1. Field of the Invention
The present invention relates to an imaging apparatus, imaging optical unit, and imaging system such as a video camera, television camera, and digital still camera.
2. Description of the Related Art
When a subject image is picked up through an imaging optical system lens, the peripheral brightness of the picked-up image is lower than the central brightness. This phenomenon is mainly caused by the cosine biquadratic rule and vignetting.
In order to correct such a drop in peripheral light quantity, for example, as proposed in Japanese Unexamined Patent Publication No. 165023 of 1994, it is considered that a ratio of the peripheral light intensity to the central light intensity on the light receiving surface of an image pickup device is determined based on the positions of a zoom lens and a focus lens that comprise an imaging apparatus and a stop value, the reciprocal of the determined ratio is outputted as a correction coefficient, and an image signal is multiplied by the correction coefficient, whereby the light quantity distribution on the image pickup surface is electrically corrected.
However, the method for correcting a drop in peripheral light quantity proposed in the above-mentioned publication is not sufficient for the following reason.
That is, in the image pickup device, the output value is not always in proportion to the incident light quantity, and the output value has a peculiar gamma property as shown in FIG. 8. Therefore, even if the peripheral incident light quantity is 10% lower than the central incident light quantity, it is not proper that 10% correction of the entirety of output from the image pickup device is corrected. That is, in many cases, the actual brightness distribution in the shooting range cannot be accurately reproduced.
Another method is also considered in which gamma correction is made for outputs from the image pickup device so that the gamma property becomes a straight line, and then the light quantity correction is made. However, the desired gamma property of the imaging apparatus is not a straight line, so that in this case it is necessary to make gamma correction again, resulting in an increase in noise.
Furthermore, the image pickup device has a characteristic by which the light receiving ratio changes depending on the incidence angle of an incident light flux. That is, the device has light receiving ratio distribution in accordance with the position of the exit pupil, and this also causes the light quantity to lower at the peripheral portion at which the incident light flux greatly tilts with respect to the light receiving surface of the image pickup device. Specifically, this light receiving ratio distribution greatly influences an image pickup device provided with micro lenses at the light receiving surface.