A telescopic sight for a firearm, comprising a housing , an optical system housed by the housing for presenting a telescopic image and an aiming point in said image to a user of the telescopic sight, and at least one sighting adjustment mechanism for making a sighting adjustment of said aiming point relative to said image, said sighting adjustment mechanism comprising a movable plunger and a manual adjustment control configured to move said movable plunger, the manual adjustment control comprising a rotatable turret knob , the turret knob being accessible to said user on the housing for manually effecting said sighting adjustment, wherein: - the optical system comprises within the housing at least one movable member , movement of said movable member causing said sighting adjustment; - the housing has a port therethrough, the plunger being slidably engaged in said port and linking the manual adjustment control with the movable member whereby, in use, the manual adjustment control causes the plunger to slide in said port when the turret knob is rotated resulting in said movement of the movable member to make said sighting adjustment, said slideable engagement being such that the housing interior remains atmospherically sealed as the plunger slides in said port; and - the manual adjustment control is removably affixed to the housing , characterised in that the manual adjustment control comprises an actuation surface that is an end surface of a substantially cylindrical post or a movable piston , which substantially cylindrical post or movable piston rotate with the knob , said actuation surface makes abutting contact with an outer end surface of a stem of the plunger so that when the manual adjustment control is to be disconnected from the housing in order to remove the manual adjustment control from the housing, the actuation surface separates from said outer end surface to enable the manual adjustment control to fully separate from the plunger whereby the plunger remains slidably engaged in said port in order to maintain said atmospheric seal of the housing interior when the manual adjustment control is removed from the housing. - the optical system comprises within the housing at least one movable member , movement of said movable member causing said sighting adjustment; - the housing has a port therethrough, the plunger being slidably engaged in said port and linking the manual adjustment control with the movable member whereby, in use, the manual adjustment control causes the plunger to slide in said port when the turret knob is rotated resulting in said movement of the movable member to make said sighting adjustment, said slideable engagement being such that the housing interior remains atmospherically sealed as the plunger slides in said port; and - the manual adjustment control is removably affixed to the housing , characterised in that the manual adjustment control comprises an actuation surface that is an end surface of a substantially cylindrical post or a movable piston , which substantially cylindrical post or movable piston rotate with the knob , said actuation surface makes abutting contact with an outer end surface of a stem of the plunger so that when the manual adjustment control is to be disconnected from the housing in order to remove the manual adjustment control from the housing, the actuation surface separates from said outer end surface to enable the manual adjustment control to fully separate from the plunger whereby the plunger remains slidably engaged in said port in order to maintain said atmospheric seal of the housing interior when the manual adjustment control is removed from the housing. <EOS>