they are not exactly the same, though they do share some components. they have different pickup patterns due to both the windscreen and, iirc, some small holes on the 57 capsule itself. more importantly, they sound different.
it's oft-said that the 58 is more suited for vocals, but i find that's not always the case. like any mic choice, it depends on the situation at hand and it's best to try both.
I should have explained myself better. Yes with the windscreen on, it may change pick up patterns due to proximity but without the windscreen on, and just the bare microphone, I stand by what I said, they are the same exact thing. I can guarantee you that.
Straight from a Shure Engineer himself:
"The SM57 and SM58 are essentially the same, except that the SM57 was intended mainly for instruments and the SM58 for vocals. The SM58 has a ball grille that acts as a pop filter. The two mics have the same proximity effect at the same miking distance.
Because of its smaller grille, the SM57 lets you get closer to it for more bass boost. But if you are 2 inches from the diaphragm in either mic, they have the same bass boost.
They have a slightly different response at high frequencies because of the acoustical effects of the different grilles." - Shure Microphone Enginee Bruce Bartlett
And if someone still doesnt believe me, the guy has a book written about microphone techniques.
http://www.amazon.com/Stereo-Microphone-Techniques-Bruce-Bartlett/dp/0240800761
A little background information on Bruce Bartlett to verify the validity of his quote.
Bruce received a degree in physics from the College of Wooster, and studied electrical engineering at Gannon College and the University of Akron. Then he worked as a microphone engineer at Astatic Corp. and at Shure, where he worked alongside the designers of the SM57, SM58, and SM81 microphones.
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This should end the discussion about the difference between the two. Without the windscreen, they are the same exact microphone, same specs (look them up).
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