Aperture, Shutter speed, and Depth of Field

I was always interested in photography, and used to have alot of point and shoot cameras,
finally i got my first DSLR camera.
It's a Nikon D90 and i got 2 lenses (Nikon 135 zoom lens, and Sigma 105 macro lens)
Currently i'm trying to learn alot about it and about photography
3 things that confused me allot and that is why i wanted to write about them and the relation between them

1- Aperture
According to Wikipedia : In aperture is a hole or an opening through which light travels.

The aperture is the size of the “hole” formed by the shutter when it opens to let light in, and is measured in “f-stops.”. A “small” aperture means a small hole, but a big number, like f11 on the right.  Since the hole is smaller, less light gets in with small apertures.  A “big” aperture means a big hole, but a small number, like f2.8 on the left.  Since the hole is bigger, more light gets in with big apertures.
for example of different aperture see image below:

 here is another example:

 as you see the smaller the f number the bigger the aperture the more light goes to sensor

2- Shutter Speed
According to Wikipedia : shutter speed is a common term used to discuss exposure time, the effective length of time a camera's shutter is open.The total exposure is proportional to this exposure time, or duration of light reaching the film or image sensor.
Shutter speeds are measured in more obvious terms: fractions of a second. A shutter speed of 1/8, for example, means that the shutter opens for one-eighth of a second. Try to capture a moving object at that speed and you wind up with a big blur. You need a shutter speed of about 1/500 to capture action clearly.
 On cameras that offer aperture and shutter speed control, you manipulate the two settings in tandem to capture just the right amount of light. For example, if you're capturing fast action on a bright, sunny day, you can combine a fast shutter speed with a small aperture (high f-stop number). To shoot the same picture at twilight, you need a wide-open aperture (small f-stop number) in order to use the same fast shutter speed.

here is an example for controlling exposure:

here is another example controlling movement:

usage of shutter speed values:

3- Depth of Field (DoF)
The Depth of Field is the distance wherein objects are in focus.
The range of depth of field (DOF) varies with focal length, aperture size and distance from the camera to the focus point.

here is an example of relation of aperture and DoF:

f/1.4

f/2.8

f/5.6

f/8

f/11

f/15

f/20

At last:

For a shallow depth of field:

• Set the aperature value as low as possible.
• Get as close to the primary subject as possible.
• You'll need lots of light.
• You may have to increase the ISO setting.

For maximize the depth of field:

• Set the aperature value as high as possible.
• The farther away you are the greater the depth of field.
• A longer exposure will maximize the depth of field; you may need a tripod, or other steady surface for your camera.

Nikon D3000 10.2MP Digital SLR Camera with 18-55mm f/3.5-5.6G AF-S DX VR Nikkor Zoom Lens
Nikon D5000 12.3 MP DX Digital SLR Camera with 18-55mm f/3.5-5.6G VR Lens and 2.7-inch Vari-angle LCDNikon D300s 12MP CMOS Digital SLR Camera (Body Only)