Toronto. … my lens would unscrew. Photographers know that the infinity focus setting sets the lens at its closest to the film/sensor plane. For close-ups, the lens is extended further from the film/sensor plane. Box cameras for film often had small lens openings relying on the physical depth of field to keep subjects over a wide range of distances more or less in focus. An of course the shorter the focal length of the lens, the wider its depth of field.
At the time the tiny Leica was introduced, most lens close settings were 1 metre. More modern film lenses come to within about 20 inches and with auxiliary lens elements, extension tubes, or a bellows, they can go even closer to the subject.
As lens technology advanced, a physical stop was set to keep the lens from extending beyond the manufacturer’s design setting. For example, this mid last century 35mm Summaron lens uses a screw post to block rotation of less than 1 metre.
Traditional projector lenses had no physical stop, relying on the long tube length after the threads to keep the lens from being unscrewed by accident and dropped. Of course modern day lenses set to auto focus keep the owner’s hands off. Even with a manual setting, most stops to focussing travel are hidden from view.
Note: The post title is George Harrison’s 1970 song, sung here by Bob Dylan. The image shown is a slightly sharpened iPod Touch photo cropped for both the 1200 pixel and 225 pixel sizes.