Shutter speed, aperture, F-Stop, depth of field, film speed, ISO, the list goes on and on of terms that can make a new photographer’s head spin. The good news is that none of it is as complicated as you might think. Let’s focus on depth of field (DOF) right now. Depth of field in photography refers to how much of your photograph is in focus from the closest object to the farthest object. In other words, depth of field tells you if the foreground and background are in focus at the same time or not.
What Creates Depth of Field
Depth of field is determined by a combination of three things:
- Subject Distance
Aperture and Depth of Field
The term aperture describes an adjustable opening inside your lens that opens and closes to adjust the amount of light that reaches your camera sensor. This change in aperture size also changes the angle of light hitting the camera sensor. That angle change, just like eyeglasses bend light, creates changes in the depth of field.
Aperture is measured by the F-Stop setting on your camera. F-Stop numbers are calculated from a ratio of the lens opening and focal length. Because it is a ratio, the numbers of F-Stop are actually counter-intuitive with regards to the aperture sizes they represent. Bottom line is this. A big F-stop number denotes a small aperture opening. A small F-stop number denotes a large aperture opening.
A small aperture opening (large F-Stop number) will result in a larger depth of field (like squinting your eyes to focus better). A large aperture opening (small F-Stop number) will result in a smaller depth of field.
Until you are very comfortable with aperture and F-stop it may help you to remember it this way:
- Large F-Stop = Large Depth of Field = More Light Needed
- Small F-Stop = Small Depth of Field = Less Light Needed
Lens Strength and Depth of Field
The next part of the depth of field determination is your lens. The focal length of the lens (basically the lens length) magnifies the effect of the aperture size. So, in essence, the stronger your lens, the smaller the depth of field possible. This difference may not be obvious on large scenes such as landscapes when comparing two similar lens strengths but the larger the differences in lens, the more pronounced the change. This effect is especially strong in macro photography where close proximity to the subject and high focal lengths result in depths of field that are sometimes less than an inch.
Depth of Field Relationship in a 70-300mm Lens
- 70mm = largest DOF
- 100mm = large DOF
- 200mm = small DOF
- 300mm = smallest DOF
Distance to Subject and Depth of Field
Just like the distances inside the lens play a part in depth of field, so does the distance to your subject from the camera and the distance between subjects. Think of it this way. Hold your arm out in front of you with your arm straight. Focus on your hand. Chances are you see a pretty good bit of the surrounding area in clear focus as well. Now, move your hand slowing towards your face until it is about half an arm length away. Now when you focus on your hand you see much less of that same surrounding area. The closer you move your hand to your face, the more distance between the hand and the surrounding area so less of the surrounding area is in focus for your eyes.
This happens with your camera as well. The closer your subject is to your lens, the smaller depth of field you generally have. Now, to throw a background out of focus, you use this to your advantage by placing your subject near you and far away as possible from the background.
Shape of the Depth of Field
One final note about depth of field (DOF). Depth of field is not even. That is, depth of field is not equal in front of and behind the focus point. Approximately 1/3 of the depth of field extends in front of your chosen focus point while 2/3 of the depth of field extends behind your chosen focus point.