Depth of Field Explained
Depth of field is the amount of the photo that is in focus. A shallow depth of field is what creates those nice out of focus background portraits you see in professional photos. It can also help tell a story by directing the viewers eye to important parts of the picture.
Depth of field is determined by four different elements:
F-Stop or Aperture
Format (sensor size or film size)
1. F-Stop / Aperture
The aperture of the lens determines depth of field much like your eyes do. When it's dark and your pupils are dilated they are "wide open." This allows more light to come in, but only a limited amount of your field of vision is in focus. Oppositely, when it is super bright outside and your pupils are really small you can see for miles.
The same is true for a lens. When a lens is "wide open" it has a limited range of focus, or a shallow depth of field as we call it. A wide open F-Stop number is a small number. Typically F2.8, F4, or smaller would create the shallow depth effect.
2. Focal Length
The length of your lens also factors into depth of field. A longer lens will create a shallow depth of field, whereas a wide lens will typically create a situation where everything is in focus.
3. Focal Distance
The distance of the subject from the lens is another contributing factor to depth of field. Subjects closer to the lens (that fall within the lens' minimum focusing distance) will create a shallower depth of field, where subjects further from the lens will not.
4. Format / Sensor Size
Much like the aperture discussion, a larger sensor (think "wide open") will create a shallower depth of field than a smaller one (think "stopped down"). So a 35mm full frame sensor will create a shallower depth of field than an APS-C sensor which is considerably smaller. The smaller the sensor, the greater the depth of field. The larger the sensor, the shallower the depth of field.
So in short, if you are looking for those nice out of focus background portraits that you see in magazine you need to know how to obtain it. The most extreme shallow depth of field situation would be a wide open lens with a small aperture number paired with a long telephoto lens, a large sensor, and a person or thing that is very close to the lens. This would create the ultimate in shallow depth. Other factors may prohibit this, but for the sake of this discussion that is how you would achieve the shallowest depth of field.