They are the same: 1200mm = 215mm
215mm is tha ACTUAL value.
1200 is the equivalent in 35mm film world... is it simply a common reference point (it assumes everyone if familiar with 35mm film equipment!).
Why the difference? It pretty much boils down to sensor size. A smaller sensor gives higher magnification for the same lens when compared to a larger sensor. You trade magnificataion for quality.
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