Reposted here my answer to a private message about my use of a long SLR lens, in case it is useful to others.
For extended range (plates at 200+ feet) I am using a fully manual old-style Nikon SLR lens, no auto iris, no auto-focus and that is what I recommend. Also needs F-mount to C-mount adaptor. For example
NIKON NIKKOR-H 50MM F2.0 LENS (NON AI NIPPON KOGAKU) | eBay plus
Camera Adapter For Nikon F Mount Lens To 16mm C Mount Film Movie Adapter | eBay
There is no way to activate a focus motor in a SLR lens without the SLR camera body, even if it was an autofocus model. Fortunately, you don't have to! That's the good news. The Dahua IPC-HF5421E camera I have has its own focusing motor that instead moves its image sensor back and forth by remote command (script), so you can remotely adjust focus the camera even with a fixed manual lens that never moves. I've never tried to do this from Windows, I only use Linux for this stuff. I do not attempt autofocus, I just have my script switch at sunrise & sunset between two fixed settings found by trial & error (one for day, another for night with 850nm
IR illuminator).
Yes, for less money you can get one of those 6-60 mm CS mount lenses. I did myself, and I also tried one of the few available 100mm CS lenses, before I ended up with an SLR lens. They do work but mine had glare / flare on bright spots, not really sharp/crisp. Maybe I'm a perfectionist but I was never happy with it. If you care about image quality the Nikon SLR lens is *much* better, although it is big and heavy and never designed to mount on an IP camera the way I did. I wonder a little about the mechanical stress on the lens mount, but my 100mm lens + long hood has been OK so far.