Can you notice what is the different between the pictures of peanuts above? The were actually taken at the same position, only the setting on the camera was different. The F number for first picture was f/2.8, and the second picture was f/8.

Yes, the F number can affect the Depth of Field of the pictures.

“In optics, particularly film and photography, the depth of field (DOF) is the distance in front of and beyond the subject that appears to be in focus.” - Quoted from Wikipedia.

The camera I used only allow me to use the F number from f/2.8 up to f/8, therefore the DOF is still limited. A good lense sometimes can have F number from f/1 up to f/32 or f/38. The F number is actually indicate the aperture of the lense. The smaller the F number, the larger the aperture.

One of the reasons to control the Depth of Field is to emphasis the subject. A good photo will have its subject sharp and the rest of the background blur. That means a shallow Depth of Field allow you to emphasis the subject better.

The other reason why to increase the Depth of Field is to increase the range in focus. It is useful when taking a portrait while want to keep the background scenery clear and sharp. This can be done by increase the F number.

Actually most of the automatic camera by default, have been manufactured in this way so that it produces clearer pictures without the need of focusing. But you cannot reduce the Depth of Field to emphasis a subject on an automatic camera.

Let’s see some other examples of controlling the Depth of Field with the pictures of peanuts.