In microfiche blowback systems it is highly desirable to use a magnification that can be varied readily. This is because various forms of microfiche use different reduction ratios, and also because specific users of the system have various preferences in the blowback magnifications that are utilized. The magnification of the system adheres to the fundamental relationship ##EQU1## WHERE D.sub.2 IS THE IMAGE-TO-LENS LENGTH (THE LONG CONJUGATE LENGTH) AND D.sub.1 IS THE OBJECT-TO-LENS LENGTH (THE SHORT CONJUGATE LENGTH). The lens equation ##EQU2## RELATES FOCUS TO THE LONG AND SHORT CONJUGATE LENGTHS. In systems where the focal length (f) of the lens is fixed, magnification is often achieved by changing the long conjugate length d.sub.2. From equation (2) it is seen that changes in the long conjugate length will affect focus and that to maintain focus such a change will require a corresponding change in the short conjugate length.
Generally, magnification control is achieved by manual control of one of the path lengths d.sub.1 or d.sub.2 with automatic control of the second path length being maintained. In this type of magnification control of the prior art, the second path length is moved in strict accordance with the lens equation (1), it being assumed in those systems that focus will be achieved if the relationships of the lens equation (1) are maintained. A disadvantage of this type of system is, for example, a lack of compensation for variations, such as temperature expansion or contraction of the support structure for the projection lens or the object. Since at high magnification and low f/numbers, tolerances may be very small between the lens and the object plane, on the order of 0.001 inches, systems which rely solely on the solution of lens equation (1) are not always satisfactory.
In automatic focus systems, such as described in the U.S. Pat. Nos. 2,968,994 and 3,421,815, means are provided to evaluate the focus of an actual image and to control the movement of some element of the optical system to maintain a focus condition once it has been achieved regardless of extraneous movement. These systems, known as optical probe focus servo systems, utilize a light source which projects a target image of high resolution backward through the optical system to the object plane, with the target image reflected from the object being projected back through the image system and reimaged at the target plane. If the long and short conjugate lengths are correct in relation to the optical probe imaging lens focal length, the return image will be superimposed on the target and no focus adjustment need be made. If the conjugate lengths are incorrect, that is, not in accordance with lens equation (1), the condition occurs in which the returned target image is in a defocused condition at the target plane. This condition can be sensed, and used to generate a servo-control signal to effect repositioning of the image plane.
Several problems relate to the auto focus system described in the referenced patents. One problem is that the system only maintains focus after it has been manually set, and that it must be manually set each time magnification is changed. A second problem involves the focus control system, which varies the optical path length between lens and object by direct movement of the object plane. Within the tolerances of the system, this direct movement may be difficult to control. A third problem which exists is stray light, there being no means to differentiate between light arriving at the sensor as a result of a focusing condition and light that arrives at the sensor due to extraneous sources of light such as light which will be scattered in the optical system.