Nikon Macro Lens, effective aperture

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Bill Janes Senior Member • Posts: 1,906
Nikon Macro Lens, effective aperture
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As you focus closer towards 1:1 magnification with a classic Nikon macro lens (Micro Nikkor in Nikon speak) the lens is extended and the effective aperture decreases. With more recent Nikon macro lenses the front element of the lens does not move and the overall length of the lens does not change. Close focusing is obtained by internal movement of lens elements, resulting in a decrease of focal length.

The classic formula of determining the effective focal length is fe = fm * (m+1), where m is the magnification, fm is the marked or nominal aperture and fe is the effective aperture. Chipped Nikon lenses report the effective aperture. With an f/2.8 macro focused at 1:1, the effective aperture is reported as f/5.6.

A more accurate formula for the effective aperture is fe = fm * (m/p +1), where p is the pupil magnification ratio (see here for details). For normal focal length macros (55 or 60 mm) the pupil ratio is often near 1:1 and the simpler formula gives reasonably accurate results. However, with telephoto macros, p is often considerably less than 1:1).

Bill Claff's Optical Bench gives p for many lenses. For the Nikon 105 mm f/2.8 OS micro Nikkor it is 0.74, 0.65, and 0.58 at infinity focus (zero magnification), 0.5 m, and 1:1 m respectively. I'm not sure which of the several 105 f/2.8 Micro Nikkors this is, but presumably one that extends the front element on close focusing (such as the AF Micro Nikkor 105 mm 1:2.8D which is screw driver focus with extension of the front element on close focusing.

With the latest AFS Micro Nikkor 105 mm f/2.8 G (an internal focusing lens) p is 1.06, 0.56 and 0.3 at infinity, 0.5 m, and 1:1m respectively. P changes more with close focusing with this lens, and in this case, internal focusing increases the variability of p.

With both of these lenses focused at 1:1, the effective aperture is reported as f/5.6 in accordance with the simplified effective aperture formula. Accounting for p, the effective aperture for the former lens at 1:1 with the aperture set at f/2.8, is f/7.6. With the latter AFS lens, the effective aperture with the lens set at f/2.8 is f/12.1. With this lens set at f/8, the effective aperture is f/34.7, well in diffraction territory.

I don't know how the chipped lenses account for p, but the above considerations indicate that it is ignored. These differences are significant with everyday photography, and additional discussion is invited.

Bill Janes

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