What is the focusing mechanism on a Sigma 18-35mm F1.8?

Question

I'm considering buying a Sigma 18-35mm, f/1.8 DC HSM ART but I am not sure about the focusing:

1. Is it a "direct manual" focus, or does the focus ring just control a servo?

2. Does it have hard-stops?

3. How big is the focus throw? (Degrees between close up and wide focus)

Answer 1

1. I can find no reference anywhere that the lens is manual focus-by-wire. When the lens was introduced in 2013, there were relatively few such lenses on the market, and virtually none offered by third party lensmakers. Bryan Carnathan does not mention it in his review of the lens at The-Digital-Picture, and he almost certainly would have if that was the case.
2. The Canon and Sony A-mount version have electronically controlled apertures. The Nikon version has the clunky mechanical linkage left over from the late 1950s that often leads to more variability between still frames taken at the same aperture setting. In all three cases, there is no aperture control ring on the lens. The aperture is controlled by the camera. For all practical purposes, the aperture can only be controlled in 1/3 stop increments. In theory, if you have a camera in the Nikon F-mount that can control the aperture in stepless increments, the lens could be set to any aperture value between the minimum and maximum apertures. In practice, I think the only way to do that with some Nikon F-mount cameras² is to use an auto exposure mode that sets the stepless aperture based on metering so you wouldn't be able to control the precise setting manually.
3. According to this review, the focus ring moves just a bit over ninety degrees from close focus (at 0.28 meters/11 inches) to just beyond infinity focus.¹ As with most shorter focal length lenses, most of that movement is devoted to the near focus distances of less than 2 meters/6 feet. DPReview says the focus throw is 110°.

From your questions, it sounds like you might be interested in this lens for shooting video. This is not a "cinema" lens and does not offer the features many cinema lenses are expected to have. It is a lens designed to be used on still camera bodies with the aperture controlled by the camera, not by a stepless aperture control ring. The manual focus ring, while mechanically connected to the focus elements of the lens, has a short throw that is devoted mostly to very close subject distances.

_{¹ Most zoom lenses, and even many prime lenses with AF and LD glass, do not have a hard focus stop at infinity. The primary reasons are a) To allow for thermal expansion/contraction of the LD glass, which is more sensitive to temperature variations than more typical optical glass, that will alter the exact position of infinity focus and b) to prevent the AF motor from bumping against a hard stop when attempting to focus the lens at or near infinity.
² In the late film era, most Nikon 35mm cameras with automatic exposure modes calculated exposure steplessly. If the camera was set to 'A' mode, the shutter speed the camera actually used was stepless to give the exposure indicated by the reading of the light meter, but the viewfinder displayed the shutter speed to the nearest whole stop. There are some who contend that Nikon digital cameras continue this practice, while others contend that they do not. There are even some who have done test that seem to indicate some Nikon digital bodies are stepless in automatic exposure modes and other Nikon digital bodies are not. Apparently Nikon has never definitively stated which is correct with regard to digital cameras. The last time they officially mentioned it was with regard to film bodies before digital bodies were available to most normal consumers.}