Great Article On Lens Variance
No two 'anythings' are 'exactly' the same...
In other words, how big is the variation in practical terms? How much of a visual difference do common lens variations across units of the same model make?
Realistically, it depends on the manufacturer's QC policies. If they work to tight tolerances, then you shouldn't see enough variation to really make a difference. If the manufacturer is sloppy or they switch vendors or they just experience a run of bad luck, one lens' MTF could be wildly different than another. Which is why reputable camera shops will always have a good return policy on lenses. Sometimes you'll get red shift or trouble focusing or something on one lens in a box full of good ones.
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