I've been using a Canon 2x II Extender with a 70-200 f2.8 non-IS lens for several months. the results are ok but not great. one thing that makes the photography difficult for this amateur is that the DOF at 400 f5.6 is incredibly short; roughly one centimetre.
I'm wondering. is the DOF shorter when using a 70-200 with a 2x extender at f5.6 than it would be when using a 400mm at f5.6?
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Depth of field is determined by:
1. Focal length. That is the focal length of "the entire lens". Remember, when we put an extender on a lens, we create a "new lens" of greater focal length, and that is the focal length of importance here.
2. Distance to the object plane of focus (to the subject, generally)
3. The aperture as an f/number. In the case of the "new lens" created by the addition of the extender, that is the f/number of the combination (for a 2.0x extender, that would be 2.0x the f/number to which the main lens proper was set). [Note that this isn't an "effective f/number" - it is the actual f/number of the "new lens".]
4. The circle of confusion diameter limit (COCDL) we adopt as our criterion of "acceptable blurring".
Thus, if we have a 70-200 mm lens set to 200 mm, with a 2.0x extender, and the "new lens" is set by the camera to an aperture of f/5.6 (that would mean that the main lens proper was set to f/2.8 - the lens takes care of that itself once it learns that a 2.0x extender is in the circuit), and assuming that the subject distance is the same and that we use the same COCDL, then the depth of field should be the same as for a 400 mm lens at f/5.6.