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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). Optics: 10 elements in 9 groups. Traditional spherical design, multicoated. Traditional front-group focusing. 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. 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.
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. 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. 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. 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. This 35-70mm f/3.5 AI was Nikon's first professional midrange zoom from 1977. It is very sharp at every setting and has a constant f/3.5 aperture. It has less distortion than any of Nikon's f/2.8 zooms, and that means much better than the 24-70mm AF-S and 28-70mm AF-S, each of which costs over ten times as much.
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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. 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. 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. 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.
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. 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. I prefer distinct focal length differences. A 24mm is perspectively easily discernable from a 40mm and a 40mm easily discernable from an 85mm. 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. 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.
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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. 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 Proper fraction button and Improper fraction button work as pair. When you choose the one the other is switched off.
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