you have it right: it is in fact based on near point. So it does in fact vary person to person. The ratings are based on a standard near point of 25 cm.
________________________________________
From: Phys-l [phys-l-bounces@phys-l.org] on behalf of Anthony Lapinski [Anthony_Lapinski@pds.org]
Sent: Friday, May 17, 2013 3:20 PM
To: tap-l@lists.ncsu.edu; phys-l@phys-l.org
Subject: [Phys-L] lens magnification
I've always wondered about this "simple" idea, and a student just asked
me. Magnifying glasses (biconvex lenses) are sold with a power rating (3x,
4x, etc.). How is this determined? Since m = -di/do, as do changes so does
di and m.
Is the di somehow a "fixed" value, corresponding to a person's near point?
And then the object is moved until it's in focus to find the "maximum"
magnification? If so, the m would change depending on the observer!
Wondering if someone has a simple explanation for this and/or an optics
demo related to this idea.