So when does a zoom make sense? When it covers more than one easily discernable FL. But 28-70 doesn't do that. It has a single useful perspective for me. A "normal" perspective. I would need to augment it on both sides with a lens that is wider and a lens that is more tele.

Proper fraction button and Improper fraction button work as pair. When you choose the one the other is switched off. Sunstars are relatively pleasing, with 18 rays thanks to the nine aperture blades. They're not as tight as they could be – each ray splits and diverges into two rays – and as expected stars on the wide end look better than those on the telephoto end where they can appear a bit 'messy'. A little bit of longitudinal chromatic aberration can be seen as magenta and cyan color fringing around high contrast edges in the image above. It's subtle enough to be a non-issue for the most part, and goes away as you stop down the lens. Life is all about compromises. With a bright, continuous aperture and minimal size and weight being the key elements of its design, it's not surprising at all that the Sigma 28-70mm F2.8 DG DN's image quality can't quite compete with larger, more expensive alternatives like Sigma's own 24-70mm F2.8 DG DN Art. If you can take your time, the newer 35-70mm f/3.5 AI-s lens is even better optically, although I find this AI version easier to focus.

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Vignetting is not a concern for the Sigma 28-70mm F2.8. It's only really noticeable at telephoto, and even there is minor and easily corrected. Bokeh Sharpness drops off a little at the corners on the wide end at F2.8, but stopping down to F5.6 gives a relatively flat field of focus and improved corner sharpness. That said, focusing in the corner yields higher corner sharpness than focusing in the center and stopping down, indicating a curved field of focus. Focusing in the corner and stopping down yields even better corner sharpness, as expected. The not-so-flat field of focus, at least in part, contributes to the peripheral softness when focusing centrally. Up front, you'll find 67mm filter threads. That's the same size as used by its Tamron rival, while the Sigma 24-70mm F2.8 opts instead for a larger 82mm filter thread.

Production History: The 35-70mm f/3.5 AI was introduced in 1977 and built until it was replaced by the 35-70mm f/3.5 AI-s in 1981. Nikon made about 50,000 of these AI versions. It holds focus while zoomed. Of course if you focus at 35mm you don't have enough precision to ensure focus at 70mm. Focus at 70mm and the focus holds as you zoom wider. If you prioritize outright image quality and durability over size, weight and cost, we'd recommend the fully weather-sealed Sigma 24-70mm F2.8 DG DN Art. And for E-mount shooters who are more size, weight and cost-conscious but who need to shoot regardless of the elements, the Tamron 28–75mm F/2.8 Di III RXD also offers a compelling alternative if you can live with its more distracting bokeh. It doesn't hurt that it's also among the most affordable F2.8 full-frame standard zoom options for the E- or L-mounts. For the size, weight and cost-conscious, it's definitely worthy of consideration. Focal Length: 35-70mm. Used on a DX camera it gives angles of view similar to what a 50-105mm lens would give on an FX or 35mm film camera. See also Crop Factor.Shooting wide-open at F2.8 (which you'll quite likely want to spend much of your time doing if you've bought this lens for its bright maximum aperture), sharpness is very acceptable in the center of the frame at 28mm and remains pretty good even once you zoom in to the 70mm telephoto. NIkon made a 43-86mm f/3.5 zoom before this, but since it is neither a normal zoom (43-86mm is normal to tele), and since it was never pitched as a professional zoom, I'm not counting it as Nikon's first pro normal zoom. The 43-86mm was a convenience zoom, not a zoom with which to earn one's living.