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Good question, Fred.
I'm not sure if you are concerned about subject motion (e.g. - moving leaves) or a moving camera (with long lenses, this can be an issue even with a solid tripod), so let me deal with both.
Regarding subject motion, this rarely turns out to be a problem for several reasons. First, since I'm typically blending different areas from the two (or occasionally more) exposures, the objects within the areas usually come entirely from one or the other exposure. Take a surf shot, for example. One of the component exposures might include the water and the other exposure might include, say, the sky - so there really isn't a point in the frame where I get a doubly image with different points in the motion of the subject. In other cases, some care in where the boundaries are created can overcome any potential problems. With foliage - say a sunny section and a shaded section - I'm most likely to blend in a bit of the longer exposure in shaded areas, and I can either be careful about the boundaries, or I find that these areas include less motion. Also, it is good to keep in mind that it isn't always necessarily to completely stop motion - lots of fine landscape work has moving leaves, for example.
As far as camera stability issues go, I have a little live view trick that I use when wind is a concern. I frame the composition, focus, then leave the live view display zoomed in at 10X. If there is any vibration from wind I'll see it in the display - and I simply wait for a lull in the motion to make the exposures.
Hope I'm answering the right questions!
Fred Miranda wrote:
How do you deal with windy conditions? I could never blend exposures where wind is a factor. In these situations, a gradual ND is helpful.