In a focus detection apparatus, an accumulation-type sensor formed of a
plurality of sensor cells accumulates an image signal, and a component of
the image signal is read from each cell for a focus detection
calculation. A signal is read from a part of the cells in the sensor, and
it is determined from the signal whether an image signal has a low
contrast. When it is determined that the image signal has a low contrast,
reading signals from the other cells is disabled to omit unnecessary
signal reading operations.