Method and apparatus for focusing a photographic apparatus

Claim 1

In a photographic apparatus having an objective and a device for determining when the objective has focused an image in a predetermined plane, said device comprising an optical system having a single optical axis extending only along a single straight line, said optical system having along said optical axis thereof an image plane normal to said optical axis and in which a given image is situated when the objective has properly focused an image corresponding to said given image in said predetermined plane, said optical system having along said optical axis spaced from said image plane a detecting plane which is also normal to said optical axis and including in said detecting plane a pair of identical elongated photosensitive detecting means which have respectively optically equivalent positions with respect to said optical axis, each of said detecting means providing an output which varies as an image is moved longitudinally along each detecting means, and optical means situated along said optical axis between said image plane and detecting plane for transmitting, when said given image is in said image plane, a pair of identical separate images, each of which is derived from said given image, respectively to identical positions along said pair of detecting means, and said optical means displacing the identical images transmitted to said pair of detecting means in opposite directions, respectively, along said pair of detecting means as said given image is displaced with respect to said image plane, so that the outputs of the said pair of detecting means will be different if said given image is before or behind said image plane along said optical axis while the outputs will be the same if the latter image is situated in the said image plane, said optical means including a relay lens means and a projector lens means spaced from each other along said optical axis with said relay lens means being situated nearer to said image plane than said projector lens means, said relay lens means receiving said given image from said image plane and transmitting said given image toward said projector lens means while the latter projects said identical images toward said detecting plane, and said optical means further including between said relay lens means and projector lens means a refracting means for receiving said given image from said relay lens means and for providing from said given image said pair of identical images which are transmitted by said refracting means to said projector lens means to be projected thereby to said detecting plane, said refracting means including a pair of prism means spaced from each other along said optical axis with one or said pair of prism means being situated nearer to said relay lens means than the other of said pair of prism means, said one of said pair of prism means acting on an image received from said relay lens means to provide from the latter image a pair of identical images which are in line with each other, and which are situated in the plane of the other of said pair of prism means, and said other of said pair of prism means acting on the pair of in-line identical images received thereby to arrange said identical images received thereby into a pair of identical images which are parallel and spaced from each other, said projector lens means receiving from said other of said pair of prism means said parallel identical images to project said identical images respectively to said pair of detecting means at said detecting plane, and indicating means operatively connected with said pair of elongated detecting means for indicating when the identical images transmitted by said optical means to said pair of detecting means are respectively situated at identical positions therealong, for thus determining when the objective has been set to focus an image in said predetermined plane.