No.6 x 2.75 (3.5mm x 70mm) TX Countersunk Self-Tapping Screw - Stainless Steel (A2) (Pack of 20)

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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. 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. 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. 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. 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.

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.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.

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 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. 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.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.