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Since we're being geeky here, isn't it the focus distance, not the focal length, that effects DOF secondary to aperture?
ie.: if you're using 55mm at f/5.6 and you focus on something that is physically 20 feet away from you, the focus distance will be close to the far end, causing a larger DOF. But if you're standing in the same spot, focusing on same subject 20 feet away from you, but you're using 200mm at f/5.6, the focus distance will close to the near end, causing a shallower DOF.
ie.2: if you now stand 5 feet from the subject (I'm guessing at the math here) so that the subject at 55mm fills the same the same way that it does at 20 feet and 200mm (both still using the same aperture), the depth of field will be the same.
...right?
ie.: if you're using 55mm at f/5.6 and you focus on something that is physically 20 feet away from you, the focus distance will be close to the far end, causing a larger DOF. But if you're standing in the same spot, focusing on same subject 20 feet away from you, but you're using 200mm at f/5.6, the focus distance will close to the near end, causing a shallower DOF.
ie.2: if you now stand 5 feet from the subject (I'm guessing at the math here) so that the subject at 55mm fills the same the same way that it does at 20 feet and 200mm (both still using the same aperture), the depth of field will be the same.
...right?