Why Is My Field of View (FOV) Moving While Focusing the Lens#
You may notice that the field of view (FOV) is moving up and down while focusing the lens by rotating its ring clockwise and counterclockwise.
Some mechanical hysteresis while focusing the lens may be the reason for the moving FOV.
Lenses are build from several individual lenses. When focusing a lens there is a change in length from the first to the last lens but also a change in position of the lenses to each other. The latter is necessary to assure that the different wavelengths meet again at one point. Since these changes are made via mechanical slide rails, a different anchor point is hit when the direction of rotation is reversed and ensures a slightly different overall focal length and lens positions.
Different lenses from different manufacturers most likely have different tolerances for that. Unfortunately, Basler doesn't have any details about that.
As a workaround, Basler recommends focusing the image only from one direction or by changing the distance of the camera and lens to the object.