(all calculations based on DX sensor, with DOFmaster LE for PalmOS)
It is a combination of aperture, focal length and focus distance.
When you use the 18mm end of the lens and focus at 10m a change from f4 to f5.6 is not very noticeable. The dof changes from:
f4: 2.89m-inf
to
f5.6: 2.23m-inf
When you use the 70mm end of the lens the change is bigger, let's assume a change from f5.6 to f8, focus distance is 10m again. The dof changes from
f5.6: 8.13m-13.00m
to
f8: 7.55m-14.8m.
Notice that with the longer focal length (18mm to 70mm), at the same focus distance (10m), at the same aperture the dof changed from:
18mm: 2.23-inf
70mm: 8.13-13.00 (all in meters)
When you shoot at 70mm f5.6 and change the focus distance from say 5 m to 20m the follow dof change takes place:
5m: 4.49-5.64
20m: 13.7-37.1
So it is a combination of all factors. When you do landscapes with a wideangle and focus far into the field, like using the 18mm end of the lens, f8 and focussing at 40m, the resulting dof will be 1.93m to infinity. There is no need in this situation to go to f16 or narrower apertures.
Would you do the same thing with a focal length of 50mm the dof becomes 11.3m to infinity. In that situation it might be useful to stop the aperture down to f16 and bring the dof to 6.54m to infinity.
For shallow dof it is therefore useful to use a long focal length, a wide aperture (f1-f4) and focus nearby.
Edit: The use of a different focal length has some side effects, like a change of perspective. In the calculations above the photographer remains in the same place and that is also not always possible when you change the focal length.
Message edited by author 2005-01-22 07:43:48.
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