U.S. Pat. No. 5,727,236 that issued to Frazier on 10 Mar. 1998 describes an optical system that has the features of a wide angle lens, a deep field lens and a close focusing lens. The optical system aims to achieve a deep focus image, i.e. to make it possible to film macro subjects in focus in the foreground, while holding infinity also in focus.
The system of U.S. Pat. No. 5,727,236 comprises an objective lens, a field lens and a relay lens aligned sequentially on an optical axis and arranged in a lens cylinder or barrel.
The objective lens forms an intermediate image at or near the field lens, either in front or in back of the field lens. The objective lens may be fixed at infinity focus and have a wide open aperture for forming the intermediate image of a larger size than is otherwise normal for an objective lens of that focal length. The field lens and relay lenses transmit that same image to a smaller final image at a film plane. The relay lens is a macro lens and may have an iris and focusing mechanism, so that the objective and field lenses are not required to be used for aperture control and focusing. A Pechan prism, a roof prism, and a mirror for inverting and reversing (reverting) the intermediate image are provided in the barrel between the field and relay lenses. The Pechan prism, the roof prism, and the mirror ensure that the final image at the film plane has the normal orientation of the final image (instead of being inverted and reversed). Such an optical system requires a substantial amount of light to provide good depth of field. Also, the optical system has a substantial number of optical surfaces (i e., air to optical media surfaces). Every time a light ray has to travel from air to glass to air again, there is some degradation of the image. There may be as many as 50 air-to-glass surfaces in such an optical system. Further the Pechan prism or its optical equivalent and the roof prism are bulky and heavy making the optical system larger and heavier than otherwise would be the case.
Snorkel-type lenses have been used for some time, but all types have necessitated alteration of the camera setup and separately attach the lens systems directly to the camera body. Snorkel lenses allow access to difficult to photograph situations, including table-top photography or ground-level access photography. The depth of field is basically the same as with normal lenses.
Disadvantageously, such lenses require removal of a zoom lens from the camera. Further this requires time to set up the camera Still further, such snorkel-type lenses require higher light levels and consequently have greater lighting costs.
Double-axis swivel optical lens systems with image rotators have been used to deal with the placement and filming of difficult situations. The image rotator corrects image orientation to correct camera geometry, even if the camera is upside down or sideways oriented. The swivel tip of the optical lens system allows low, ground level shots to be easily accessed by keeping the camera off the ground, or either underslung or overhead shots, while the camera remains in a horizontal orientation.
Disadvantageously, however, such double-axis swivel optical lens systems are expensive to produce because such systems have a large number of lens elements and prisms. Typically, such systems have up to 15 or so objective lenses for handling different angles of acceptance. Further, such optical lens systems place a zoom lens on the front of the system. The various lenses add a significant amount of weight to the front end of a camera to which the lenses are attached and extend the overall length, with the swivels being too far back. This makes the optical lens system and camera awkward to use. Therefore, such lenses are difficult to manufacture economically and are cumbersome to use. Further, a large amount of light is required to operate such lenses. Typically, the optical lens system has an F stop of F 5.6 or smaller (e.g., F 8).