Focusing Issue

My Sigma 135 f/1.8 is a nice size lens which I use for photographing the cats, butterflies and flowers etc. It sometimes just will not focus however and it’s causing me to miss shots.  Most times it’s fine but occasionally it just refuses to focus. If I move the focus point to something a good distance from the chosen subject it will focus. If i manually defocus it tends to reaquire the focus. 

I use a Nikon D850 with back button focusing only. AF-C with D-9 typically. I don’t have this problem with my Sigma 150-600 or my 300PF. 

Should I reset the calibration to default and try that? Any suggestions?

A super large aperture telephoto lens, if the image is too far out of focus, the Phase Detect system has no way of knowing which way to go and just won't initiate focus. That is fairly common and a limitation of the focus system. That can happen even with native lenses. In some situations if you switch from AF-S to AF-C, the camera will try harder to focus. If that isn't it, it is possible the lens needs a firmware update to work perfectly with a D850 although the Sigma site does not indicate this. But then it didn't indicate that My D500 and the 150 Macro would have an issue either but it did and a FW update fixed it. If you have the Sigma dock, you can do it yourself. If not, the Sigma dock is pretty inexpensive or alternately you could send them the lens to update but I'd recommend checking FW before you do anything else.

Thanks E.J.

I never thought it might be firmware but yes it could be. I have the dock so I’ll give that a go first. 

I have this problem with my Nikon 200-400. If I focus say at 20 feet then try to focus on something distant, it won't do it. Too much of a jump I guess. My 400 f/2.8 will do this though perhaps because of the larger aperture.

As I said, an image can be so far out of focus that the Phase Detect system doesn't know which way to go and so it doesn't move. A slight bump to the focus using the manual focus ring will usually resume function. Some cameras have a FW setting that allows you to sepcify that the camera will just run the lens focus back and forth until there is enough data to focus. One setting that may help is to turn off Focus Tracking with lock on (a3 on a D500).

E.J. Peiker wrote:As I said, an image can be so far out of focus that the Phase Detect system doesn't know which way to go and so it doesn't move.  A slight bump to the focus using the manual focus ring will usually resume function.  Some cameras have a FW setting that allows you to sepcify that the camera will just run the lens focus back and forth until there is enough data to focus.  One setting that may help is to turn off Focus Tracking with lock on (a3 on a D500).

I've been doing the manual bump and that's fine. At first I thought it might be a lens problem but notice a loaner 200-400 did the same thing so figured it must be the nature of the beast. Turning off the Focus Tracking with Lock On doesn't help, at least not with my D4.