1. Field of the Invention
This invention relates to a focus detecting system.
2. Description of the Prior Art
As a device for detecting the amount of in-focus error, there is known a device as shown in FIG. 1 of the accompanying drawings wherein light receivers 2, 3 and 4 such as photoelectric conversion elements are disposed at optically equally spaced three points in the predetermined focal plane (film surface) of a photo-taking lens 1 and forward and rearward thereof. The words "equally spaced" include "substantially equally spaced". The light receivers 2, 3 and 4 are elements such as CCDs which convert the states of images formed thereon into electrical signals, and processing circuits 5, 6 and 7 are conventional circuits which put out sharpness signals F.sub.1, F.sub.2 and F.sub.3, respectively, proportional to the sharpnesses of the images. These sharpness signals F.sub.1, F.sub.2 and F.sub.3 vary as shown in FIG. 2 of the accompanying drawings for the movement of the focal plane from the predetermined focal plane. FIG. 2 may be schematically represented as shown in FIG. 3(A) of the accompanying drawings. In the Figure, O represents the predetermined focal plane position. The difference (F.sub.1 -F.sub.3) between the sharpness signals F.sub.1 and F.sub.3, as shown in FIG. 3(B) of the accompanying drawings, exhibits a linear characteristic for out-of-focus in the range from the vicinities of the peak values of the sharpness signals F.sub.1 and F.sub.3 toward the predetermined focal plane side. Accordingly, the difference (F.sub.1 -F.sub.3) can be used as an in-focus error signal.
However, the difference (F.sub.1 -F.sub.3) is varied by the brightness of the object to be photographed or the like and therefore, it has already been proposed in the United States by applicant (Ser. No. 310,373) to divide the difference by the sum of the three signals F.sub.1 to F.sub.3 to thereby normalize it so as not to be varied by the brightness of the object to be photographed. However, the sum (F.sub.1 +F.sub.2 +F.sub.3) of the three signals, as shown in FIG. 3(B), varies more or less also in the relatively narrow vicinity of the predetermined focal plane and the value thereof sharply varies outside the vicinity of the predetermined focal plane and therefore, it has been unsuitable for normalizing said difference (F.sub.1 -F.sub.3).
Also, the section in which the value of the sum (F.sub.1 +F.sub.2 +F.sub.3) of the three signals is relatively stable in the vicinity of the predetermined focal plane is limited to the section in which the value of the signal F.sub.2 is greater than the values of the signals F.sub.1 and F.sub.3 and therefore, such section has been relatively narrow.
If the spacing between F.sub.1 and F.sub.3 is increased, the range in which the value of F.sub.1 +F.sub.2 +F.sub.3 does not sharply vary can be widened, but if the spacing between F.sub.1 and F.sub.3 becomes greater, the inclination of the graph showing the signal between the focal point positions at which the signals F.sub.1 and F.sub.3 of the signal indicative of the difference (F.sub.1 -F.sub.3) shown in FIG. 3(B) becomes smaller, and this has led to the disadvantage that the ratio at which the value of the in-focus error varies becomes small relative to the rate at which the photo-taking lens deviates from the in-focus position.
If an attempt is made to apply a conventional in-focus error amount detecting device having the above-noted disadvantage to an automatic focusing apparatus including an auto focus lens, the range in which the auto focus operation of the automatic focusing apparatus including the auto focus lens can be effected becomes very narrow because the range in which the amount of in-focus error can be accurately detected is relatively small, and this in turn has led to the disadvantage that the operation of the automatic focusing apparatus is stopped even when the position of in-focus error amount detecting device deviates only relatively slightly from the in-focus position.