Zoom lens system for use in microfilm projection apparatus

Claim 2

A zoom lens system for use in a microfilm projection apparatus in which the image projection is conducted at a condition of a definite object-image distance, the system comprising from the enlargement side to the reduction side: an aperture stop disposed fixedly; a first lens group of a positive refractive power located near the aperture stop at the largest focal length side; a second lens group of a negative refractive power; and a third lens group of a positive refractive power, wherein the first to third lens groups are moved along the optical axis all toward the shortest focal length side while increasing a distance between the first and second lens groups and decreasing a distance between the second and third lens groups in an operation of zooming from the longest focal length side to the shortest focal length side.

Claim 3

A zoom lens system as claimed in claim 1, wherein the first lens group includes, from the enlargement side, a positive lens element, a biconcave lens element and a positive lens unit having at least one positive lens element.

Claim 4

A zoom lens system as claimed in claim 1, wherein the second lens group includes, from the enlargement side, a positive lens unit having at least one positive lens element and a negative lens unit having at least one negative lens element.

Claim 5

A zoom lens system as claimed in claim 1, wherein the third lens group includes a compound lens unit composed of two lens elements cemented together.

Claim 6

A zoom lens system as claimed in claim 1, wherein the magnitification is varied between 20x and 28x in accordance with the zooming operation.

Claim 7

A zoom lens system as claimed in claim 1, wherein the second lens group includes, from the enlargement side, a positive lens unit having one positive lens element and a negative lens unit having one negative lens element.

Claim 8

A zoom lens system for use in a microfilm projection apparatus in which the image projection is conducted at a condition of a definite object-image distance, the system comprising from the enlargement side to the reduction side: an aperture stop disposed fixedly; a first lens group of a positive refractive power located near the aperture stop at the longest focal length side, the first lens group including, from the enlargement side, a positive lens element, a biconcave lens element and a positive lens unit having at least one positive lens element; a second lens group of a negative refractive power, the second lens group including, from the enlargement side, a positive lens unit having at least one positive lens element and a negative lens unit haivng at least one negative lens element; and a third lens group of a positive refractive power, the third lens group includes a compound lens unit composed of two lens elements cemented together.

Claim 9

A zoom lens system for use in a microfilm projection apparatus in which the image projection is conducted at a condition of a definite object-image distance, the system comprising from the enlargement side to the reduction side: an aperture stop disposed fixedly; a first lens group of a positive refractive power located near the aperture stop at the longest focal length side, the first lens group including, from the enlargement side, a positive lens element, a biconcave lens element and a positive lens unit having at least one positive lens element; a second lens group of a negative refractive power, the second lens group including, from the enlargement side, a positive lens unit having at least one positive lens element and a negative lens unit haivng at least one negative lens element; and a third lens group of a positive refractive power, the third lens group includes a compound lens unit composed of two lens elements cemented together.

Claim 10

A zoom lens system for use in a microfilm projection apparatus in which the image projection is conducted in a condition of a definite object-image distance at a range between 20x and 28x, the system comprising from the enlargement side to the reduction side: an aperture stop disposed fixedly; a first lens group of a positive refractive power located near the aperture stop at the longest focal length side, the first lens group including, from the enlargement side, a positive lens element, a biconcave lens element and a positive lens unit having at least one positive lens element; a second lens group of a negative refractive power, the second lens group including, from the enlargement side, a positive lens unit having at least one positive lens element and a negative lens unit having at least one negative lens element; and a third lens group of a positive refractive power, the third lens group includes a compound lens unit composed of two lens element cemented together; wherein the first to third lens groups are moved along the optical axis while increasing a distance between the first and second lens groups and decreasing a distance between the second and third lens gropu in an operation of zooming from the longest focal length side to the shortest focal length side.

Claim 11

A zoom lens system for use in a microfilm projection apparatus in which the image projection is conducted in a condition of a definite object-image distance at a range between 20x and 28x, the system comprising from the enlargement side to the reduction side: an aperture stop disposed fixedly; a first lens group of a positive refractive power located near the aperture stop at the longest focal length side, the first lens group including, from the enlargement side, a positive lens element, a biconcave lens element and a positive lens unit having at least one positive lens element; a second lens group of a negative refractive power, the second lens group including, from the enlargement side, a positive lens unit having at least one positive lens element and a negative lens unit having at least one negative lens element; and a third lens group of a positive refractive power, the third lens group includes a compound lens unit composed of two lens element cemented together; wherein the first to third lens groups are moved along the optical axis while increasing a distance between the first and second lens groups and decreasing a distance between the second and third lens gropu in an operation of zooming from the longest focal length side to the shortest focal length side.

Claim 12

A zoom lens system for use in a microfilm projection apparatus in which the image projection is conducted at a condition of a definite object-image distance, the system comprising from the enlargement side to the reduction side: an aperture stop disposed fixedly; a first lens group of a positive refractive power located near the aperture stop at the longest focal length side, including from the enlargement side, a positive lens element, a biconcave lens element and a positive lens unit having at least one positive lens element; a second lens group of a negative refractive power, and a third lens group of a positive refractive power, wherein the first to third lens groups are moved along the optical axis while increasing a distance between the first and second lens groups and decreasing a distance between the second and third lens groups in an operation of zooming from the longest focal length side to the shortest focal length side.

Claim 13

A zoom lens system for use in a microfilm projection apparatus in which the image projection is conducted at a condition of a definite object-image distance, the system comprising from the enlargement side to the reduction side: an aperture stop disposed fixedly; a first lens group of a positive refractive power located near the aperture stop at the longest focal length side, including from the enlargement side, a positive lens element, a biconcave lens element and a positive lens unit having at least one positive lens element; a second lens group of a negative refractive power, and a third lens group of a positive refractive power, wherein the first to third lens groups are moved along the optical axis while increasing a distance between the first and second lens groups and decreasing a distance between the second and third lens groups in an operation of zooming from the longest focal length side to the shortest focal length side.

Claim 14

A zoom lens system as claimed in claim 9, wherein the second lens group includes, from the enlargement side, a positive lens unit having one positive lens element and a negative lens unit having one negative lens element.

Claim 15

A zoom lens system for use in a microfilm projection apparatus in which the image projection is conducted at a condition of a definite object-image distance, the system comprising from the enlargement side to the reduction side; an aperture stop disposed fixedly; a first lens group of a positive refractive power located near the aperture stop at the longest focal length side; a second lens group of a negative refractive power, and a third lens group of a positive refractive power including a compound lens unit composed of two lens elements cemented together, wherein the first to third lens groups are moved along the optical axis while increasing a distance between the first and second lens groups and decreasing a distance between the second and third lens groups in an operation of zooming from the longest focal length side to the shortest focal length side.

Claim 16

A zoom lens system for use in a microfilm projection apparatus in which the image projection is conducted at a condition of a definite object-image distance, the system comprising from the enlargement side to the reduction side; an aperture stop disposed fixedly; a first lens group of a positive refractive power located near the aperture stop at the longest focal length side; a second lens group of a negative refractive power, and a third lens group of a positive refractive power including a compound lens unit composed of two lens elements cemented together, wherein the first to third lens groups are moved along the optical axis while increasing a distance between the first and second lens groups and decreasing a distance between the second and third lens groups in an operation of zooming from the longest focal length side to the shortest focal length side.

Claim 17

A zoom lens system as claimed in claim 11, wherein the second lens group includes, from the enlargement side, a positive lens unit having one positive lens element and a negative lens unit having one negative lens element.