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Generally you shouldn't be going slower than 1/(focal length)(x sensor size multiplier, according to some people) in terms of shutter speed. So, depending on who you ask, at 400mm you should go no slower than 1/400 (w/o sensor size adjustment) or 1/600 (w. sensor size adjustment)
With IS, they have varying levels of claimed effectiveness, ranging from 2 stops to 4 stops. Taking a claim of 3 stop advantage at face value as an example, at 300mm you would need to be faster than 1/75 (1/300 x 4)
With IS, they have varying levels of claimed effectiveness, ranging from 2 stops to 4 stops. Taking a claim of 3 stop advantage at face value as an example, at 300mm you would need to be faster than 1/75 (1/300 x 4)