Nikon Macro Lens, effective aperture

Started 2 months ago | Discussions thread
Leonard Shepherd
Leonard Shepherd Forum Pro • Posts: 19,619
Re: Nikon Macro Lens, effective aperture

Bill Janes wrote:

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.

This is complex territory

"P" is usually only an issue when focussing closer than 1/10 life size

I am not sure if you have the 105 VR macro - but if not try to handle one on a DSLR.

Put the lens on a camera, aperture priority at aperture f4, look into the front of the lens, press and hold the depth of field preview and watch the physical size of the aperture change from well stopped down at infinity to about wide open at in 1:1 focus.

Then repeat looking at the top plate read out and note the exposure time between infinity and 1:1 stays the same - even though if the lens was a symmetrical design (which it is not) the exposure time should increase approaching 1:1 magnification.

Next set the lens at infinity f2.8 (aperture wide open), change focus to 1:1 and (as the lens cannot open wider than wide open) watch the exposure time increasing.

What the Nikon system seems to do with the the 105 VR macro lenses is at f4 at 1:1 focus is to open up the aperture to get an f2.8 exposure when showing f4 on the exposure read out.

Nikon "gets" the exposure right at 1:1 by opening up the aperture by 1 stop, with the other 1 stop being from focus breathing - to get the 2 stop 1:1 exposure increase needed with a symmetrical lens.

Back to the top plate read-out - you can set f51 at 1:1 focus and observe it changes to f32 by changing focus to infinity.

Summing up - at 1:1 you get a correct exposure with 1 stop less depth of field than for the 1:1 aperture read out indicates - and a 1 stop brighter aperture than with a symmetrical lens.

Why do the 60mm AF-S, the 105 G and the 200 D macro work in this unusual way?

I have not been able to find out from Nikon.

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Leonard Shepherd
In lots of ways good photography is much more about how equipment is used rather than anything else.

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