A macro lens marked is used at unit (life-size) magnification,

PHYSICS · Multi-part · Fermi University Physics · Optics & Photonics · Geometrical optics and imaging · Geometrical optics and imaging

What an Instrument Can See, Ch Pupils: the aperture of a linear system

Problem

A macro lens marked f/2.8f/2.8 is used at unit (life-size) magnification, ∣MT∣=1|M_T| = 1. Treat it as a thin lens, so that its entrance and exit pupils are the same disc. Someone argues: "The working f-number only matters at very high magnification, and at unit magnification it is found from f/#working=f/#1+∣MT∣=2.82=1.4,f/\#_{\text{working}} = \frac{f/\#}{1+|M_T|} = \frac{2.8}{2} = 1.4, so at unit magnification the lens becomes effectively a full stop faster." ((a)) Find the error in this working. ((b)) State the correct working f-number at ∣MT∣=1|M_T| = 1.

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