USRE17995E - Erecting finder - Google Patents

Description

March 10. 1931. 3. MITCHELL Re. 17,995

ERECTING FINDER I Original Filed May 22, 1925 i I nz/entar 6' 6079916. IfZL' tcizell.

Reissued Mar. I 10, 1931 UNITED STATES PATENT OFFICE A. mTOHELL, 01 LOS .ANGELES, CALIFORNIA, ASSIGNOR, BY MESR'E ASSIGN- MENTS, TO MITCHELL CAMERA CORPORATION, A CORPORATION OF DELAWARE ERECTING FINDER Originalllo. 1,887,080, dated October 9, 1928, Serial No. 110,866, filed May 22, 1926. Application for reissue filed October 7, 1930. Serial No. 487,075.

I Referring first to Fig. 1 I'show at a suit- This invention relates to finding instru ments for cameras and the like. The invene tion is useful as a'finding instrument in connection with other optical instruments, but as it has been particularly designed for use in connection with cameras it will be explained in that connection without, however,

limiting the invention to such-particular use. Particularly in motion picture cameras an 10' accurate and reliable finder is desired, as

' rect right and left relation, so as to avoid all great accuracy is required in getting exactly the correct field of view on the film. It is also highly desirable that the image in the" finding instrument be shown upright and in corpossible confusion in the mind of the operator. An object of the present invention is to provide a simple form of finding instrument that will show the field of view without inversion or reversal and will show it of a size suitable for easy inspection. It is a further particular object to provide an optical structure simple and effective in itself for erecting the image,.which would other 2 wise be inverted and trans osed right for by. the ordinary objective lens of the n er. 1

Other objects and correspondin accomplishments of the invention will e better understood from a consideration of the following detailed description, reference-for I ing another mo ified form.

It will be understood, and particularly so from what I say hereinafter, that the speclfic and particular forms herein shownand described are intended merely to be typical and illustrative of my -inventi on, andnot s cifically limitative thereof; the invention ing only limited as is set forth in the appended claims.

able translucent surface on which the image isreceived. This translucent surface may be any surface suitable for the purpose, many such being known in the art. Without intending any limitation, 1 shall here refer to it as a. ground glass; Preferably I form this ground-glass surface on the front face of a lightcollecting or slightly magnifying lens 11. This lens and its front image receiving shape, its proportions corresponding to the proportions of the picture being received in the camera to which the finding instrument is attached. The ground glass thus forms a 1 frame showing the outlines of the image actu ally received in the camera.

The ground glass 10 may be'mounted in any suitable tube or other housing 12, which also carries the erecting objective combination 13. This combination may be carried in a. separate casing 14, and part 14a of which may be tubular 1n shape so that it may slide in the housing 12 to adjust the distance between the objective combination and the ground glass. It will be understood, however, that to adjust this focal distance it is only necessary to move the ground glass and .face may be conveniently. rectangular in the objective combination relatively, that either one may be moved with reference to the housing. To keep'the objective combination always in one relative position (up-'- right, as shown in the drawings) a pin 146 may play in a longitudinal slot 140 as shown in Fig. 1..

The objective J combination comprises an objective made up of one or two lenses 15. For instance, the objective may be formed of two lenses 15 of suitable configuration to obtain an image ofthe size desired on ground glass 10; and the two lenses maybe aflixed, as b cementing, to the opposite plane faces 7 of t e prism. structure of the objective combinatiomas is shown in the various views. It

will also be well understood that a single lens could be used as an objective, but that use of double lenses is usually preferable in anobjective for various well known reasons, and also for the reason that, in this case, the light beam through the prism structure is practically a parallel beam and the prisms therefore need only be of such size as to have reflecting faces substantially the same size as the lenses.

