1.  Field of the Invention
This invention relates to a camera having a finder optical system disposed independent of a photographic optical system, and in particular, to an active autofocusing type rangefinder camera which projects a beam of light coming from a light source, through a part of the finder optical system, on an object and receives the light beam reflected from the object, thereby measuring automatically an object distance to drive a lens to a focused point. 
2.  Description of the Related Art
Range measuring devices used in compact cameras make use of an infrared projecting, active autofocusing type trigonometrical range finding technique.  This technique is such that Infrared light emitted from an infrared-emitting diode (which is hereinafter abbreviated to IRED) is projected through a projection lens on an object, and reflected light from the object Is received, through a light-receiving lens, by a semiconductor position sensor device (which is hereinafter abbreviated to PSD) disposed at the distance of a base length from the projection lens so that the object distance is measured in accordance with the position of incidence of the light.  Since the projection lens is usually provided independence of the finder optical system, parallax is produced between tile optical axes of the projection lens and the finder.  Consequently, a focus frame indicative of a range measuring area in the finder and the actual position of spot light caused by the projection lens may vary with the object distance. 
An arrangement for obviating such inconvenience is set forth, for example, in Japanese Patent Preliminary Publication No.  Sho 57-64217.  This arrangement is made so that one optical path on the object side is divided by a path splitting member into two, one taken as a projection lens system and the other as a finder optical system, and the optical axis of emergence of the projection lens system is made to coincide with that of incidence of the finder optical system for parallax correction. 
If, as the path splitting member, a wavelength selective mirror is used which transmits infrared Light and reflects visible light to reduce the loss of the amount of light, the characteristic of wavelength after the optical path has been split will change with the angle of incidence of a ray of light because the path splitting surface of the wavelength selective mirror is constructed with a multilayer thin film. 
Since in general the finder optical system has a wide angle of view, the angle of incidence of light on the path splitting surface also changes in a wide range.  Hence, where the light split by the wavelength selective mirror is viewed through the finder, the wavelength characteristic varies with the position of a finder picture, namely the angle of incidence of light on the wavelength selective mirror, with the resultant problem of causing coloring to an image.