One aspect of working with macro lenses which was new to me was the “effective aperture” which is reported in camera when using the lens. In the past, my cameras and lenses have either not been of a superior quality or of the type that I am using now, so I was not exposed to this phenomena. I have often seen apertures reported as high as f67 with this lens. I was a bit confused at first, as I had never heard of apertures this small. It would seem amazing that any light whatever could reach the sensor through such a small opening. However, a bit of research revealed that “Macro lenses change their ‘effective aperture’ as you focus closer. This effective aperture gets smaller as most macro lenses actually change focal length too, as you focus close. Some lenses do not report this, but all Nikon brand macro lenses and some 3rd party ones like your Tamron do.”* It might be hard to comprehend how and why this occurs but once you have a grip on the concept, it makes sense. Basically, the closer you get, the less light is allowed into the lens and, even though we are using an fixed aperture f2.8 lens, the amount of light entering is “effectively” the same as an aperture of f51, for instance. The camera is then relaying that information to you in your viewfinder.
More information than you ever wanted to know and a good starting point for further research concerning this subject can be found in the photomacrography.net forums. There are also discussions about the relationship between effective aperture and diffraction blur.