A lens evaluation

[william_carnahan]

Hi.  Two questions, please. 1)  Should I be concerned about a used lens I might purchase, when that lens on my D850 only indicates F/3 in any mode, though the lens (Nikon 105mm Macro VR G) is a F/2.8?   2)  When I tried to fine tune several Nikon 'G' f-mount lens, using the auto fine tune function on the D850, using exactly the steps published in a well-known guidebook, I always get a fail message referencing focus settings.  Can the fact that I have back-button focusing set up interfere with the auto fine tune function?  Neither the Nikon manual or the other guidebook address this.  Thanks.

[ShunCheung]

Macro lenses loses max aperture when you focus to the macro range.

[bgelfand]

3 hours ago, william_carnahan said:

1)  Should I be concerned about a used lens I might purchase, when that lens on my D850 only indicates F/3 in any mode, though the lens (Nikon 105mm Macro VR G) is a F/2.8?   

Depending upon the magnification (a function of the distance the subject from the front of the lens), probably not. When you get close to the subject, the effective aperture gets smaller (larger f-stop number) e.g. f/3 rather than f/2.8 Rather than go into detail in this reply, here is a link that explains it better than I could:

Effective Aperture and Macro | B&H eXplora (bhphotovideo.com)

Try focusing on an object 10 or 20 feet away. Now does the camera report the aperture as f/2.8?  Then focus on an object at the minimum focus distance. Is the aperture reported as f/3 (or perhaps smaller? If so, the lens is operating correctly.

 

As to the second question, I suggest you turn off back button focus, perform the auto fine tune for the lens, then turn back button focus back on for normal use. The fine tune data for the lens should be stored and used.