BRIGHTNESS LEVEL CONVERTING APPARATUS, BRIGHTNESS LEVEL CONVERTING METHOD, SOLID-STATE IMAGE PICKUP APPARATUS, BRIGHTNESS LEVEL CONVERTING PROGRAM, AND RECORDING MEDIUM

A brightness level converting apparatus improves a contrast of an image represented by an image pickup signal output from a logarithmic conversion solid-state image pickup device, so as to obtain a natural image. For this purpose, the brightness level converting apparatus converts brightness levels in the image pickup signal output from the solid-state image pickup device which logarithmically converts an incident light quantity. In the brightness level converting apparatus, a brightness level converting section converts the brightness levels in the image pickup signal by using a brightness level converting function expressed by an S-shaped curve which indicates the relation between an input brightness level and an output brightness level and includes one inflexion point. Therefore, the contrast in a low brightness level range in the image pickup signal is reduced. At the same time, the contrast in a high brightness level range is reduced while appropriately keeping a gradation. With this, the contrast in an intermediate brightness level range in the image pickup signal is improved considerably. Therefore, the image represented by the image pickup signal whose brightness levels are converted becomes a further natural image.