Zoom lens barrel and method for adjusting back focal distance thereof

A method for adjusting a back focal distance of a zoom lens barrel which includes a front lens holder frame including an outer peripheral surface with a helicoid and holding a front lens group, a rear lens holder frame including a radially extending cam follower pin and holding a rear lens group, a linear movement guide mechanism which guides the front lens holder frame and the rear lens holder frame to move in the optical axis direction, and a rotatable drive ring including a helicoid which engages with the helicoid of the front lens holder frame and a cam groove in which the cam follower pin of the rear lens holder frame is engaged with cam groove being provided with a plurality of spaced back focal distance adjusting sections parallel with the lead of the helicoids and a plurality of zoom sections that connect adjacent ones of the spaced back focal distance adjusting sections and that are not parallel with the lead of the helicoids,

• the method comprising:
• detecting whether the cam follower pin is located in any one of the back focal distance adjusting sections, and
• stopping the drive ring when the cam follower pin is located in any one of the back focal distance adjusting sections.