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Old 12-08-2012, 08:12 AM
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bojan
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Join Date: Jul 2006
Location: Mt Waverley, VIC
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There is no "effective" f-number, and no "effective" FL - those terms are wrong and highly misleading, and this tread is the proof of that.

"Effecive FL" was "invented" to help people wrap their minds around the lens focal length and their field of view when used with standard Leica format film (24x36mm) and/or APS format (16x22mm).
It indicates what FL on APS sensor (or film) would be needed on Leica format to have the same viewing angle.
A little better and more accurate term is "Crop factor" (1.6 in APS size case, but it involves mental arithmetic and it seems people do not like that..)... it tells us how much smaller is the field of view for a given FL and sensor size, compared to standard Leica format (24x36mm)
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