Depth of field

In photography, depth of field is huge.  It is defined as the distance that is in focus, front to back (away from the camera at right angles), in the image. Depth of field allows you to control what is in and out of focus, and by how much.  It is used to help draw your eye to a desired location.  It can make the difference between an average, good, or excellent shot.  You can control depth of field using several methods.  
#1:  The size, or f/number, of the aperture is very important.  A small f/number means a wide open aperture.  It also means less depth of field.  A large f/number means a small aperture; it gives you more depth of field.  
#2:  The focal length of the lens has a great impact.  Wide angles settings (28 mm relative to a 35 mm film camera) gives you more depth of field than telephoto settings (105 mm).  
#3:  The point of focus is also a factor.  As you get closer to your subject, the depth of field decreases.  
#4:  Film or sensor size is important.  Small sensors have to enlarge an image more to produce a print.  That means the circles of confusion have to be smaller.  This means larger sensors have better depth of field at any actual (not relative) focal length.  
There is a lot more to this topic, it will be explored in other blogs.