F-Number the bare minimum

If you are trying to get out of the "Auto" mode and try new things here is a brief introduction on what the F number means and the use of setting the aperture value.

How well exposed your final picture is depends on primarily three things (a) aperture (b) shutter speed and (c) ISO.

Light passes through the aperture. So the size of the aperture matters.

Light will be allowed to pass and will be captured by the sensors for the duration of time set by the shutter speed. So the value of the shutter speed matters. If the shutter speed is 1/100 sec then the camera captures whatever it sees for 1/100 sec. This is even referred to as exposure time.

Light will be received by the sensors (for the duration set by the exposure time). So the sensitivity of the sensors matter. This can be set by the ISO value. Lower the ISO value lower is the sensitivity.

The size of the aperture is mentioned in terms of F number. All that you should remember is  (1) larger is the F number more is the distance over which things appear to be in focus (2) larger the F number smaller is the aperture opening. This is illustrated in the picture below. 

The left picture was taken at a F number of 5.6 and the right one was taken at a F number of 36. The focal length and ISO values were same for both the pictures. For the smaller F number (5.6, left) only a small strip of the ground seems to be in focus and everything else is out-of-focus. For the larger F number (36, right) a larger area is in focus and the blur is not so much. A technical term you might hear in this relation is "depth of field" that's the depth over which everything is in focus. So basically,

1. Smaller F number (left image) ==> Smaller area in focus and more blur = smaller depth-of-field
2. Larger F number (right image) ==> Larger area in focus and less blur = larger depth-of-field

But you will note that  the exposure time (or the shutter speed) for the left picture is 1/125 sec which is shorter than the exposure time of the right picture 1/3 sec. This is because of the following:

1. Smaller F number (left image) ==> Larger aperture opening through which light can pass ==> To get a well lighted image, exposure time can be small.
2. Larger F number (right image) ==> Smaller aperture opening through which light can pass ==> To get a well lighted image, exposure time has to be longer.

That's all. What exactly does "F/5.6" mean ? Why does a larger aperture causes only a small area to be in focus ? etc. etc. all that for a later time.