Image sensor and sensing method thereof

An image sensor includes a pixel array including a plurality of unit pixels in a matrix including rows and columns, a selection unit configured to select outputs of some of the columns of the pixel array and output selection output signals, and an analog-digital conversion block including a plurality of analog-digital conversion units corresponding to the columns of the pixel array. First ones of the plurality of analog-digital conversion units include analog-digital conversion blocks configured to convert the selection output signals and output digital data. When the first analog-digital conversion units convert the selection output signals, second ones of the plurality of analog-digital conversion units are turned off.