Shariya, Michel's graph is a bit confusing. The 'npp' varies from the centre to the edge of the lens. The 12mm (gold ring) is the 'npp' for the centre of the lens etc The 20mm is the 'npp' for the edge of the lens. Thus any 'npp' you use is a compromise but clearly 12-15mm ie around the 'gold ring' point covers the widest area of the lens (centre +/- 45 degrees) and is what most people use. Hope this makes sense.
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