Lens system and imaging device

An imaging device comprising:

• a lens system;
• and
• an imaging element that receives an optical image formed by the lens system and converts the received image into an electric image signal,
• wherein the lens system comprises a plurality of lens groups, each including at least one lens element, the lens system comprising, in order from an object side to an image side:
• a first lens group that is fixed with respect to an image plane in focusing from an infinity in-focus condition to a close-object in-focus condition;
• an aperture diaphragm;
• a second lens group that moves in a direction of an optical axis in the focusing;
• a third lens group that is fixed with respect to the image plane in the focusing;
• a fourth lens group that moves in the direction of the optical axis in the focusing;
• and
• a lens group closest to the image side,
• wherein the following conditions (1) and (2) are satisfied:
• 2.5<|f2/f4|<6.5 (1)
• 0.5<|f/f4|<1.5 (2)
• where
• f2 is a focal length of the second lens group,
• f4 is a focal length of the fourth lens group, and
• f is a focal length of an entire system in the infinity in-focus condition.