New answers tagged focal-length
http://photographylife.com/equivalent-focal-length-and-field-of-view I have to contradict with the reply's to this post The only thing that changes is the field of view when you use a cropped sensor. But the angle of view is what causes the distortion. And it remains the same for a lens irrespective of the cropped factor.
I think the only possible answer to your question is "no such rule of thumb exists". The problem is just too complicated - as Matt indicates in a comment, how "distracting" the background is depends immensely on the content of the background, so you're first of all going to have to capture that. And if you can do that, you're pretty close to having reduced ...
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