Imaging optical system

An imaging optical system comprising, in a sequential order from an object side to an image side:

• a first lens having negative refractive power;
• a second lens having a meniscus-shape, positive refractive power, and a light input surface that faces toward the object side, the light input surface having a concave shape;
• a third lens having positive refractive power, a light input surface that faces toward the object side, and a light output surface that faces toward the image side, each of the light input and output surfaces having a convex shape toward a corresponding one of the object side and image side;
• a fourth lens having negative refractive power, a light input surface that faces toward the object side, and a light output surface that faces toward the image side,
• the fourth lens being configured such that
• the light output surface has an aspherical concave shape;
• the light output surface has a center portion through which an optical axis passes, and a periphery thereof;
• and
• the light output surface has a portion where the negative refractive power becomes gradually weaker from the center portion toward the periphery thereof;
• and
• a fifth lens having positive refractive power, a light input surface that faces toward the object side, and a light output surface that faces toward the image side, each of the light input and output surfaces having a convex shape toward a corresponding one of the object side and image side, wherein:
• the light input surface on the object side of the fourth lens has an aspherical shape;
• the aspherical-shaped light input surface of the fourth lens has a center portion through which an optical axis passes, and a periphery thereof;
• the aspherical-shaped light input surface of the fourth lens has a portion where the negative refractive power becomes gradually stronger from the center portion toward the periphery thereof;
• the aspherical-shaped light input surface of the fourth lens has one or more inflection points, wherein a level of the negative refractive power at each of the one or more inflection points of the aspherical-shaped light input surface changes;
• a change of a curvature of the light input surface on the object side of the fourth lens has an inflection point;
• and
• a change of a curvature of the light output surface on the image side of the fourth lens decreases substantially monotonically form the center portion toward the periphery, and has no inflection points.