Patent ID: 7768714

Claim:
An extender lens apparatus detachably attached to an image-forming optical system including a plurality of main lens units that includes a first main lens unit disposed closest to an object and a second main lens unit disposed between the first main lens unit and an image plane, the apparatus comprising: a first extender lens unit to be inserted in a space between the first main lens unit and the second main lens unit; and a second extender lens unit to be inserted in a space between the second main lens unit and the image plane, wherein the first extender lens unit has a magnification increasing effect, wherein the image-forming optical system includes an aperture stop, and wherein the following condition is satisfied: 0 < a + b c < 0.7 where a represents a distance from a lens surface closest to an object in the extender lens apparatus to the aperture stop, b represents a distance from the lens surface closest to the image plane in the extender lens apparatus to the image plane, and c represents a distance from the aperture stop to the image plane.