This invention relates to a distance metering device which detects a distance to an object based on an object image, and further to an optical apparatus provided with a distance metering device such as camera.
In lens built-in cameras such as compact camera, a distance metering device of external light passive system is provided adjacent to a viewfinder optical system. This distance metering device includes a pair of line sensors and an objective lens for focusing an object image within a distance meter frame provided in a viewfinder on these line sensors.
The distance metering principle adopted by such distance metering device is basically according to a trigonometric metering method. Specifically, a relative displacement of an image sensed by one line sensor with respect to an image sensed by the other line sensor is detected, and an object distance is calculated using a detected displacement.
Conventionally, the distance metering device of the external light passive system adopts the following multi-spot metering method. Light receiving areas of a pair of line sensors SR, SL are each divided into, e.g., three small areas (hereinafter, distance meter areas) AR(1), AR(2), AR(3) as shown in FIG. 14. A displacement of one image (first image) with respect to the other image (second image) is detected for each of the pairs of distance meter areas AR(1) to AR(3), and an object distance is calculated using detected displacements. An object distance for auto-focusing control or AF control is calculated based on object distances of the respective pairs of distance meter areas AR(1) to AR(3).
The displacement of the first image with respect to the second image in each pair of distance meter areas is detected by comparing a first image PR(i) in the pair of distance meter areas AR(i) (i=1, 2, 3) of the line sensor SR with a second image PL(i) in the pair of distance meter areas AR(i) of the line sensor while alternately shifting these images by one pixel and deciding a shift amount where a degree of coincidence between the first and second images PR(i) and PL(i) is at maximum.
The degree of coincidence between the first and second images PR(i) and PL(i) is determined by calculating a sum F of level differences (density differences) of the pixel data in the corresponding pixel positions each time the image is shifted and by comparing the sums F for the respective shifts.
Referring to FIGS. 15 and 16, in the distance metering device of external light passive type, an optical axis L1 of a distance metering sensor 20 including the line sensors SR, SL differs from an optical axis L2 of a viewfinder optical system 21. Accordingly, a spatial parallax is created between a field of view .theta.1 of the distance metering sensor 20 and a field of view .theta.2 of the AF frame 22 provided in the viewfinder. Thus, in close-up photography, for example, pairs of distance meter areas AR(1), AR(2) of a sensing area 23 of the distance metering sensor 20 fall outside an AF frame 22, as shown in FIG. 16. If an object distance is calculated by usual multi-spot metering method, the reliability is reduced due to an error.
In order to prevent reduction in the reliability of the calculation result, it is desirable to, in close-up photography, calculate an object distance for the AF control using the pair of distance meter areas AR(3) falling within the AF frame 22.
On the other hand, in the case that only the pair of distance meter areas AR(3) falling within the AF frame 22 is used for the calculation of the object distance in close-up photography, distance metering performance can be further improved if the viewfinder optical system 21 and the distance metering sensor 20 have a fixed relative positional relationship. In other words, if the spatial parallax of the pair of distance meter areas AR(3) at least with respect to the AF frame 22 is made smaller, the measurable closest distance can be further shortened (see FIG. 15).
As a means of improving the distance metering performance in view of the above-mentioned relationship, it could be appreciated to increase the size of the respective pairs of distance meter areas AR(1) to AR(3). However, this will cause the problem that the sensing area 23 of the distance metering sensor 20 becomes larger than the AF frame 22.