Photography – With exposure objective focusing means – focusing aid – or... – Having auxiliary illumination
Reexamination Certificate
1999-10-07
2001-02-13
Perkey, W. B. (Department: 2851)
Photography
With exposure objective focusing means, focusing aid, or...
Having auxiliary illumination
C356S003040
Reexamination Certificate
active
06188843
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of adjusting a rangefinder apparatus for measuring the distance to an object to be measured; and, in particular, to a method of adjusting an active type rangefinder apparatus suitably used in a camera or the like.
2. Related Background Art
In active type rangefinder apparatus used in cameras and the like, an infrared light-emitting diode (IRED) projects a luminous flux toward an object to be measured, the reflected light of thus projected luminous flux is received by a position sensitive detector (PSD), a signal outputted from the PSD is arithmetically processed by a signal processing circuit and an arithmetic circuit and then is outputted as distance information, and the distance to the object is detected by a CPU. In general, since errors may occur when the distance is measured upon a single light-projecting operation, light is projected a plurality of times so as to obtain a plurality of distance information items, and an integrating capacitor of an integrating circuit is discharged at predetermined intervals according to the plurality of distance information items, so as to effect integration (first integration), thereby averaging the plurality of distance information items. Thereafter, this integrating capacitor is charged at a constant rate (second integration), the time (second integration time) required for regaining the original voltage is determined as the result of integration, and a distance signal for causing a taking lens to effect a focusing action is computed in conformity with a predetermined converting expression according to the second integration time. This converting expression is determined for each camera before its shipment from a factory.
SUMMARY OF THE INVENTION
In active type rangefinder apparatus such as the one mentioned above, it has been desired that inexpensive ceramic capacitors be used as the integrating capacitor in view of the demand for cutting down manufacturing costs. However, ceramic capacitors have a problem of dropping the charging voltage due to dielectric absorption. Namely, a ceramic capacitor forms an equivalent circuit such as the one shown in
FIG. 1
immediately after the charging is started. As a consequence, if the switch SW is opened after the charging, a voltage drop will be seen due to the resistance component R
x
in FIG.
1
. Such a phenomenon is known as dielectric absorption. Due to such dielectric absorption, even when the distance to the object to be measured is constant, the result of integration by the integrating capacitor, i.e., second integration time, would change upon every distance measuring operation.
FIG. 2
is a graph showing the change in second integration time in the case where distance measuring operations are repeatedly carried out. This graph shows the respective second integration times obtained when distance measuring operations are repeatedly carried out with the distance to the object to be measured being fixed at 608 mm. As shown in this graph, the second integration time increases from about 17.03 msec to about 17.07 msec from the first distance measuring operation to the 40th distance measuring operation, becomes about 17.09 msec around the 100th operation, and is about 17.10 to 17.12 msec at the 200th or later operation. In particular, the fluctuation of second integration time is greater within the range from the first distance measuring operation to the several-tenth distance measuring operation.
Thus, although the distance to the object to be measured is constant, if the result of integration by the integrating capacitor, i.e., second integration time, changes upon every distance measuring operation, the distance signal computed in conformity with a predetermined converting expression according to the second integration time will vary. Such a problem may occur not only when the rangefinder apparatus is being used but also when the converting expression employed for computing the distance signal from the second integration time is being determined.
Namely, for determining a converting expression, while an object to be measured is successively disposed at respective positions with different distances, their corresponding second integration times are determined by carrying out operations similar to the distance measurement, and the converting expression is determined from the relationship between thus determined second integration times and the actual distances to the object. Since distance measuring operations are repeatedly carried out when the converting expression is thus being determined, the second integration time may change upon every distance measuring operation in this case as well.
Meanwhile, in such a rangefinder apparatus, it is preferred that the period of each accumulating operation and the number of accumulating operations in the integrating circuit be set to values corresponding to the external light luminance. Namely, from the viewpoint of improving the accuracy in distance measurement, it is preferable that the period of each accumulating period be elongated when the external light luminance is lower than when it is higher.
However, for example, when different converting expressions are used in the cases where the period of each accumulating operation is shorter and longer, respectively, if the converting expression for the case where the period of each accumulating operation is shorter is initially determined and then the converting expression for the case where the period of each accumulating operation is longer is determined, the latter converting expression will yield a greater conversion error than the former converting expression does. Also, for example, when a common converting expression is used for both of the cases where the period of each accumulating operation is shorter and longer, if the respective second integration times for the cases where the period of each accumulating operation is shorter and longer are determined in this order and then the common converting expression is determined according to these second integration times, this converting expression will yield a greater conversion error.
In order to overcome the above-mentioned problems, it is an object of the present invention to provide a rangefinder apparatus adjusting method which can determine a converting expression with a reduced error in conversion from the result of integration (second integration time) to the distance signal even if the integrating capacitor has a problem of dielectric absorption when the period of each accumulating operation in the integrating circuit of the rangefinder apparatus is changed.
The rangefinder apparatus adjusting method in accordance with the present invention is a method of adjusting a rangefinder apparatus comprising: (1) light-projecting means for projecting a luminous flux toward an object to be measured; (2) light-receiving means for receiving reflected light of the luminous flux projected to the object at a light-receiving position on a position sensitive detector corresponding to a distance to the object, and outputting a signal corresponding to the light-receiving position; (3) arithmetic means for carrying out an arithmetic operation according to the signal outputted from the light-receiving means, so as to output an output ratio signal corresponding to the distance to the object; (4) integrating means, having an integrating capacitor, for discharging or charging the integrating capacitor according to the output ratio signal outputted from the arithmetic means, so as to accumulate and integrate the output ratio signal and output an integrated signal corresponding to the result of integration; (5) adjusting means for adjusting a period of each accumulating operation and the number of accumulating operations such that an integration time in the integrating means becomes a substantially constant value; and (6) detecting means for detecting the distance to the object in conformity with a predetermined converting expression according to the integrated signal ou
Fuji Photo Optical Co., Ltd.
Leydig Voit & Mayer Ltd
Perkey W. B.
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