Nice pictures! I like the forget-me-nots; I was just photographing some with an f/2.8 lens the other day as well! But it was a 28mm f/2.8 lens…
Anyway, on to the technical question! My first thought was that zoom lenses are often variable-aperture, like f/3.5-5.6. It varies depending on the focal length that you select. However, the lens in question is a prime, so that’s out of the question! So, I fired up my search engine, and after running for about .5 seconds, it gave me a couple of results that should answer your question! 🙂
Here are the results, in the order that I found them:
First, was a person who had the same question as you.
My Nikon 60mm D Micro or my D800E (with the 60mm attached) have a strange behavior, if i set aperture to f2.8 up to f4.5 when i focus in or out the camera shows different f numbers e.g. f2.8 shows as f.3.5 Sad. This occurs only from 2.8 to 4.5 if i set f5 and above the camera shows the right f number.
And here’s the answer:
You might be confusing the effective and real aperture.
When focussing close, there is light loss due to magnification, and the combination of actual aperture and light loss is known as the effective aperture. Nikon cameras display the effective aperture on the LCD, which confuses a lot of people. (emphasis mine)
– https://www.imaging-resource.com/lenses/nikon/60mm-f2.8g-ed-af-s-micro-nikkor/review/ –
The lens is marked as a constant ƒ/2.8 lens, but in fact the effective aperture will be smaller as you focus at shorter and shorter distances. The following table lists the aperture to distance differences:
Subject distance 6” 7” 8.5” 9” 10” 1′ 3” 3′ infinity
Largest aperture ƒ/4.8 ƒ/4.0 ƒ/3.8 ƒ/3.5 ƒ/3.3 ƒ/3.2 ƒ/3.0 ƒ/2.8
Smallest aperture ƒ/57 ƒ/51 ƒ/45 ƒ/45 ƒ/40 ƒ/36 ƒ/36 ƒ/32
So, to answer your question, it is a combination of camera and lens! The lens loses light when focusing up close, so Nikon cameras show a different aperture to compensate so that you can calculate your flash exposure correctly. That’s not very handy for someone who doesn’t use flash! 🙁
Here’s a thread that goes into more details if you’re interested. http://www.dpreview.com/forums/thread/3901125
