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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. I prefer distinct focal length differences. A 24mm is perspectively easily discernable from a 40mm and a 40mm easily discernable from an 85mm. 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. 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. 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.
Proper fraction button is used to change a number of the form of 9/5 to the form of 1 4/5. A proper fraction is a fraction where the numerator (top number) is less than the denominator (bottom number).There's both good news and bad news on the bokeh front. The Sigma 28-70mm F2.8 DG DN's bokeh has a really pleasing, smooth look to it, with only minimal onion ring effect and smoothly-rounded, step-free edges even when stopped down to F4. 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.
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. 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. 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. Sure, it has some caveats in the image quality department. Most notably, corner sharpness isn't stellar at telephoto while wide-open; close-up telephoto shots wide open can have a soft dreamy look , and it's also quite prone to cat's eye bokeh effect. But honestly, depending on your subjects, those may not be major concerns for you. And image quality is otherwise solid, with very good sharpness across much of the focal range, pleasing bokeh, and good resistance to aberrations and ghosting.
Please note:
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. The 35-70mm f/3.5 AI is a great lens for film and FX Digital. It's silly for DX digital because there are far more modern DX lenses that do a much more convenient job. 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. 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 There's just a little bit of longitudinal chromatic aberration, visible as color fringing, around high contrast edges of slightly out-of-focus regions, but as you can see in the sample below, it's not really anything to concern yourself over. It disappears immediately upon stopping down.
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. 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'. Proper fraction button and Improper fraction button work as pair. When you choose the one the other is switched off. 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. 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.
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. 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. 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. 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. 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.
When it comes to distortion, we need to discuss the Leica L-mount and Sony E-mount versions of the lens separately. That's because if you're an L-mount shooter, distortion is corrected automatically in both JPEG and Raw files, but if you're shooting Raw on the E mount variant on a Sony body and using Adobe software, there's (currently) no correction applied for distortion. Distortion isn't an issue for the L-mount version thanks to automatic correction, but Sony E-mount variants show some barrel distortion at wide-angle and prominent pincushion at telephoto. So if I have to augment the 28-70 with two additional lenses, I might as well save weight/space and gain capability with a single prime. 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. 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. 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. Cat's eye effect is quite noticeable when shooting wide-open and can appear quite a long way towards the center of the image frame.
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