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On a heavily used Nikon D750 w/ AF-S NIKKOR 24-70mm f/2.8G ED:

When using live vision, ussualy while holding the camera up over my head, autofocus focuses nowhere. This happens right after having shooted a correctly focused photograph using viewfinder, with slight subject distance variation.

When doing so, I hardly ever want a shallow DoF, as I'm focusing on groups of people, so F-number is always between F5.6 and F16. Focusing mode is AF-S-wide at center point, and I don't use shutter button focusing, I use only dedicated AF-ON button focusing. So it should just focus at center of image as I press AF-ON button, and keep this focus distance until I shoot.

On this example, it seems to be getting stuck at its shorter focusing distance:

(f/7.1, 1/80s, ISO-4000, I couldn't look at the screen while focusing, but it should undoubtly have focused some of the people in the central area, or at least the wall behind them) enter image description here

On this subtle example, it fails to focus the central situation, setting focus on closer, peripheral subjects (as if I was using multiple point AF selection, what I am NOT doing):

(f/7.1, 1/125s, ISO-5000, I pressed AF-ON button while pointing to people speaking at the table. Zooming in image shows that the focus is set on the heads of the people close to the camera, speaking persons are completely out of focus.) enter image description here

Is this caused by a fail, A D750 common issue, or a mispractice?

  • I don't have a D750 to be certain of this, only a D5500, but Live View tends to make up its own mind what to focus on; it ignores your centre setting, which only works in TTL mode. Check it sometime when you can see the screen. – Tetsujin Aug 13 at 16:54
  • I've done it, focusing on Live view to well contrasted subjects. Sometimes it works just fine, but some others it can't reach focus, for no apparent reason. – Lisan Aug 13 at 16:56
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    1/80 second while holding the camera over your head. Are you sure this is primarily an AF problem instead of a camera movement problem in this shot? – Michael C Aug 13 at 21:28
  • @MichaelC the image posted is clearly out of focus, there's no visible motion blur on it. – Lisan Aug 13 at 23:35
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You are using the contrast based AF system and it is quite different from the phase based AF system used otherwise.

It appears you simply do not understand it or have it set up appropriately. There is no AF-C when in live View. There is AF-F with a few options. And when AF-F is used autofocus is continuous until you press the shutter button or AF-on button... which locks the focus. It sounds to me like you are actually locking the focus when you think you are initiating autofocus.

https://imaging.nikon.com/support/digitutor/d750/functions/focusmode_liveview.html

  • Thanks. It's not the case, but this info helps to improve the question. I was using AF-S mode on Live View (I made a mistake when writing the question, as I'm used to Canon naming of AF modes). The AF-Area mode was Wide-area AF and the area selected was the center spot. On examples shown, I have pointed the AF-Area at the desired focused subject, pressed AF-on button, released it, recomposed and then shooted. – Lisan Aug 20 at 19:23
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    Then my next guess is the ambient light levels... I'm seeing an EV level of about 9 from the settings, which is about typical indoor lighting. But it's not strong lighting that will deliver a lot of contrast. And you're pointing it at areas that seem to be more uniform in color/tonality which also means lower contrast. Both make it hard for the contrast detection AF system. I just tried using it in an average lit room in my house and I couldn't get it to be reliable/consistent at all (D810, 2 different lenses). If that's the situation then I would just use regular PDAF focus/recompose. – Steven Kersting Aug 20 at 21:17
  • That seems to be the issue. I've had no chance yet to test CDAF on controlled different light conditions, but at least I can say I found it to have a much lower error rate under better lighting. – Lisan Aug 23 at 15:00

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