1. Depth of Field
This chart
shows that the depth of field (DOF) does not change (in practical use)
if magnification is kept constant.
- "S" is the "subject" distance.
- Magnification was defined as 0.25X, the threshold for a macro photograph.
- Circle of Confusion (C of C) was defined as 0.025mm, one of the accepted values for 35mm small format.
- Lens focal lengths range from 18mm to 300mm, inclusive.
The DOF under these conditions is about 11mm from near boundary
to far boundary. The DOF does actually change, but not enough to
affect practical application. It is about 11.04mm with an 18mm lens,
and about 11.00mm with a 300mm lens, a change of 4 parts in 1000.
If magnification is held constant, for all practical purposes changing
focal length of the lens has no effect on the DOF.
2. Circle of Confusion Growth Behind the Depth of Field
This graph
shows how the Circle of Confusion (CofC) grows behind the depth of field
(DOF) for five lenses (50mm, 90mm 135mm, 200mm and 300mm). These
lenses were selected as they represent popular macro focal lengths, and
those lenses most often used with extension tubes. As with the DOF
chart shown in the previous section, magnification is held constant at
0.25X with a lens aperture of f/11 which would commonly be used for macros
of this magnification. As before, the CofC limits for the DOF are
defined as 0.025mm. Note the distance behind the DOF is in centimeters.
This graph shows how the CofC grows to create an "out of focus background"
in an image behind the DOF.
Its purpose is to demonstrate that increasing focal length does not increase the size of the CofC unless the background is sufficiently far enough behind the DOF. What does this mean in practical application? For an increase in focal length to visibly enhance an "out of focus background," the background must be sufficiently far enough behind the DOF. If there are potentially distracting objects just behind the DOF, using a longer lens will not make much, if any, improvement. The CofC does not begin to spread much between focal lengths in this example until it is at least 10cm (100mm) behind the DOF. Keep in mind this is a 0.25X macro using a practical aperture of f/11 giving a DOF of about 11mm.
3. Field Growth Behind the Depth of Field
This
chart shows how the perspective changes with focal length behind the DOF
for five focal lengths (50mm, 90mm 135mm, 200mm and 300mm). The horizontal
field is the dimension in (real) space that will land on the 36mm film
frame width, and it grows with distance from the lens. As with the
other two sections, the magnification was held constant at 0.25X and CofC
was defined as 0.025mm to find the rear of the DOF. The field width
at the subject distance is 144mm (0.25X magnification).
Thus, while increasing focal length will not change the DOF, and may
not help with making the background more out of focus unless the background
is sufficiently distant, a change in focal length does (very noticeably)
affect how much background is included in the image.