AF Micro Nikkor 105 mm f/2.8D and Aperture Shift???

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dave gaines
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AF Micro Nikkor 105 mm f/2.8D and Aperture Shift???
10 months ago

I recently purchased a used AF Micro Nikkor 105 mm f/2.8D. This lens preceded the current 105 mm f/2.8G AF-S VR Micro. It is considered by many reviews to be equally sharp and suffers less CA wide open. SLR Gear has a good review of both of these macro lenses.

http://slrgear.com/reviews/showproduct.php/product/102/cat/12

What I just noticed when close focusing is that the maximum aperture at 1:1 is not f/2.8 but f/5. As you focus the lens out toward infinity the max aperture moves toward f/2.8. I found this easy to check in manual exposure mode and manual focus. I get the same results it with the D800e camera aperture control setting changed from the command dial to the aperture ring. You cannot set f-stop below f/5. And oddly, the range of apertures is in whole stops (per the manual) and is covered by just the middle of the aperture ring range, not from end to end.

I don't find any mention of this Aperture Shift in the review linked above. On the Nikon website the spec's for this lens does not list the maximum and minimum apertures.

Here's the question: Has anyone else noticed this or is it an anomaly of just my lens? Does this also occur on the newer, current lens, the 105 mm f/2.8G AF-S VR Micro? It's easy to check if you have either lens. I can't see why it's physically or mechanically necessary to close down the aperture at very close distances. Does anyone know why a macro lens would do this?

If the lens really does focus breathe to an equivalent 60 mm at 1:1 (see below) then the same physical size of aperture would yield a smaller f-stop number, like f/1.2, not larger.

Of course, using f/2.8 at 1:1 would yield very thin DOF. According to the on-line DOF calculator at DOF Master, DOF is 0.9 mm at f/2.8 and focused at 300 mm or 1.6 mm at f/5 focused at 3000 mm. That doesn't sound likemuch difference but I'd prefer the later DOF. For most purposes you'd want to shoot very close macro stopped down anyway. With the R1C1 twin flash or a ring flash you'd probably want to stop down to between f/5.6 and f/11 for more DOF. Given all the light you need with a flash, you may be tempted to risk minor diffraction and shoot as small as f/32 in order to avoid OOF areas in your main subject.

The user reviews follwing the review linked above also claim that the lens" breathes" to a shorter focal length at 1:1 from 105 mm to around 60 mm. I can't see this on quick inspection and use. I don't know how these people would know? AFAIK, the only way to be sure of this is to measure focus distance versus width of view and calculate the angle of view at various focused distances and compare that AOV to known focal lengths.

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Dave

 dave gaines's gear list:dave gaines's gear list
Olympus C-8080 Wide Zoom Olympus E-330 Nikon D800E Nikon AF-S Nikkor 70-200mm f/2.8G ED VR Nikon AF-S Nikkor 14-24mm f/2.8G ED +7 more
Nikon D800E
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