Does IS help with slow moving subjects?

Sorry, no, wrong.  There plenty of times that IS is vastly superior to a tripod.

This is 1200mm-equivalent, and it's been heavily cropped beyond that.  This thing is going around 8/10ths of the speed of sound and travels through a stationary frame in a tiny fraction of a second.  But I can hold it nearly dead stationary in a frame by hand-tracking with the help of two-axis stabilization, and get this shot at only 1/50th.  You couldn't do this with a tripod and a fast shutter speed, and even using a very good tripod head to help with the tracking isn't as good as good IS is in the same shooting environment.

A tripod is only like perfect IS when you are shooting stationary subjects.  IS can be very helpful on moving subjects when you want to freeze solid-body motion, as I did above.

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Lee Jay
(see profile for equipment)