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In terms of video autofocus, the Sigma 28-70mm DG DN has the potential to be a really great option. Not only does it offer silent autofocus drive and very nicely-damped manual focus, but it also has well controlled focus breathing. There's only a bit at the 28mm wide-angle end and very little at all by the time you zoom in to 70mm. 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. The 35-70mm f/3.5 AI is a masterpiece of precision. You have to feel one to appreciate it. The focus flips with a fingertip from 3 feet (1m) to infinity. All the zoom action happens inside the barrel: the front and rear groups move around inside the barrel while the exterior doesn't move at all. 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. And since it is so lightweight, balance is very good. Regardless of the mount variant you choose, it shouldn't feel front-heavy on any body you might want to pair it with.
For the most part, bokeh is very pleasing, with only very slight onion ring and a nice, polygon-free shape even when stopped down to F4. 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.On the telephoto end the maximum magnification ratio is 0.22x. Close-up subjects shot at 70mm can appear soft and dreamy. 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. 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 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. Stopping down to F5.6, we see a slight improvement in sharpness at wide-angle and a bigger improvement at telephoto, since the lens is softer wide open at 70mm than it is at 28mm. You'll appreciate this improvement in sharpness more if you're shooting with a high-resolution camera. We tested with both the 42-megapixel Sony A7R III and 47-megapixel Panasonic S1R; with a 24-megapixel body, that difference would be much less noticeable.
Proper fraction button and Improper fraction button work as pair. When you choose the one the other is switched off. The Nikon 35-70mm f/3.5 AI zooms by moving the elements inside the barrel. Nothing moves externally except the zoom ring. Roll your mouse over to see what happens. I moved the focus a little between shots; pay attention not to the focus ring but to the glass which is moving up and down. As you might expect, the featherweight Sigma 28-70mm F2.8's body is predominantly constructed from polycarbonate, although it does still have a metal mount and build quality is good. And while it isn't fully weather-sealed like its nearest rivals, the mount still includes a seal that should help protect your camera body from the elements, if not the lens itself. It's aimed at photographers and videographers who want a bright walkaround zoom and the bokeh possibilities that a wide aperture brings but who don't want the size, weight and cost typical of many F2.8 zooms. Travel and landscape photographers in particular will find its modest size and weight appealing, and it also offers potential as a portrait lens or for video capture. It won't couple well to the cheaper digital ( D80 and below) and cheaper film cameras ( N80 and below). It works perfectly every professional film camera ( F, F2, F3, F4, F5, F6), with Matrix metering on the FA, F4 and F6.As a two-ring zoom it's best for use on a tripod. Hand-held, I constantly need to grab zoom, then focus, then zoom, then focus, etc. I can't do both at once as I can with a one-touch zoom. Compatibility: On the D2, D3, D200, D300 and F6, use the "Non-CPU Lens Data" menu option to input the zoom setting and f/3.5, which will give you full matrix metering and EXIF data, and finder read-out of set aperture. It works great in aperture-preferred as well as manual modes on these cameras. We didn't see any major issues with lateral chromatic aberration for this lens. There's a truly minute amount of it, perhaps 2-3 pixels wide on a 42MP image (that's a half a millimeter on a 40" x 60" print) that clears up easily if you enable CA corrections in-camera or in your Raw converter. Results at 70mm follow a similar pattern, but with softer results overall, particularly at close focus distances. Wide open, portraits can often look a little dreamy. But if what you need most of all is portability and you understand the compromises necessary to achieve it, the Sigma 28-70mm F2.8 DG DN is hands-down the smallest and lightest of the bunch and still offers solid image quality.
Distortion isn't an issue for the L-mount version thanks to automatic correction, but the Sony E-mount variant shows some barrel distortion at wide-angle and quite prominent pincushion at telephoto.On the other hand, it's quite prone to cat's eye effect when shooting wide-open, giving the bokeh more of a football (or for non-Americans, rugby ball) shape the closer it gets to the corners. And that problem is not limited just to those corners but extends quite a long way towards the center of the frame. 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. On a Nikon D3, it's perfectly sharp in the center at f/3.5 at 35 and 50mm. At 70m, stop down to f/5.6 and it's perfectly sharp. It's only a little softer at f/3.5. 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. This lens is a joy to use. It just works, never gets in the way, and yields great results. Just be careful not to let the sun shine into it for fear of veiling flare.
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