Focus problems with D750

[J.davis]

While chasing birds/aircraft in flight, I find that focus is quick and accurate when there is a plain background, as soon as the focused item dips below the skyline,
focus is lost and usually not acquired again until the background is clear again.
Still objects are not a problem or objects that are close enough to fill the frame.
Is this the same problem as Mongo is having?

[MissionMan]

Are you using 3D tracking?

[J.davis]

Spot metering, single point focus and manual with auto ISO.
Someone has mentioned using 21 point focus (will give that a try tomorrow).

[MissionMan]

Try 9 point. It's supposed to be the most accurate of the tracking ones.

I normally use highlight metering as well but not sure how much of an impact that has.

[MattNQ]

On my d700 when shooting indoor netball (moving subjects against busy moving backgrounds) I usually had more success with 9 pt focus.

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[arthurking83]

The issue may be a bit confusing here.

I think the issue is focus itself, as opposed to focus tracking if I'm reading the OP right.

That is, you easily get focus (say) against a plain background, such as a sky .. but once the object is in amongst some distracting backgrounds you easily lose focus and the camera/lens hunts for focus and sometimes acquires the background instead?

The two focus issues can be:

1/ maintaining focus(as described above).

2/ focus tracking.

1. can be a problem whether you are using one of the tracking features or not.
2. is an issue only in maintaining tracking, not focusing itself.

JD said that he's using single point focus, which implies that he's not using the tracking feature. So the number of points used for focus will have no bearing on the issue(he's only chosen one point).

If the issue is that you are using the 3D tracking feature, and the tracking focus point is skipping about wildly, then the number of focus points chosen in the AF setup system can have a bearing on how accurately the AF system maintains it's tracking ability. This is related to #2.
In this instance if you choose all 51points for maintaining tracking, this can confuse the AF system is the subject and background are not too dissimilar.
If the subject and background are very different(ie. the plane/bird against a plain background), then the tracking feature has an easier time of differentiating the two, so using more focus points doesn't confuse the system easily and tracking is more stable.

One of the setup systems I'd recommend for the OP to adjust would be the focus tracking lock.
(but I'm assuming that by the description you've written in your OP, you're using AF-C or AF-A mode, not AF-S .. that is, not single shot focus mode).

Look for CSM menu item a3(it may be different for the D750, but most modern Nikon's use a similar CSM menu order). It's called Focus Tracking with Lock On
What this feature does is to delay(or not) the time in which the AF system waits to change the focus distance.
It may be described in a way where it's a setting for when something crosses the path between you and your subject, but it works the same for your situation too.
By default it's set to 3(medium).
Smaller numbers mean that the delay between shifting focus is shorter, and larger numbers means that the delay is longer.

So .. if you change to Off, 1 or 2, the af system will shift focus quicker for those times when you get to the focusing issue situation again. Not what you want.
If you change to 4 or 5, then the delay between the focus systems wanting to change the focus point is longer. This gives you more of a chance to maintain the focus point.
This could help a bit, but the downside is that it makes acquiring focus a slower overall proposition.

The other aspect you could look into is on the lens itself too.
The Sigma has the USB docking port, so if the issue is predominantly with the Sigma(I'm assuming again, as you said planes and birds which implies a long lens).
While I don't have any of the dockable Sigma lenses, check to see if there is a setting in the software to adjust the speed at which your Sigma lens delays focus changes.
It's basically a similar ideology to that of the cameras focus lock adjustment, but the beauty here is that this only affects the one lens.
ie. if you use the camera setting, all of your lenses will feel slower to acquire focus, or shift focus .. with the Sigma USB dock setting, only the one lens is altered to suit.

[J.davis]

Thanks all, 9 points done the trick, Back to where I was a few months back, and happy.