The function of the prisms in the objective combination-is to erect and rectify the image finally thrown upon the ground glass. The

form shown in Figs. 1, 2 and 3 consists of a right angled prism 20 to whose hypotenuse face are cemented two smaller right angled prisms 21 and 22 in the arrangement and relation best shown in Fig. 2. Supposing the object to be at 0, a light ray from the object traverses the front lens 15, strikes the face 21a of prism 21 and is reflected downwardly to strike one of the 45 faces of prism 20 and thence reflected to the other 45 face of that prism and thence upwardly to strike the 15" face 22a of prism 22 and thence rearwardly through'the rear lens 15 to ground glass 10. The reflection through the prism structure causes a complete reversal of the image from what it would be if it were transmitted by the lens alone; the result is that the image at I is in correct relative .position,

being correct as for right. and left erect. a

In Fig. 3 Ishow the same system but with and being the addition of a simple magnifying glass 25 infront of groundglass 10, the realaiinage on ground glass 10 being viewed through the magnifying glass 25. In this case the lens at 11 performs the function of a light collector forthe magnifying glass 25, giving good illumination over the whole image.

In Fig. 451 show an objective combination I 13a of a somewhat different specific type. -Here the two lenses 15 are cemented to the flat front and reanfaces of a prism of special form, this prism having two upper angular reflecting faces 30 and two lower angular ie fleeting faces 31. This form of erecting prism is known as the Brashear-Hastings erecting prism. Its function is substantially the same as the prismatic system shown in Fig. 2; and acting in combination with the lens '15 it obtains the same result as before described. InFig. 4 the dash-line 35 shows how a ray would proceed through prism structure without lens 15, indicating how the image, without lens 15, would be-inverted and reversed; and dotted-line 35a shows how the same ray proceeds through thecomplete combination including the lenses, indicating that the image is erect and in correct position right for left.

There are other prismatic systems that may.

' be used in connection with an objective lens for the purpose of rectifying-and erecting the image; but I prefer to use a system of such physical proportions that it may be placed between the two lenses of the objective 'without separating those lenses too far to prevent their acting as a single objective lens. It 18, of course, well known that the several individual lenses of a complete objective lens may be designed so as to be separated somewhat from each other; and I make use of this possibility in my present design and separate the two lenses. These observations, of course, apply only to the use of an objective having two lens glasses. If a single lens glass is used, that single glass may be cemented to the front face of the prismatic system.

Also, for the purpose of this erecting obpect'ive combination, I prefer to use a prismatic system which has flat parallel front and rear faces so that the objective lens glasses may be cemented directly to those faces. By so doing I provide a simple, convenient and compact erecting lens combination with the prismatic system between the two lens glasses. In such a position, as will readily be understood, the prismatic system need not be verylarge, as the pencil of light passing or the whole objective combination may be composed or formed up, in any suitable way. have, for instance, shown in Fig. 4 how all ,t e prismatic reflecting faces maybe formed on a single piece of glass, Without the necessity of cementing together the three prism pieces shown in Fig. 2. Likewise the lens or lenses 15, although conveniently cemented to the prism faces, maybe applied thereto in any other convenient and suitable manner, It is only necessary, in the optical system,

that the lens elements be located at the end faces of the prism structure.

I claim:

1. In an erecting finder, an erecting obpective combination .comprising anerecting prismatic system having flat parallel front and rear faces each substantially at right angles to the optical, axis of the system, and an objective 'lens comprising two lens glasses cemented respectively to said front and rear faces of the prismatic system, the two lens glassesof the objective acting as a single objective lens which would without the interpoe sition of the prismatic erecting system throw an' inverted and reversed image; and an image receiving surface spaced from the objective combination and on which surface the objective throwsits ima e.

2. In an erecting-fin er, an erecting objective combination comprising an erecting prismatic system, and, an objective of two lenses secured directly to the front and rear faces of the prismatic system; the lenses being of such focal lengths as to act as a single objective with a focal length that throws an image behind-the rear lens; and an image receiving surface behind the rear lens.

3. In an erecting finder, an'erecting objective combination comprising an erecting prismatic system, and an objective of two lenses secured directly to the front and rear 5 faces of the prismatic system; the lenses being of such focal lengths as to act as a single objective with a focal length that throws an image behind the rear lens.

4, In an erecting finder, an erecting objective combination comprising an erecting prismatic system having lenses at the front and rear ends thereof, the lenses being'of such focal lengths as to act as a single objective with a focal length that throws an image behind the rear lens; and an image receiving surface behind the rear lens.

5. In an erecting finder, an erecting objective combination comprising an erecting prismatic system having lenses at the front and rear ends thereof, the lenses being of such focal lengths as to act as a single objective with afocal length that throws an image behind the rear lens.

GEORGE A. MITCHELL.

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