Lens optical system

A lens optical system, comprising:

• a first lens group in which a first lens of an object side is composed of a meniscus lens having a negative refractive power, and having a positive refractive power as a whole;
• a second lens group arranged at an image side I than the first lens group, the second lens group being a focusing group for correcting a change in image distance depending on a change in object distance, being composed of two or less lenses, and having a positive refractive power as a whole;
• and
• a third lens group arranged at the image side I than the second lens group, the third lens group having a negative refractive power as a whole, in which a first lens of the image side I is composed of a concave or meniscus lens,
• wherein when the second lens group is focused while moving, the first lens group and the third lens group are fixed to have a constant length of an overall length.