A rule of tumb is just a rule of thumb. It only applies to those who invented it.
In fact if you have steady hands, you'll probably be able to get sharp pics with slower speeds. I can generally half or even quarter the speed based on this rule and still get sharp pics
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Remember, there are lots of other factors affecting camera shake.
Shake is highly dependent on how you hold the camera. Both hands on the body with a heavy telephoto lens attached is pretty shaky compared to with the left hand underneath the lens and the right hand on the body. Anyone tried it with their hands the other way round
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The actual weight of camera and lens has an impact. The heavier, the more difficult to keep steady, especially when sustaining the same position some time before taking the shot.
A grip with vertical shutter button will improve stability when shooting in portrait format.
How do you press the shutter button? Ever tried taking a shot with a 200mm at 1/125 or 1/60 handheld with and without the self timer. You'll definitely see a difference there.
Apart from shaky hands, body posture and temperature play their part as well. Having a relaxed pose or even leaning against a tree or wall will help steadying anyone. In freezing weather conditions, I'm shaking anyway, irrespective of what I'm doing

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If you have altzheimers, you'll need a tripod as well as a zimmer frame

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If you've taken plenty of pictures, you'll now at what speeds you can handhold while still getting sharp results. If you haven't, get out there and get snapping.
Maybe this is all academic, as Canon's IS lenses don't abide by the rule. In fact, it may be time to dump the Canon and switch to Konica Minolta with it's in-body imaga stabaliser technology.