I noticed this on both CCTV and "TV lens" C-mount lenses.

Is this for manual black level calibration, or protecting some early sensor or *icon tube style from wear, or entirely another reason?

It does give interesting photographic results when stopped down to a hair's breadth, you get visible spheres from all the dust in the system :)

Also, some "TV" lenses have a square vignette inside that seems to be only effective if the lens is wide open - if this helps with stray light or similar problems, why has this not been implemented in still camera lenses - while you risk the vignette being misoriented, modern (post-screwmount) camera mounts do force an orientation...

(while these are primarily videography lenses, C mount lenses can be used for still photography too, so I did not consider this completely OT).

  • Whether a lens has a rectangular frame built into it depends on the lens design. They're usually not needed in modern cameras because the frame is built into the camera as part of the film gate or shutter mechanism. Some modern lenses, made for digital still photography, have a frame built into them. Some C-mount lenses do not have the frame.

    XF 18-55/2.8-4 Fujian 35/1.7

  • Clickless apertures without a minimum stop size predate clicked apertures. The features are retained in film lenses because cinematographers find them useful. They can be adjusted smoothly and silently while filming, and they can fade to black.

    Photographers use lenses with click stops because the clicks are convenient. The aperture doesn't need to move smoothly between stops when capturing still images. The clicks let the photographer "know" what aperture is set, and they keep the aperture from changing while other settings are adjusted.

    See Early Photography: Lens Diaphragms.

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