The aperture is the main determinant of depth of field when using a camera. Open the aperture
up and you will get less, close it down and you will get more. The question is: Why do you get
more depth of field with a small aperture? To simplify things I will only be looking at the object
side of the camera for this discussion. The same concepts apply on the object side as they do on
the image side, so I will omit the image side for now.
Before we can get going, we need to define the circle of confusion in relation to the object side of
the equation. We started by defining the circle of confusion in terms of the image. For a typical
Nikon dSLR the circle of confusion is about 0.02 mm and the detector is about 23.6 mm wide.
That translates into about 1/1200 of the width of the detector. To translate this to the object side
you just need to know the width of your field of view and divide by 1200. This can also be written
in terms of magnification as
c(o) = c/m (where c(o) = object side circle of confusion, c = circle of confusion, m =
magnification)
As magnification goes down (i.e. The field of view enlarges) the c(o) proportionally enlarges. As
the magnification increases the c(o) proportionally decreases.
up and you will get less, close it down and you will get more. The question is: Why do you get
more depth of field with a small aperture? To simplify things I will only be looking at the object
side of the camera for this discussion. The same concepts apply on the object side as they do on
the image side, so I will omit the image side for now.
Before we can get going, we need to define the circle of confusion in relation to the object side of
the equation. We started by defining the circle of confusion in terms of the image. For a typical
Nikon dSLR the circle of confusion is about 0.02 mm and the detector is about 23.6 mm wide.
That translates into about 1/1200 of the width of the detector. To translate this to the object side
you just need to know the width of your field of view and divide by 1200. This can also be written
in terms of magnification as
c(o) = c/m (where c(o) = object side circle of confusion, c = circle of confusion, m =
magnification)
As magnification goes down (i.e. The field of view enlarges) the c(o) proportionally enlarges. As
the magnification increases the c(o) proportionally decreases.
| Depth of Field and the Aperture |
Now we can get about discussing the role of the aperture in DOF. The top part of the diagram
shows a large aperture with a relatively narrow depth of field. The object circle of confusion is
located at the focus plane and listed as c(o). The bottom diagram shows the effect of closing the
aperture. The c(o) remains the same, but the angle of the rays extending from the lens to c(o)
has decreased and this causes the depth of field to increase. At a narrow angle, it takes longer
for the blur circle to enlarge to the size of the c(o) and thus the DOF is increased.
You will also notice that the increase is mostly on the far side of the focus plane. Because of the
angles involved, there will always be more depth of field on the far side of the focus plane
compared to the near side. As the field of view gets larger (lower magnification), that difference
will increase. Typical photographers say that 1/3 of the DOF is in front of the focus plane and 2/3
is behind the focus plane. This is more of a rule of thumb than a fact as the actual distribution
varies quite widely.
As the magnification rises into the macro photography range, the distribution of DOF becomes
more equal on the far and near side of the focus plane. The distribution always favors the far
side, but the difference becomes minimal at high magnification. Because of this it is commonly
assumed the distribution is equal far to near with macro photography.
Next: Why is there so little DOF at high magnification?
shows a large aperture with a relatively narrow depth of field. The object circle of confusion is
located at the focus plane and listed as c(o). The bottom diagram shows the effect of closing the
aperture. The c(o) remains the same, but the angle of the rays extending from the lens to c(o)
has decreased and this causes the depth of field to increase. At a narrow angle, it takes longer
for the blur circle to enlarge to the size of the c(o) and thus the DOF is increased.
You will also notice that the increase is mostly on the far side of the focus plane. Because of the
angles involved, there will always be more depth of field on the far side of the focus plane
compared to the near side. As the field of view gets larger (lower magnification), that difference
will increase. Typical photographers say that 1/3 of the DOF is in front of the focus plane and 2/3
is behind the focus plane. This is more of a rule of thumb than a fact as the actual distribution
varies quite widely.
As the magnification rises into the macro photography range, the distribution of DOF becomes
more equal on the far and near side of the focus plane. The distribution always favors the far
side, but the difference becomes minimal at high magnification. Because of this it is commonly
assumed the distribution is equal far to near with macro photography.
Next: Why is there so little DOF at high magnification?