1. Field of the Invention
This invention relates to cameras, and more particularly to cameras with automatic focusing devices.
2. Description of the Prior Art
Usually, the automatic focusing device is arranged in the camera so that after the end of a focusing operation, a transit to a shutter operation occurs. Since actuation of a shutter release immediately follows setting to the in-focus condition, and because the time the focusing operation takes to complete is a variable factor, the timing of the start of a shutter release, or the instant of shutter operation shifts unpredictably. In more detail, it is common practice that the scanning operation of the lens always goes in one direction, either from close up to infinity, or from infinity to close up. Therefore, in an example of the former case, when shooting a nearby object, the shutter starts to open as soon as a camera release actuation takes place. Conversely when shooting an infinite distant object, after the elapse of some appreciable time from a camera release actuation the shutter is released.
From the standpoint of insuring adequate shutter operation, it is desirable that every exposure operation has, despite differences in object distance, to proceed just after the release button is pushed down. In considering the up-to-date available automatic focusing mechanism, this object is very difficult to attain. Rather, the dependence of the initiation of the exposure operation on the object distance gives the photographer needless trouble when shooting a moving object. With this in mind, according to a proposal, for example, in Japanese Laid-Open Patent No. Sho 55-40438, shutter opening is initiated in a predetermined period after a camera release is actuated, thereby eliminating the above-described drawback.
The above-cited proposal makes use of a logic product ("AND") condition of a signal representing the end of an automatic focus adjustment (hereinafter referred to as "AFEND" signal) and a timer-up signal produced in a predetermined period from the camera release actuation in determining when the shutter starts to open. Since this period of timer actuation is so chosen to cover almost all the different times the automatic focus adjustment for different object distances will take to end, for almost every object, the time interval until the shutter opens becomes constant. However, this proposal also has the following drawback.
That is, in application to the passive type of automatic focusing devices using CCD, for example, when the object brightness is low, or when the object is of a vertical stripe pattern, the production of the AFEND signal often fails. Also, in application to the active type of automatic focusing devices using infrared light-emitting diodes (hereinafter abbreviated as "IRED"), for example, when reflection of the object is low, often no AFEND signal is produced. When the AFEND signal is not given, it follows that where transit to the next sequence is conditioned by the "AND" of the aforesaid timer-up signal and the AFEND signal, that because of the lack of the AFEND signal, there is no opportunity for any of the subsequent steps of operation to proceed. This implies that, in the system using an electrical power latch and the IRED, the electrical power source remains in the latched state, permitting the aforesaid IRED to continue lighting with the result that the amount of current wastefully consumed greatly increases.
Certainly, allowing for situations where the aforesaid automatic distance measurement is difficult, an equivalent to the AFEND signal may be formed. That is, in the passive type, that signal represents a certain fixed object point, and, in the active type, it represents that the lens has reached the focusing position for an infinitely distant object. However, much remains to cause production failure of the AFEND signal, many of which are accidents. For example, motion of the lens mounting mechanism is not smooth, or may be interrupted. If so, the above-described problem will arise.
It is an object of the present invention to provide an automatic focusing device using a reference light or like signal for projection onto an object to be photographed in forming object distance adjustment, wherein when an AFEND signal is not obtained within a prescribed time, the distance finding operation is automatically inhibited to avoid wasteful consumption of electrical energy.
Still another object of the present invention is to establish a system which can eliminate the above-described drawbacks by providing a 2nd timer means actuated when counting of a 1st timer means ends.
In one embodiment, when an AFEND signal appears within the earliest prescribed time (hereinafter called "1st" short time) defined by the 1st timer means, the procedure is similar to that in the conventional device. When the AFEND signal appears in another prescribed time contiguous to the 1st one defined by the 2nd timer means (hereinafter called "2nd" short time), a subsequent operation proceeds almost immediately. Furthermore, when the AFEND signal is not produced even after termination of the 2nd short time, actuation of the shutter release is prohibited, for example, by releasing the latching of the electrical power source, and the system is reset to the initial position assumed before actuation of the camera release.
The present invention has the following advantages:
(1) The use of the 1st timer means of a similar function as that in the conventional device provides the assurance of obtaining a certain shutter operation in ordinary photography.
(2) As the governor controlling the speed of movement of the focusing mechanism in the lens mounting is slow in action due to lowering of the ambient temperature, even when timing the appearance of the AFEND signal is delayed, it is possible by provision of the 2nd timer means that, as long as its period of actuation goes on, the accuracy and reliability of the focusing control mechanism operation is maintained at an acceptable level.
(3) Even when the AFEND signal does not arrive due to some accident, the initial state prior to camera release is regained in automatic response to termination of a counting operation of the 2nd timer means, thereby increasing consumption of current which would otherwise occur as in the conventional device, and a rapid shortage of electrical energy in the battery can be avoided.
These and other objects, features and advantages of the invention will become apparent from the following detailed description of embodiments thereof by reference to the drawings.