A method for thermally compensating lenses in an optical system for high power lasers, comprising the steps of: providing a high power monochromatic laser, said high power monochromatic laser emitting a high power monochromatic laser beam that causes heat-induced changes in an index of refraction of a lens (26, 28, 30, 32) through which said high power monochromatic laser beam passes; providing a positive dn/dT collimating lens (26, 28, 30, 32) formed from a first optical glass material and being thicker at a center than at an edge to collimate said high power monochromatic laser beam and positioning said positive dn/dT collimating lens (26, 30) in collimating relation to said high power monochromatic laser beam, said positive dn/dT collimating lens (26, 30) having an index of refraction; providing a focusing lens assembly to focus said collimated laser beam and positioning said focusing lens assembly in focusing relation to said collimated laser beam; and including as a part of said focusing lens assembly at least one lens (28, 32) formed from a second optical glass material that is different than the first optical glass material and being thicker at a center than at an edge, the at least one lense (28, 32) having a negative dn/dT to offset change in the index of refraction of said positive dn/dT collimating lens (26, 30) induced by the high power monochromatic laser, wherein the index of refraction of the positive dn/dT of the collimating lens becomes more positive in response to the heat-induced changes and the index of refraction of the negative dn/dT of the focusing lens becomes more negative in response to the heat-induced changes. providing a high power monochromatic laser, said high power monochromatic laser emitting a high power monochromatic laser beam that causes heat-induced changes in an index of refraction of a lens (26, 28, 30, 32) through which said high power monochromatic laser beam passes; providing a positive dn/dT collimating lens (26, 28, 30, 32) formed from a first optical glass material and being thicker at a center than at an edge to collimate said high power monochromatic laser beam and positioning said positive dn/dT collimating lens (26, 30) in collimating relation to said high power monochromatic laser beam, said positive dn/dT collimating lens (26, 30) having an index of refraction; providing a focusing lens assembly to focus said collimated laser beam and positioning said focusing lens assembly in focusing relation to said collimated laser beam; and including as a part of said focusing lens assembly at least one lens (28, 32) formed from a second optical glass material that is different than the first optical glass material and being thicker at a center than at an edge, the at least one lense (28, 32) having a negative dn/dT to offset change in the index of refraction of said positive dn/dT collimating lens (26, 30) induced by the high power monochromatic laser, wherein the index of refraction of the positive dn/dT of the collimating lens becomes more positive in response to the heat-induced changes and the index of refraction of the negative dn/dT of the focusing lens becomes more negative in response to the heat-induced changes.