Continuous focus issues in live view mode. (Nikon d5100)

[Seventen]

In live view mode doing video the continuous focus has issues sometimes it works fine sometimes it only works when live view mode is enabled and sometimes it dont work at all.

Anyone have any ideas what I may have done wrong?

Just quite annoying as need to edit the videos to cut out the focus searching each time i auto focus, i do use manual in times it dont work but sometime only using one hand so the continuous is quite important

Thanks

 

[KmH]

You not only have to set the Focus Mode (AF-F), you also have to set a Focus Area Mode.
What Focus Area Mode are you using?

You might want to review pages 99-102 of your D5100 Reference Manual.

As noted in the specifications for the D5100, when using live view for video the D5100 uses contrast-detection AF instead of the phase-detection AF it uses to make still photos when not using live view.

 

[Seventen]

I am in the AF-F mode for it, have tried all the focus area modes when I get the problem, normal area, large area, face priority and the subject tracking.

Its quite random how it works sometimes but dont other times, but if i switch the live view of it works fine, it happens with all my lenses but like i say it is random and can go a whole day with no problems and then the next week it wont work.

 

[Seventen]

Last night I was filming and all was perfect put camera down made coffee returned turned camera back on, no settings had changed but wouldnt focus without me pressing the shutter button half way each time i needed to refocus or had to use manual, seems quite weird and so confused.

 

[Seventen]

Starting to be quite silly I took it to shop where its still under warranty and they said nothing wrong with it it worked fine, (Typical really) they said could have it sent back to Nikon for checking over but will take some time, I couldnt take this offer at moment as skies are clearing so got so astro photography to concentrate on.
But again tonight worked fine then used it just now and nothing. Could it be some connection issue?

 

[KmH]

There is a minimum amount of light that is required for AF to work.

 

[Seventen]

These are very well lit situations day time out in sun and inside in well lit rooms, shutter speed in P in the house is selecting over 300th of a second and sometimes it stops starts stops without the light or location changing

 

[KmH]

What may look well lit to your eye inside may be quite dark to the camera.
Outside, a small lens aperture can make even bright daylight seem dim to the camera when it's used in contrast-detection AF mode for shooting video.

Making it even darker to the camera, when using live view in video mode the lens has to be stopped down to the lens aperture setting.
In still shooting the the phase-detection AF is done with the lens at it's widest aperture, then just before the shutter curtains are opened for the exposure the lens is stopped down..