Depth of Field and Focal Plane
One part of the exposure triangle, as we discussed, is aperture. Aperture controls our depth of field but not all depth of fields are the same. The amount of depth of field is influenced by many things including the type of lens, it's focal length, the focal plane and so much more than we can get into in this post. So to keep things simple to understand we will discuss depth of field in how it relates to focal distance and its relative focal plane.
For this discussion we will refrain from discussing specific lenses or indicating specific aperture settings. Those numbers will vary widely depending on many variables anyway. I will simply keep it to a shallow depth of field based on the different focal distances used in these examples. Shallow depth of field is obtained by using a wide aperture and following the basic rules of exposure.
I also want to establish the definition of focal plane and depth of field. Focal plane refers to the principal where the point of sharpest focus on your subject extends outward, from the subject, parallel to the camera sensor. Therefore something beside your subject and parallel to your sensor will be in focus as it sits in line with the focal plane (the illustration above shows this principal) while something in front of or behind the subject will start to loose focus. That area of relative sharpness is your depth of field.
This area of sharpness degrades in front of and behind the focal plane and the amount of perceived degradation is determined by the physical properties of the lens, distance of the subject from the camera and distance of adjoining objects. However there is a certain amount of sharpness immediately in the vicinity of the focal plane and it is this area of relative sharpness that becomes the depth of field (this is illustrated by the red area in the following examples).
In our first example we have an area of 60 feet with our subject placed mid way at 30 feet. In the foreground we have some flowers and our background is a line of trees. The focal plane is the thin line splitting the subject. The red area represents the depth of field or the area of the scene that remains in relative sharpness of focus. You can see that the depth of field is quite large (about 15 feet in this illustration). You also need to understand that this gradual softening of focus will be more gradual behind the subject than in front. So our field of flowers will be mostly out of focus as well as our tree line.
In this next image we have reduced our scene even more. Our subject is now about 15 feet in front of a tree. A lens with good depth of field at a wide open aperture will now compress the area of relative sharpness to about 10 feet. A big difference than our 15 or so feet from our previous example. Depending on the lens you will have the edges of the flowers and the front of the tree encroaching on that field of sharpness.
Now we move the scene closer to the camera. Placing our model in front of a wall 10 feet from the camera we see that the wall sits within the depth of field. We will also have the flowers closest to our model in focus because our focal distance has now been reduced. Our depth of field is comparatively reduced as well from 10 feet down to about 3 feet.
Moving in even closer, on the flowers for example, we have reduced our depth of field considerably to a matter of only a foot because we have reduced our focal distance down to about 6 feet (these are large flowers).
Keeping with the flow, zooming in with a macro lens on a single flower will show that our depth of field is now measured in inches. If we get even closer it gets reduced even further to the point where the are of relative sharpness can be measured in parts of an inch. And so on...
The best way to fully understand this is to take your camera and experiment with it. A handy little exercise is explained in a previous article, "Depth of field - A DIY Workshop".