Hi Steve
When refering to lenses Magnification would be the term to use
However with a barlow I believer multiplier for the magnification is quite appropriate 
multiplicity is definitely not the correct word in this case.
> From: azstefano@me.com
> Date: Fri, 8 Mar 2013 06:54:47 -0700
> To: stellarium-pubdevel@lists.sourceforge.net
> Subject: [Stellarium-pubdevel] Optical terminology
> Hi Bogdan:
> The best English for for Barlow and telescopes is "magnification" or "magnification factor." In English, multiplicity would be used for the number of instances of something, while magnification is the power that something appears larger.
> Steve
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