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I photograph with a Nikon D7100 with a Tamron 18-400 mm lens. My camera is set to autofocus but when I try to photograph a bird sitting on a branch that is BEHIND near branches, the camera vacillates (snapping back and forth) and will not adjust and focus on the distant bird. Why is this happening?

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The camera does not know what you want it to focus on.

Most pictures are easy to focus correctly. Everything is roughly at the same distance. Or, for portraits, there is a big face in the foreground. Here, there is no clear main object, and the camera has to guess.

You can help the camera by using different autofocus modes (e.g. single point), and by selecting a focus point.

The camera uses one or more focus points to measure focus. Note that the area indicated in the display is only an approximation of the area covered by the sensor.

The branches might move due to wind. The camera is also moving slightly in your hand. So, branches are moving in and out of the sensor area(s), and the camera tries to adjust accordingly.

Workarounds:

  • Use manual focus.
  • Switch to manual focus when the bird is in focus.
  • Usually, there is a button to hold the focus. Or:
  • Assign autofocus to a button, so you can tell the camera when to focus and when to stop.
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  • \$\begingroup\$ Thank you for your quick response and help; I will try the single focus point mode and see if that helps. \$\endgroup\$
    – Hol
    Commented Dec 20, 2022 at 11:48
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This might be awkward on that Tamron, because you cannot just grab the focus wheel & manually override on the fly. You must physically switch it to manual before you can change it manually, or you risk breaking the mechanism.

The camera isn't aware that you want a picture of a bird rather than a twig, so without giving it a hint first it will either be incapable of making any decision at all, or come to the 'wrong' conclusion by focussing on the nearest.

On a similar super-zoom I have here [the Nikon 18-300mm] I find if there's a potentially confusing focus between two objects at dissimilar distances, if I 'help it out' by manually focussing towards the desired object, it will then persuade the camera to make up its mind. I can do this in either direction, focus towards the near or far object & it will pick that one when I then let it auto-focus.
To do this on the Tamron, of course, you will have to switch it out of auto-focus, then back again; which might be a lot of extra effort in the long run.

You may have more luck if you can use a single-point autofocus point & find an 'edge' of the bird as your focus point, or a hard delineation in the plumage. I'm aware that is not necessarily a simple task.

One additional consideration is that because of the lens design you are actually quite near the absolute lower limit on aperture required for auto-focus to work at all. Add to that the Tamron's notoriously slow auto-focus mechanism even in ideal conditions, and that may be why it's so easy to confuse. I have a Tamron 70-300mm which is also easy to confuse [which is why I ended up spending all the money on the Nikkor instead.]

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    \$\begingroup\$ Thank you, I agree, the Tamron is slow, and I have tried your idea of picking a far object, like the edge of the wing and waiting for it to focus, but it still flickers; not knowing what it should focus on and of course, but that time, the bird has gone. But thank you, I will try your suggestions. \$\endgroup\$
    – Hol
    Commented Dec 20, 2022 at 11:54

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