Camera Focus - The Physics Behind it

Camera focus happens when light rays from your object converge precisely onto the image sensor. Getting this right is the foundation of every machine vision system.

The distance between lens and sensor where sharp focus occurs is called the back focal length.
When you adjust focus, you're changing this distance to match your working distance (how far away your object is).

For M12 S-Mount lenses, each full rotation moves the lens 0.5mm (thanks to the M12×0.5 thread pitch). This shifts the focus plane by roughly 10-50mm at the object, depending on your focal length.
C-mount lenses work differently - they use an internal focus ring rather than threading.

Buzz Words & Meaning

Effective Focal Length (EFL):
Determines your field of view and depth of field. This is what lens specs typically list (e.g., "4mm lens").

Mechanical Back Focal Length:
Distance from the lens rear housing to the sensor. This determines if a lens physically fits your camera.

Flange Focal Distance:
For C-mount systems, this is standardized at 17.526mm from the mount flange to the sensor.

Note:
Optical back focal length (distance from last optical element to image plane) is rarely used in camera design.
The mechanical back focal length is what determines whether a lens will physically fit and focus with your camera. Consider The Imaging Source's M12 S-Pro or S-Ultra lenses for compact applications or C-Pro or C-Ultra C-mount lenses for standardized flange focal distance systems.