Tube lens, imaging optical system and microscope

A tube lens used in combination with an objective of an infinite correction type that enlarges an image of an object, the tube lens consisting of, in a following order from an object side;

• a first lens group including a cemented lens and having a positive power;
• a second lens group having a negative power;
• and
• a third lens group including a positive lens and a negative lens and having a positive power as a whole,
• wherein:
• the first lens group includes a lens having a concave surface facing an image side;
• the second lens group includes a lens having a concave surface facing the object side, and
• the tube lens satisfies the following conditional expressions:
• 0.3<D2/FL<1.3;
• 0<|RG2/RG1|<3;
• 1.5<NdG2;
• 70<νdG1;
• NdG3p>1.7;
• and
• νdG3n<40,
• where FL is a focal length of the tube lens, D2 is a distance from a lens surface of the tube lens closest to the objective to an exit pupil position of the objective, RG1 is a radius of curvature of the concave surface of the lens in the first lens group, RG2 is a radius of curvature of the concave surface of the lens in the second lens group, NdG2 is a refraction index of the lens in the second lens group having the concave surface facing the object side with respect to a d line, vdG1 is a highest Abbe number in Abbe numbers of a lens having a positive power included in the first lens group, NdG3p is a refraction index of the positive lens included in the third lens group with respect to the d line, and vdG3n is an Abbe number of the negative lens included in the third lens group.