Image forming optical system and electronic image pickup apparatus equipped with same

An image forming optical system comprising, in order from its object side:

• a first lens unit having a negative refracting power;
• a second lens unit having a positive refracting power;
• a third lens unit having a negative refracting power;
• and
• a fourth lens unit having a positive refracting power, wherein
• the air gaps between the lens units are variable during zooming,
• the first lens unit comprises one positive lens,
• the second lens unit comprises one negative lens,
• the third lens unit consists of one or two lens components,
• the fourth lens unit consists of one lens component,
• and the following conditional expression (1) is satisfied:
• 0<M3/M2<0.55 (1),
• where M2 is the amount of movement of the second lens unit during zooming from the wide angle end to the telephoto end in the state in which the zoom lens is focused on an object point at infinity, M3 is the amount of movement of the third lens unit during zooming from the wide angle end to the telephoto end in the state in which the zoom lens is focused on an object point at infinity, and the term “lens component” refers to a single lens or a cemented lens.