An autofocus device for a folded optic imaging system having an image sensor (105; 125) and a primary light folding surface (122, 124) configured to redirect a portion of light from a target image scene in a first direction along an optical axis (121, 123,) the autofocus device comprising: an optical element having an input surface configured to receive light from the primary light folding surface (122; 124), a secondary light folding surface (110; 135) configured to redirect the received light in a second direction toward the image sensor (105; 125), and an output surface through which light redirected by the secondary light folding surface (110; 135) propagates in the second direction toward the image sensor (105; 125); a lens assembly comprising: a stationary portion having a first surface connected to the output surface of the optical element and a second surface connected to the image sensor (105; 125), and a movable portion (115; 130) positioned in an optical path between the image sensor (105; 125) and the primary light folding surface (122; 124); an actuator (180; 181) configured to move the movable portion (115; 130) of the lens assembly along the optical axis (121; 123) in a direction substantially parallel to a plane formed by the image sensor (105; 125); characterised in that the lens assembly further comprises at least one guide rail (184; 185) coupled between the actuator (180; 181) and the movable portion (115; 130) of the lens assembly, the at least one guide rail (184; 185) positioned to slidably engage another surface within the folded optic imaging system to constrain motion of the movable portion (115; 130) of the lens assembly away from the optical axis (121; 123) or rotating around the optical axis (121; 123), and in that the stationary portion of the lens assembly comprises a field corrector lens. an optical element having an input surface configured to receive light from the primary light folding surface (122; 124), a secondary light folding surface (110; 135) configured to redirect the received light in a second direction toward the image sensor (105; 125), and an output surface through which light redirected by the secondary light folding surface (110; 135) propagates in the second direction toward the image sensor (105; 125); a lens assembly comprising: a stationary portion having a first surface connected to the output surface of the optical element and a second surface connected to the image sensor (105; 125), and a movable portion (115; 130) positioned in an optical path between the image sensor (105; 125) and the primary light folding surface (122; 124); an actuator (180; 181) configured to move the movable portion (115; 130) of the lens assembly along the optical axis (121; 123) in a direction a stationary portion having a first surface connected to the output surface of the optical element and a second surface connected to the image sensor (105; 125), and a movable portion (115; 130) positioned in an optical path between the image sensor (105; 125) and the primary light folding surface (122; 124); an actuator (180; 181) configured to move the movable portion (115; 130) of the lens assembly along the optical axis (121; 123) in a direction substantially parallel to a plane formed by the image sensor (105; 125); characterised in that the lens assembly further comprises at least one guide rail (184; 185) coupled between the actuator (180; 181) and the movable portion (115; 130) of the lens assembly, the at least one guide rail (184; 185) positioned to slidably engage another surface within the folded optic imaging system to constrain motion of the movable portion (115; 130) of the lens assembly away from the optical axis (121; 123) or rotating around the optical axis (121; 123), and in that the stationary portion of the lens assembly comprises a field corrector lens.