Photography – With exposure objective focusing means – focusing aid – or... – Having viewfinder
Patent
1997-11-19
1998-11-17
Perkey, W. B.
Photography
With exposure objective focusing means, focusing aid, or...
Having viewfinder
G03B 1324
Patent
active
058390049
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a device and a method for facilitating the focusing of a camera, more particularly for the focusing of a camera during a so-called tilt operation in which an image plane/focusing plane of the camera is angled relative the optical axis of an objective lens or vice versa, in order to obtain an inclined plane of focus in accordance with the so-called Scheimpflug theorem.
BACKGROUND OF THE INVENTION
The viewfinder of a camera includes a focusing screen, on which the image of the selected object is projected. Said focusing screen is located in the image plane/focusing plane of the camera and the projected image on the focusing screen may thus be used for focusing adjustment. The focusing screen normally comprises a matte surface and a thin, positive fresnel lens abutting said matte surface. The fresnel lens serves to redirect the light from the matte surface (and especially from its edges) towards an observer's eye or an eyepiece, through which the image on the focusing screen is observed. The fresnel lens thus helps to obtain an evenly distributed light-intensity across the entire image-area.
The focusing screen is normally positioned perpendicularly to the optical axis of the camera. In some cameras, however, it is possible to angle or "tilt" the image plane/focusing plane relative the optical axis of an objective lens or vice versa, in a so-called tilt operation in order to obtain an inclined plane of focus. This helps the photographer to decide more easily which parts of the object that are to be in focus.
However, a serious problem arises when the incident light falls at an angle to a focusing screen comprising a fresnel lens.
The problem lies in that the exiting light has virtually the same direction as the incident light, which means that the light is directed away from the observer's eye or the eyepiece normally located directly above the center of the focusing screen. Thus, only a small fraction of the exiting light from the focusing screen can be viewed by the photographer, which makes focusing very difficult or--at larger tilt angles--impossible.
If the eyepiece is moved sideways so as to catch the exiting light, the photographer is forced to look through the eyepiece at an angle, which means that the eyepiece is used in an optically very unfavourable manner.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a device and a method for facilitating the focusing of a camera during a tilt operation, in order to compensate for the drawbacks described above.
This object is achieved in accordance with the present invention by means of a device and a method for facilitating the focusing of a camera during a so-called tilt operation, in which an image plane/focusing plane of a camera is angled relative the optical axis of an objective lens or vice versa, in order to obtain an inclined plane of focus, the camera being provided with a focusing screen located in said image plane/focusing plane and an optical viewing location, the optical axis of which coincides with the optical axis of said focusing screen, said tilt operation resulting in the exiting light from the focusing screen being directed away from said viewing location, wherein an optical wedge is located between said focusing screen and said viewing location in such a way as to redirect the exiting light to said viewing location at which an eyepiece or an observer's eye is located.
In a preferred embodiment, said optical wedge is a fresnel prism plate located adjacent to said focusing screen on the side of the exiting light.
In an advantageous embodiment of the invention, a plurality of fresnel prism plates having progressively larger deflection angles, are removably insertable adjacent to said focusing screen.
Said deflection angles preferably range from 0 to 30 degrees.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will now be described in more detail, by way of example only, and with reference to the attached drawings, in which:
FIG. 1 schematic
REFERENCES:
patent: 3864712 (1975-02-01), Von Stwolinski
patent: 4429964 (1984-02-01), Schaefer
patent: 4496231 (1985-01-01), Heitmann et al.
patent: 4602860 (1986-07-01), Hoffman
patent: 5034764 (1991-07-01), Inabata
patent: 5534959 (1996-07-01), Keelan et al.
Nordlund Per
Pettersson Erland
Perkey W. B.
Victor Hasselblad AB
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