christo™ Offline Upload & Sell: Off
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Using an extender does impact the DOF. The answer "how" is not simple because DOF depends on aperture, focal length, circle of confusion diameter, and subject distance. Assuming the same aperture setting for the lens alone or lens + extender, and ignoring the circle of confusion impact (which must be real, but is unspecified, and should be small), and assuming your subject is closer than the distance at which the DOF really extends to infinity, the DOF will be shorter. If the subject is far enough away that the DOF extends to infinity with the lens+extender, in which case it will with the lens alone, by definition it cannot be numerically different.
In your situation, the practical answer is that when shooting the same subject at the same F number, your DOF will be decreased if you are using all of the additional zoom. I think the best approach is to ignore the fact that your lens+extender is two pieces, and perform calculations based on your using a theoretical lens that has a 98-280mm focal range.
Things get sticky when you take into account "shooting wide open" because you have a change in wide open F stop, and the smaller F number with the extender counteracts the DOF decrease of the additional focal length. As the focal length of the two lens makeups overlap, it is trivial to come up with an effective focal length for which the DOF actually increases with the combo lens with both set wide open. Getting any more detailed would take a graph.
As far as the F numbers you mention: you will not be able to shoot at f/4.5 or f/5.0: the 1.4x subtracts a whole stop from the maximum aperture (adds one stop numerically to the F number). As your lens has a maximum aperture of f/4, your maximum aperture with the extender will be f/5.6.
The camera will readout the F number of the lens combination -- again, everything is treated as if it were just a lens with a longer focal range. Ditto for the EXIF data. Whether it has the equivalent DOF depends on things you have not specified, but, making assumptions about what you are talking about for subject distance, you will actually have shorter DOF as with everything else equal, the DOF decreases as the square of the focal length, and that is where you are making the biggest percentage change.
In summary: I think you will like the combo for the purpose you state if you feel you want more reach -- if anything, the DOF will decrease. The only potential problem I see is that shooting baseball so often means shooting in low light, and having a smaller max. aperture means slower shutter speed. Slower shutter speed in combination with longer focal length (and longer lens, sometimes, depending on you), means camera shake can become an issue in lighting where it was not with the lens alone. The biggest effect on camera shake becoming a problem in lower light would likely be due to the extended focal length, as I assume you are talking about zooming in to 280mm.
Caveat: as far as the EXIF data and the F number readout on the camera, that is actually dependant on the camera firmware, not physics, and I have not read the EXIF data for a 1Ds or D30 for a shot with a lens + extender, nor shot either with same, so I can't rely on experience. However, with my D10, the EXIF data reflects the combined focal length and zoom setting, the readout and EXIF aperture reflect the true combined aperture, and it would be pretty weird if that weren't also true of the other EOS dSLR's.
Edited by christo™ on May 13, 2003 at 04:08 PM GMT
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