A method of detecting defects in an image sensor that may occur from a floating diffusion area of the image sensor, a tester using the method, and a control signal generator using the method include a photo diode generating charges corresponding to an image signal; a transmission transistor having a first terminal connected to a the photodiode and a second terminal connected to a floating diffusion area, thereby transmitting the charges generated in the photo diode to the floating diffusion area in response to a charge transmission control signal; and a reset transistor having a first terminal applied by a reset voltage and a second transistor connected to the floating diffusion area, thereby transmitting the reset voltage to the floating diffusion area in response to a reset control signal. The reset transistor is turned on during at least one sampling zone selected between reset level sampling and signal level sampling that are performed with respect to the image sensor.