D800 Help: AF-C d9,21, 51 - would the focus point track a moving object?

Started May 28, 2012 | Discussions thread
prl105
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Re: D800 Help: AF-C d9,21, 51 - would the focus point track a moving object?
In reply to tedtea, May 29, 2012

tedtea wrote:

I read the manual but couldn't understand this -

How do I track a moving object, with the focus point following the target?

Steps:

  • Reset focus point to middle

  • AF-On on a fixed object ie clock on wall (i don't use Shutter half-press to focus)

  • With the AF-On still pressed down, i pan the camera

In AF-C mode, using 9, 21 or 51 points, the focus point doesn't track the the original focused spot when I panned.

However, using 3D mode, it does.

Could someone please help me -
1. What do the 9/21/51 points AF-C modes do?

2. Is the 3D mode the only way to be able to lock focus to an initial object, then pan to follow it?
ie I tried Auto but it wanted to pick its own initial focus point.

Thank!

Hi. I hope I can help with your question here.

First of all the AF-S and AF-C focus modes control the autofocus activation type; AF-S setting will focus once on your selected focus point, whereas AF-C will focus continually on your selected focus point.
Now regarding the number of focus points, in both AF-S and AF-C focus modes;

  • single point means only the visible focus point in the viewfinder is active and used to focus

  • 9 point means the 8 focus points surrounding the visible focus point are active and support the acquisition of focus

  • 21 point means the 20 focus point surrounding the visible focus point are active and support the acquisition of focus

  • 51 point means all focus points support the acquisition of focus

You should note that none of these will track the subject as it moves within the viewfinder frame.

If you wish to track the subject, then you have to turn on the AF-C/51 point 3D tracking, as this is the only mode that will track the subject (and I think you have found this to be the case already). A simple way to test this is by setting on your tripod and focus on a red car ( for example) on one side of the frame and watch the focus point move across with the car as it crosses the viewfinder frame. Remember to keep AF-On depressed..
I understand these work the same on both D800 and D300 bodies.

The D800 has an additional Auto option. This option invokes the new 91k sensors that also support face detection. So with this option the camera will focus on what it thinks is important, namely it's looking for faces, so you have no real control with this option.

If you want further reading on this, I suggest you read the recently posted D800 review on this web-site, section 15, where this is pretty well explained. Here,s an sample;

"With AF mode set to 'C' for 'continuous', you can select '3D tracking AF'. In this mode, the focussing system uses color information to track your chosen subject across the AF array."

Final point about the number of focus points 9/21/51 and when to select them. All the surrounding focus points are being used to help focus acquisition, so these are effectively active and therefore using more of the focus processing which in turn can slow the focus acquisition down. So single point is fastest and 51 point is slowest, however, if you are panning a fast moving subject and are having difficulty nailing the focus, you need some help and that is what these options provide. So for me, when I'm close to and panning with a subject, such as a wading bird, or duck for example, then I try to use 9 point, whereas if I'm at an air show and panning a fast moving fighter aircraft, I get more reliable results from 21 or even 51 point settings. In the main I leave mine set to 9 point.

To further understand how these 9/21/51 focus point work, on your D300' set to 9 point and then using the selector pad move the selected point across the viewfinder. While your doing this, look at the focus point representation in the LCD on top and you will see the block of 9 points moving across frame. This is the same for single and 21 point too, while with 51 point all of the focuses points are selected so you can only see the main focus point in the viewfinder.

Also buried in the menu's is an option to have the focus point wrap round when it get to the edge of the viewfinder frame, and this introduces an interesting situation with 9 and 21 points selected.

Anyway, sorry for my diatribe, it took me quite a while to work this out too, and I agree the camera body manual from Nikon is not so easy to understand, but hopefully with my description above, you may find they make more sense if you read them again.
--
PRL

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