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if I'm wrong about this, someone please correct me, but an f-stop is an f-stop
I don't think the X7 factor will change the light. stand alone light meters don't ask for legnth of lens, they only ask for ASA & f-stop. so if you're wondering about 3.5 being too dark for you on a 400mm lens, see if 3.5 is too dark on any lens you already have.
but I have to agree w/ Mat Thompson in saying a 400mm fixed lens (ie no zoom) will be tough to find anything, though not impossible. I would think you might almost have to have a spotting scope added onto your camera to help "aim"
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