Abstract: A telephoto lens unit (1) having a lens barrel that supports a telephoto lens and capable of being mounted on and dismounted from a camera body (16) comprises a front lens barrel section (11) that supports the telephoto lens with the subject side defined as the front, and a rear lens barrel section (13) that can be mounted on and dismounted from the front lens barrel section. The distance from the image-most surface of the telephoto lens to the image formation plane of the telephoto lens is kept sufficiently long. Converter lens units (12, 14) for elongating or shortening the focal distance are mounted on the front lens barrel section (11) instead of the rear lens barrel section (13) to provide interchangeable telephoto lenses for a camera having different focal lengths. Also, an erecting prism unit (15) and an eyepiece unit (17) can be connected instead of the rear lens barrel section to enable the telephoto lens to function as a terrestrial telescope.

Abstract: A terrestrial telescope with digital camera has an imaging optical system having a group of objective lenses and an imaging element disposed at a position at which an optical image is formed by the group of objective lenses. An observation optical system is provided for observing the optical image transmitted by the group of objective lenses. An optical element is inserted into the optical axis so as to be retractable from the optical axis during imaging. The optical element has a transmission surface having a single-eccentric toroidal configuration with different curvatures in vertical and horizontal planes of the optical element for correcting the position on the imaging element at which the optical image is formed so that the position on the imaging element at which the optical image is formed remains substantially unchanged regardless of whether the optical element is inserted into or retracted from the optical axis of the group of objective lenses.

Abstract: A terrestrial telescope with a digital camera includes an objective lens group and a quick-return half-mirror located on the downstream side of the objective lens group. The quick-return half-mirror can be retractably inserted into the optical axis of the objective lens group. An image pickup device is provided downstream of the quick-return half-mirror, and there is an observation optical system for observing an image reflected by the quick-return half-mirror. The transmission surface of the quick-return half-mirror has a toroidal shape that corrects image-formation positional deviations caused by the insertion of the quick-return half-mirror.