f/2.6 is one-fourth stop less than f/2.8.
Your Nikon D700 only displays apertures in one-third stop increments. The next widest f-number past f/2.8 it displays is f/2.5 Even when the lens is set to f/2.6, your camera has no way to display that number. Apparently it displays f/2.8 unless the lens can open up to f/2.5 or wider.
If the lens is set to f/2.6 using the aperture ring, then I would expect that the lens is opening all of the way up when you take an image. Have you tried shooting with the ring set to f/2.8 and then set to f/2.6 under a controlled test?
- Be sure the focal length, shutter duration, and ISO are set identically
- Be sure to use a shutter duration long enough to minimize the effect of flickering lighting. In countries with 50Hz alternating current, use 1/100 or longer. In countries with 60Hz alternating current, use 1/120 or longer. To be on the safe side, extend those exposure times to 1/50 and 1/60, respectively.
- Zoom the lens all of the way out to 28mm.
Compare the results between using f/2.6 and using f/2.8. A histogram might be helpful to see if there is a noticeable difference in the two exposure values.
There no way to set the ring to 2.8, the next stop up is f4. I set the camera up manually as described then set the lens to f2.6 on the ring and then took a picture and compared it with the command dial set to f2.8. Both images were identical in illumination. So it seems it is either not possible to get to f2.6 or the command dial's f2.8 is actually a f2.6
In the absence of an f/2.8 "stop" on the lens' aperture ring you could try the following:
- If you have an available scene with uniform brightness (such as a rear illuminated scrim that is very evenly lit), you can compare the f/2.6 setting at 28mm to the f/2.6 (f/2.8) setting at 70mm.
- You've already compared f/2.6 via the aperture ring vs. f/2.8 via the camera's menu (with the lens locked at the narrowest aperture). Now do that test again using both ends of the lens' focal length range for each control method.
A difference of 1/4 stop is pretty minute - you'll likely only be able to confirm it repeatedly by comparing histograms. It goes almost without saying that for the most consistent results use tripod so the camera does not move between various exposures. A series of three shots at each setting/method will also help to see the difference in variability caused by the limitations of the mechanical aperture control and the possible difference in exposure between f/2.6 and f/2.8.