text: 1. Field of the Invention
  The present invention relates to a solid-state image sensor, a camera, and a method of driving a solid-state image sensor.
  2. Description of the Related Art
  Along with the recent requirement of a multiple pixel structure and high frame rate of a solid-state image sensor, a technique of shortening the time to read out a signal from a pixel is becoming increasingly important. Japanese Patent Laid-Open No. 2004-134867 describes a solid-state image sensor that provides one amplification element in correspondence with a plurality of photoelectric converters. One of the plurality of photoelectric converters will be referred to as a “first photoelectric converter”, and another as a “second photoelectric converter” hereinafter for the descriptive convenience. The solid-state image sensor described in Japanese Patent Laid-Open No. 2004-134867 transfers the charges of the first photoelectric converter to the input portion of the amplification element and reads out a signal (to be referred to as a “first optical signal” hereinafter for the descriptive convenience). After that, the solid-state image sensor transfers the charges of the second photoelectric converter to the input portion and reads out a signal (to be referred to as a “second optical signal” hereinafter for the descriptive convenience) without resetting the input portion. In this case, the signal read out after the charges of the second photoelectric converter are transferred to the input portion is a signal corresponding to the sum of the charges of the first photoelectric converter and those of the second photoelectric converter.
  In the method described in Japanese Patent Laid-Open No. 2004-134867, the second optical signal is larger than the first optical signal. Hence, the time needed for convergence of the second optical signal is longer than that needed for convergence of the first optical signal. If the time to read out the first optical signal and the time to read out the second optical signal are defined similarly, the times should be set such that the second optical signal can be read out at a sufficient accuracy. In this case, however, the time to read out the first optical signal is set longer than necessary. If the time to read out the first optical signal and the time to read out the second optical signal are defined similarly while giving priority to shortening the readout time, the readout accuracy of the second optical signal is lower than that of the first optical signal. Japanese Patent Laid-Open No. 2004-134867 does not describe these examinations and ideas about improvements based on them.