Sir Motley wrote:
I wish to buy at some time in the future a macro lens and a good just-walking-around lens. I am considering the Nikkor 105mm macro and the Nikkor 16-300mm. If I bought the Sigma macro that you were talking about, would that satisfy my requirements for both a macro and a walking around lens?
You'd get better responses, I suspect, posting your own subject rather than hijacking someone else's.
But, to answer your question... They are two very different things, a macro lens and a general purpose, walk-around lens.
16-300mm actually is neither. That's an extreme, 20X zoom that tries to "do it all". Typically those types of lenses have not been especially good at anything, have to compromise in some ways.... though they might be convenient since you practically never need to change lenses. I don't know about the Nikkor 16-300mm in particular... suggest you research it carefully, before spending your money. But a lens like that sort of defeats one of the primary purposes of an interchangeable lens camera... the capability to swap lenses and adapt the camera to different purposes. (Note... "20X" is an approximation referring to the range of the zoom... 300mm divided by 16mm is actually only 18.75X.)
Zooms have improved a lot in the last couple decades. But I still try to stick with 2X, 3X, 4X maybe 5X or 6X, at most: 12-24mm, 24-70mm, 70-200mm, 100-400mm. These typically compromise a lot less than the extreme zooms.
Your camera must be a DX model, if you are considering a 16-300mm. Chances are your camera came with a walk-around lens... probably an 18-55mm. That's actually a pretty good choice. But maybe you want a little wider range of focal lengths and/or faster focusing... so might want to consider the Nikkor 16-80mm or 18-140mm. These have variable apertures, not have particularly large apertures (for low light shooting or stronger background blur effects).... Some people prefer a large, non-variable aperture, walk-around zoom... such as Nikkor 17-55/2.8, Sigma or Tamron 17-50/2.8. Note that these are larger, heavier and necessarily have a narrower range of focal lengths... and typically a higher price tag.
If you already have an 18-55mm, an alternative might be to get lenses to complement that, such as a 55-200mm or 70-300mm telephoto zoom or a wide angle such as a 10-24mm.
Those are more true "walk-around" lenses. I don't have experience with them so you'll need to do some research. If they are able to focus pretty closely and at high enough magnification... or you use it with some macro extension tubes that makes a lens focus even closer... you may not need a macro lens.
But, if you want to do a lot of macro shooting, then a true macro lens would be the easiest way to do high quality work. For general purpose macro, a lens in the 85mm to 105mm range of focal lengths is what I recommend. This is a good compromise of size, price, working distance, and ease of use. There are more compact, shorter focal lengths... but they put you closer to your subject. And there are longer focal lengths that make for a lot more working space... but they are expensive and a lot more difficult to get a steady shot (more likely to need a tripod).
You can choose among Micro Nikkor 85mm and 105mm, Tamron 90mm (two versions), Sigma 105mm, and Tokina 100mm macro lenses. In Nikon mount, the Tokina is a "D" type lens, which does not have a built-in motor, so cannot autofocus on the more entry-level Nikon cameras (D3000 and D5000-series, presently). Manual focus usually isn't a problem for macro work, might even be preferable a lot of the time. But this lens might be less useful for other non-macro purposes, than the others that are able to autofocus.
Speaking of which, one reason macro lenses are somewhat specialized is that they are slower autofocusing. This is because the lens needs to move it's focusing group a long, long way to be able to focus from 1:1 to infinity! It also is because macro designs deliberately use a "long throw" focus mechanism, which emphasizes precision over speed. That's needed because at higher magnification depth of field tends to be very shallow, so focus accuracy becomes more important.