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They usually have much narrower AFOV than fixed FL EPs (throughout the entire range of the zoom). The Nagler zoom only has 50 degree AFOV (IIRC) as opposed to 82 degrees for the fixed FL Naglers. (An exception is the Speers-Waler 5-8mm zoom.) Also, the optics for the zoom puts more glass in the way, inevitably decreasing transmittance.
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