Andy: : Herschel, I think the "trial-and-error" (or as you say analogue) way of finding : the best rotation point is the most efficient and practical way. The location of : the point seems to depend on a number of things and accounting for them all is : probably not a simple matter if you don't have information about the internal : design of the lens. And even if you do it is not simple! - andy nicely put, and the only way I've ever done it :) to be honest, I've never been able to calculate nodal points in lenses especially well - and zooms (effective multifocal length lenses) at all! Given lens makers round everything anyway, '50mm' lenses are often not.. any calculations are likely to be as erronoous as aperture calculations based on convenient f numbers ;) karl
