Have you ever noticed how camera shake is much more prevalent at longer focal lengths? This is because the narrower field of view amplifies even micro-movements of the camera, which means shutter speeds that are fast enough to render images perfectly sharp at wider focal lengths aren’t always enough to freeze those shot at longer focal lengths.
To get around this problem, many photographers use what’s called the reciprocal rule (AKA hand-holding rule). At face value, this simply requires the photographer to match their focal length with their shutter speed’s denominator to mitigate camera shake. So, if you’re shooting at 400mm, you’d need a minimum shutter speed of 1/400 sec to avoid camera shake.
