Your camera is basically just a box with a hole in it and a light sensor inside. If the right amount of light gets through the hole to strike the sensor, you get a properly exposed picture. If you get too much or too little light, you get garbage. You already know that in program mode you can control the sensitivity of the light sensor itself by changing the ISO value, but in manual mode you can also control the amount of light that gets inside in the first place. You do that with the shutter and the aperture.

link: http://lifehacker.com/softwar…