I did a test a couple days ago to see just how much my TC20E hurts performance on my 80-200 AF-S Zoom...And I'd like a bit of help understanding the results! First off the test: Zoom on tripod, Focused on a tree trunk about 15 feet away under bright sun, 200mm, f5.6, took first photo. Then I mounted the 2X TC, zoomed back to '100' on the zoom, left aperture at f5.6 (which is now the minimum aperture due to the 2X) refocused and took second picture. The results: Of course the first photo (no 2x) is sharper and has more contrast, but the difference was much less than I thought it would have been. what surprised me is that the two photos have different DOF. The photo with the TC Had much less DOF than the one without. Why is that? Obviously I don't understand what a teleconverter does optically to do it's work. Is this hypothesis correct: I understand that the camera reported f5.6 because it is accounting for the 2 stop change, and the lens is actually at it's max aperture (f2.8). So is the TC actually magnifying 100 f2.8 and to do that 'costs' 2 stops of light? I'd really like to understand what's going on here! I plan to go out tomorrow and shoot again, offsetting the f stop by 2 to re- compare picture quality. Perhaps I should do it at several apertures. For what it's worth, equipment used was D70, 80-200 afs, TC20E all on sturdy tripod. I shot RAW mode under bright sunlight. In photoshop I compared center and corner crops onscreen and then printed with modest unsharp mask applied.