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An undocumented AF feature to keep an eye on

edited December 2014 Posted in » Nikon D3200 Forum
I keep learning new things about my D3200, and while playing with button settings in the menu came across another which it seems is not well documented and understood.

As we are no doubt aware, the D3200 and others of its ilk are always in what is known as "focus priority" mode. That is, if you cannot lock onto AF, the shutter will not operate. In darkness or odd circumstances one must either focus elsewhere and recompose, or go into MF.

I was looking into the possibility of doing what is called "focus trapping", whereby one puts the camera in focus priority mode, with a single focus point containing no focusable item. Holding the shutter button down, the camera fires only when something comes into focus. So, for example, you could aim at a tunnel, and the camera would fire when the train comes out, or aim at a basket, and it would fire when the ball goes through it. Alas, the D3200 cannot do this or at least not well, and in finding this out I found out something else.

In the menu, under the "Buttons" heading, one can assign the AE-L/AF-L button in various ways. The last menu option here is "AF-ON". This is an odd choice of terms because what it does is to disable the usual AF operation by the shutter button. Now, AF occurs ONLY when you push the AEL/AFL button. What is not noted anywhere, as far as I can see, is that along with this, it switches the camera to "shutter priority (aka release priority)" mode. The shutter will fire whether or not focus has been found, and if you are not focused, your picture will be blurry.

I am guessing that the D5xxx menus are similar if not the same. So, if you need shutter priority, this is the only way to get it without disabling AF. If you are having problems with AF, this is one setting to check in case somehow it got changed.

Edit to add: It seems that the D5xxx does have a separate setting for focus versus release priority. I do not know, however, how the AF ON button influences it.

Edit to add: You can still almost focus trap with this setup. If you use the AF-ON function to preset a focal point, you can then wait until the action comes toward your preset focal point and make multiple shots. The camera will fire without refocusing, so if a subject is in your sweet spot you will catch it. The arrival of the subject will not trigger the shutter. You'll have to do that yourself. It's not much of a step beyond going manual.


  • edited December 2014
    Indeed undocumented and is also something I read about just a few months ago. It's actually a concept called back-button focusing.

    It's great when AF-A fails you or you want more control. Just set AF mode to AF-C and AE-L/AF-L button to AF-On. So when you press and release the button, it acts as AF-S. Press and hold, and it acts as AF-C.

    There are other benefits. I suggest researching it in articles and videos and see if you can incorporate it into the way you shoot. I've been trying to myself but have struggled to get used to it.
  • edited December 2014
    There is nothing particuarly new about back button focusing, which has certain advantages when you are shooting slowly or prefocusing. It's a more reliable way to focus and recompose. For example, though my D3200 will recompose most of the time with the shutter button, sometimes it will refocus when I don't expect it to. If you are fussy about decoupling all operations, AE hold, AF hold, and AV operation, this is one way to go about it.

    What is undocumented here, and represents a change in the way Nikon has implemented it in the past, is the change from focus priority to release priority. In most Nikon digitals, that choice is a separate option, but in the D3200 and a few others it is not offered at all. The only way to gain release priority given in the manual is to switch off AF. The back button change is not mentioned, and most reviews presume that the D3200 is entirely focus priority in operation.

    In the past, Nikon's implementation of back button focusing has resulted in a more or less accidental side effect, which users have utilized for the trick of focus trapping (aka trap focusing). In this setup, you must have back button focusing and uncompromised focus priority. When you are set for back button focusing, the shutter actuation is completely decoupled from focusing. Here is an example of focus trapping. It might work on a D5100, but never will on a D3200, and probably not on a D750 unless the firmware has changed:

    You set the camera to back button focus, and focus priority, single point focus and AFS. Now you aim the center point at the top of a fence post and focus on it. Now, letting go of the focus button, move the camera up so that the center point is aimed at the sky. Since nothing is now in focus in your center point, the shutter will not fire. If you hold down the shutter button the shutter will not fire, but if a bird alights on the fencepost, it will. You have "focus trapped" the bird.

    This trick only works with back button focusing. Although the D3200 has focus priority, it and all other cameras I know of consider any focus acquisition done with the shutter button to be all the focusing that shot needs. It will fire even if you recompose with nothing in focus. So if you focus on a fencepost and then move the camera, it will fire even if nothing is in focus, which is appropriate for normal shutter-button recomposing. The only way to focus trap with shutter button focusing would be to find an utterly unfocusable area to aim at, which is rare. True focus trapping requires only that the area be quite out of focus, not that it be impossible to focus.

    Recent Nikons have, I hear, changed this behavior, switching to release priority as the D3200 does for back button focusing (much to the chagrin of users who complained about the change that some, like the D 8xx, had a firmware update to restore the function at least partially). I believe Canon has always done so, and that one can only focus trap with third party firmware.
  • edited December 2014
    Very interesting.

    I don’t have my D5100 anymore, so I'm not sure about it. I do know that you can only change the release priority to shutter specifically for AF-C. So in AF-S, it’s always focus priority.

    The D750 allows the release priority to be changed for both AF-S and AF-C. I played around with a few more settings and it appears it can do “focus trapping”.
    AF-S release to focus priority
    Set AF mode to Single-point AF-S
    AE-L/AF-L button set to AF-On

    Now, I’ve only tested this by waving my hand in front of the camera. So I’m not sure how well this feature actually works in the real world. I wonder if it’s fast enough to catch something like a bird or a baseball zipping past the frame.
    I also wonder what kind of impact it has on the battery. Presumably, the camera is constantly working to see if something is in-focus at the AF point and it’s constantly metering the frame. So if I held the shutter button for several minutes, I wonder if that would wreak havoc on the battery life.
  • edited December 2014
    I think it's possible that the D750 received the update that others did, after Nikon briefly disabled trapping altogether. I think I read somewhere that it had been changed, but from your test it's pretty obvious that yours does it.

    Indeed, this is likely to be battery intensive, and not something you would do constantly. I think the usual idea is to set the shutter to multiple, so that once the trapped object comes into focus, you will get a sequence of shots. Presumably a bird perching will be caught if the noise of the camera doesn't scare it away. I don't think it's fast enough for a baseball, but it's apparently used for animals and race cars and the like. Clearly you'd better be carrying a spare battery.

    Until recently the idea had not even occurred to me, but the little kid in me was a bit disappointed to find it could not be done with mine.

    On the other hand, I was glad to see that the D3200 does have an undocumented way to combine AF with release priority, even it if's a bit of a kludge.
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