I understand that using a 2X teleconverter on my Canon 70-200 f4 IS USM will disable any hopes of using AF. And...will also give me a max. aperture of f8. I can probably live with the f8; as most shooting (Yellowstone) will be in daylight. Will my T2i still offer the white dot or beep MF shooting assist aid? And just how practical is this combo in the field anyway? I'm trying to extend my range with a minimum of expense. No way I can afford the lens that I'd really like, and I'd prefer not to rent(but could). I once used a 1.4X TC on this lens, but was unimpressed with the mere additional 80mm that it gave me; though the IQ results were good. Using a 2X TC and getting me out to 400mm would be ideal. But is it practical? And how much IQ will be depreciated? And if a good idea, which specific TC would be your recommendation. I also have a 55-250 STM lens, and a Canon SX50. Would these be better options than the 2X TC on the DSLR lens? Thanks for helping out.