1. Field of the Invention
The present invention relates to an in-focus state detection device for detecting an in-focus state of an object lens by sensing a non-visual light of a light source reflected by an object by a sensor through the object lens, and more particularly to an in-focus state detection device of a TTL-active type which is suitable for use in a one-eye reflex type camera or a microscope having an exchangeable object lens.
2. Description of the Prior Art
The in-focus state detection device of this type has been well known, such as by U.S. Pat. No. 4,357,085 issued on Nov. 2, 1982. An optical system to which this device is applied is also proposed by Japanese Patent Application Laid-Open No. 22210/1982 laid open on Feb. 5, 1982 and Japanese Patent Application Laid-Open No. 58110/1982 laid open on Apr. 7, 1982.
In this type of device, a sensor has its photosensing area divided into two sub-areas, and a light source, an object surface and boundaries of the photosensing sub-areas of the sensor are in a conjugate relation when an object lens is in an in-focus state so that the in-focus state of the object lens is detected by a difference between outputs of the photosensing sub-areas of the sensor. However, since this type of device usually uses a light source which emits a near infrared light, the in-focus state for a visible light used for photographing or viewing is not detected if a chromatic aberration of the object lens in the near infrared light is different from that of the object lens in the visual light. This is highly inconvenient when the device is used in a one-eye reflex type camera or a microscope having an exchangeable object lens.
FIGS. 1A, 1B and 1C show schematic constructions of a conventional TTL active type in-focus state detection device. FIG. 1A shows a near focus state, FIG. 1B shows an in-focus state and FIG. 1C shows a far focus state. Basically, a light source 1 which emits a near infrared light and a two-split sensor 2 are arranged at equivalent positions to an anticipated focusing plane P around an optical axis of an object lens 3, and a light flux L1 omitted from the light source 1 is projected to an object plane S through a predetermined aperture of the object lens 3. A light flux L2 reflected by the object plane S is sensed by the sensor 2 through another aperture of the object lens 3. Accordingly, the projected light flux L1 and the reflected light flux L2 deviate from a center of the optical axis of the object lens 3 and pass through the respective apertures.
In the near focus state shown in FIG. 1A, the light flux L1 projected to the object plane S deviates downward from the center of the optical axis. The reflected light flux L2 sensed by the sensor S2 further deviates from the center of the optical axis. In the far focus state shown in FIG. 1C, the deviation of the light flux is opposite to that in the near focus state. In the in-focus state shown in FIG. 1B, the light flux L1 is projected to the center of the optical axis of the object plane S, and the reflected light flux L2 returns to the center of the optical axis of the sensor 2.
FIG. 2A shows an energy distribution of the reflected light flux L2 impinged to the photosensing plane of the sensor 2. In the near focus state or the far focus state, the reflected light flux L2 deviates into an area A or an area B divided by the center of the sensor 2. Accordingly, the near focus state and the far focus state can be discriminated by a polarity of a difference between signals in the area A and the area B and a zero-crossing point. In the in-focus state, the reflected light flux L2 equally distributes in the area A and the area B.
FIG. 2B shows a differential output of the area A and the area B. An X-axis represents a lens position, a Y-axis represents the differential output, C1 represents a near focus range, C0 represents an in-focus position, C2 represents a for focus range and arrows D1 and D2 represent detection detectable ranges.
The object lens 3 shown in FIG. 3 is usually connected for aberration such that the in-focus position for a visible light used for photographing or viewing is essentially constant, but the in-focus position for the near infrared light is not always constant but a relatively large infrared aberration is included. Thus, is a device having a fixed object lens, an influence by the infrared aberration can be practically eliminated by arranging the infrared light source, the object plane S and the center of the two-split sensor 2 in a conjugate relation when the photographing or viewing optical system is in the in-focus state. However, in the device having the exchangeable object lens, it is necessary to adjust the in-focus position because the residual infrared aberration varies substantially depending on a focal distance of the object lens and a focusing magnification.
For example, in a device shown in FIG. 3, if the in-focus position is set for an object lens and then the object lens is exchanged by another object lens 3, a difference is created between an anticipated focusing plane P of the photographing or viewing optical system and an actual focusing plane P' in the in-focus state of the lens. On the other hand, if the object lens 3 is set at the in-focus position of the photographing or viewing optical system, the in-focus state detection device produces a defocus state signal. Thus, when the object lens is exchanged, a reliability of the in-focus state detection is not satisfactory.