Optical element, front optical unit of an objective, objective and microscope

An optical element comprising:

• a planoconvex basic shape along an optical axis of the optical element;
• a first portion and a second portion, each extending around the optical axis;
• and
• a third portion arranged between the first portion and the second portion, wherein the third portion has a concave-convex form;
• wherein the first portion includes a plane side face for facing an object to be imaged, and the second portion includes a convexly shaped side face for facing an image plane;
• wherein the plane side face of the first portion is designed to collect and steer a radiation to be captured into the optical element, and the optical element is designed to guide rays of the captured radiation in a beam path;
• wherein the first, the second and the third portions are arranged directly adjacent to one another;
• and
• wherein a refractive index of the third portion is greater than a refractive index of the second portion, and wherein the refractive index of the second portion is greater than a refractive index of the first portion.