official patent description:
In recent single-lens reflex digital cameras, a display such as a liquid crystal display (LCD) panel is disposed on the back of a camera body such that an image inputted to an image pickup device can be viewed using the display. For example, there has been proposed a single-lens reflex digital camera, having no optical viewfinder, that allows a user to take photographs while the user checks an image using a display disposed on the back of a camera body. In such a structure, an autofocus detecting device is disposed at an upper position in the camera body and light reflected by a light-beam splitting mirror can be guided to the autofocus detecting device. However, when light-beam splitting mirrors disclosed in Patent Documents 1 to 4 are used, the light quantity of transmitted light guided to an image sensor is increased compared with that of reflected light to improve the quality of photographed images. As a result, the light quantity of photographing light guided to an autofocus detecting device becomes insufficient.“….”The present invention can provide a light-weight small-size digital camera.”
And there is one more interesting news. Depending on the light wavelength the mirror reflects more or less light: “A sufficient quantity of subject light can be guided to an autofocus detecting device by adjusting the reflectivity to light with a wavelength to which the autofocus detecting device has high sensitivity to 25% or more and 35% or less, which can solve a problem in that light quantity becomes insufficient for autofocus detecting“…”When autofocus auxiliary light is used to perform photographing on a subject having a low contrast or in a dark place, the returned light out of the autofocus auxiliary light can be reflected at a reflectivity higher than that of light having a different wavelength and guided to the autofocus detecting device.
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