken.
For this reason, focus pulling devices have been developed that allow a smooth, fluid change in focus without having to touch the lens directly. In the simplest case it is a protruding pin attached to the distance ring of the lens that can be moved back and forth.
In the professional field, however, sharpening devices have now become established in which the distance setting ring is connected to a rotary wheel via gears and ring gears on which z. B. rotates the camera assistant to change the focus level. Special lenses for cinema cameras usually have a permanently attached / integrated toothed ring , while there are clip-on toothed rings for other lenses, for example in the area of video DSLRs . A gear on the focus puller meshes with this ring gear. The gear, in turn, is connected to the rotating ring (possibly via additional gearwheels). In order to be able to adapt the focus puller to different lens diameters, the gear is mounted on a pivotable arm. The entire focus pulling device must also be mounted in a position that cannot be changed with respect to the lens. This is usually achieved by attaching both the camera with the lens and the focusing device to a support system (e.g. 15 mm or 19 mm lightweight supports), so-called rods.
In order to ensure compatibility between lenses and focus pulling devices, the tooth spacing of the lens ring gear and the gear wheel engaging in it is standardized, although there are several standards. In the field of cinema technology, teeth with a module of 0.8 (corresponding to a tooth spacing of around 2.51 mm) are common.
There is a similar device for setting the f-number on the lens.