Image sensing apparatus and method of controlling the image sensing apparatus

#The present invention relates to an image sensing apparatus having a firework image sensing mode and, more particularly, to a control method for image processing in the image sensing apparatus.

#A “firework mode” specialized for taking firework images in image sensing apparatuses such as electronic cameras has been disclosed (see, for example, Japanese Patent Laid-Open No. 2001-311977). Because the human eye has such a characteristic as to maintain an afterimage, long trails of light are perceived in a firework with the eye. In contrast, in image sensing, only trails of light corresponding to the shutter speed are imaged. For example, in still image sensing in a firework mode, therefore, image sensing control such as automatically decreasing the shutter speed is performed to express tails of light of a firework. Special exposure control such as setting the amount of exposure to a value suitable for a firework is also performed in image sensing in a firework mode, because the suitable amount of exposure cannot be obtained by the ordinary exposure control under a dark firework condition.

#When a normal image sensing mode is selected in still image sensing, the shutter speed is controlled so that an amount of exposure becomes proper, and an image signal corresponding to the quantity of light to which an image sensing unit during the shutter released period is exposed is generated. In the normal image sensing mode, a shutter speed of, for example, about 1/30 second is a limit of low shutter speed for prevention of blurriness due to camera shake or blurriness due to movement of an object. When a firework image sensing mode is selected, the diaphragm is controlled so that the shutter speed is set to a predetermined value for the firework mode, and an image signal corresponding to the quantity of light to which the image sensing unit is exposed during the shutter released period is generated. While long trails of light are perceived in a firework with the human eye since the human eye has such a characteristic as to maintain an afterimage, only trails of light corresponding to the shutter speed of an electronic camera or the like are imaged in actual image sensing with the electronic camera, as described above. Therefore, it is effective to continue exposure for about 2 seconds in image sensing on a firework to express trails of light in the firework. In ordinary cases, therefore, the shutter speed determined in advance for a firework mode is about 2 seconds. FIG. 14 shows an image of an aerial firework taken at a shutter speed of 1/30 second. A firework which will be seen in a tailed state with the human eye is imaged as dots, as shown in FIG. 14. FIG. 15 shows an image of the same aerial firework taken at a shutter speed of 2 seconds. A firework can be expressed in a tailed state as shown in the figure, if the exposure period is 2 seconds.

#The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.

#FIG. 1 is a block diagram showing the configuration of an image sensing apparatus according to a first embodiment of the present invention;

#FIG. 2 is a diagram for explaining a gamma characteristic in a normal image sensing mode according to the first embodiment of the present invention;

#Preferred embodiments of the present invention will be described in detail in accordance with the accompanying drawings.

#FIG. 1 is a block diagram of an image sensing system of an electronic camera schematically showing a first embodiment of the present invention. In FIG. 1, reference numeral 101 denotes a lens; reference numeral 102, a diaphragm for adjustment of the amount of exposure; reference numeral 103, an image sensing unit; reference numeral 104, an analog front end (AFE) that samples an output from the image sensing unit and performs A/D conversion of the sample output; reference numeral 105, a luminance/color (Y/C) separation circuit; reference numeral 106, a color signal processing section; reference numeral 107, a luminance signal processing section; reference numeral 108, a color difference matrix circuit; reference numeral 109, a mode switch section; reference numeral 111, a shutter control section; reference numeral 112, a luminance signal processing circuit included in the luminance signal processing section 107; reference numeral 113, a gamma circuit that is included in the luminance signal processing section 107, and that performs gamma conversion processing; reference numeral 110, a mode control section that supplies gamma curve data to the gamma circuit 113 and that controls the gamma circuit 113, the edge enhancement circuit 114 and the chromaticity correction circuit 116; reference numeral 114, an edge enhancement circuit that is included in the luminance signal processing section 107, and that performs edge enhancement processing; reference numeral 115, a color signal processing circuit included in the color signal processing section 106; and reference numeral 116, a chromaticity correction circuit that is included in the color signal processing section 106 and that corrects the level of a color signal.

#Light entering the lens 101 is controlled for suitable exposure by the diaphragm 102 and undergoes photoelectric conversion in the image sensing unit 103. An image signal obtained by photoelectric conversion in the image sensing unit 103 undergoes sampling and A/D conversion in the analog front end 104 and is separated into a luminance signal and a color signal in the Y/C separation circuit 105. The color signal separated in the Y/C separation circuit 105 undergoes signal processing relating to colors, such as white balancing, in the color signal processing circuit 115, then undergoes chromaticity correction in the chromaticity correction circuit 116, and is thereafter input to the color difference matrix circuit 108. On the other hand, the luminance signal separated in the Y/C separation circuit 105 is input to the luminance signal processing section 107, undergoes processing relating to luminance in the luminance signal processing circuit 112, processing for edge enhancement or the like in the edge enhancement circuit 114 and gamma conversion processing in the gamma circuit 113, and is thereafter input to the color difference matrix circuit 108. Then a color difference signal is formed in the color difference matrix circuit 108 and is output from this circuit. In the first embodiment of the present invention, the diaphragm 102 also functions as a shutter for controlling the exposure period at the time of still image sensing. In still image sensing, the exposure period is determined by closing the diaphragm 102 after the image sensing unit has started accumulating electric charge.