Back button focusing and focus points

I use back button focusing on my D7200 because it takes so much hassle out of composing a shot and also allows me to seamlessly shift from single point static focusing to continuous tracking of a moving subject. In a static scene, I pick the point I want to focus on, centre this in the viewfinder (which shows the single central focus point) press the back button to achieve focus lock, then recompose. If the subject is moving, I centre it in the viewfinder (which still shows a single central focus point), but this time hold the back button down as I track the subject.

Both these techniques work very well. However, what I'm not clear about is what setting I should use for the number of active focus points. The D7200 has a maximum of 51 points, so I normally have all of these active, on the assumption that this will allow the camera to keep focus, even if the subject moves significantly away from the centre of the viewfinder.

So the question is this: Am I right in making that assumption, or are there times when I should select a smaller group of focus points and if so, why? A secondary issue is why do I only ever see the single central focus point in the viewfinder, even when holding the back button down, which means I'm in AFC mode?

I hope I've explained that clearly enough, but let me know if more info is needed.

Hi Gewitty, Agree that back-button focus is the way to go. Just to be sure, are you talking about which AF-Area Mode to use (the mode you choose by pressing the AF button and rotating the front dial, with options like single-point, Dyn [51], Dyn [21], Dyn [11], etc.), or about Custom menu option a7, which lets you choose between 51 and 11 points?

I'm talking about the a7 options, which allow selection of either 11 or 51 focus points. Not sure why I would ever use 11 rather than 51. Although on reflection, perhaps the AF button options you mention (single-point, Dyn [51], Dyn [21], Dyn [11], etc.) are relevant to the issue.

I suppose the question should really be: When using back button focusing, what settings should be used to best enable a fast switch from single point static image shooting to continuous focus tracking of a moving subject, i.e. Using single press back button to lock focus on a static subject, then hold down the back button to enable tracking of a moving subject.

The purpose of this setting option is a simplification for people who want to be able to move the AF point quickly across the screen. When you limit the option to 11 AF points, those still spread across your camera's whole AF area, but it takes fewer presses on the multi-selector to move the AF point from, say, left to right. This mode makes sense occasionally, for instance if you're waiting for animals to show up, aren't sure where in the frame they will be, but want to take the shot as soon as possible. For most shooting situations, however, this mode offers no value. I almost never use it.

As far as your second question goes: no matter which AF area mode you use, the viewfinder on the D7200 only highlights a single AF point when you focus, namely the focus point you selected by pressing the multi-selector. Even when the AF area mode is a dynamic area one, the camera still only highlights a single point. This is the case for all of Nikon's entry and mid-level bodies. It is different on the pro bodies; for instance, my D500 shows me multiple focus points in this case, with the central one enlarged.

Hope this helps.

Which lens are you using as the larger spread of focus points may not help if its a f6.3 lens,you may be better to stay with the ones close to the center.

mikew said:

Which lens are you using as the larger spread of focus points may not help if its a f6.3 lens,you may be better to stay with the ones close to the center.

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Would like to understand this one. What does max. aperture have to do with the choice of focus point? Sure, focusing in the corners requires good glass if you want good results, but good glass and wide aperture are still different animals.

Its my understanding you need f5.6 or better to make best use of the outer focus points.you have one point that supports f8 (most likley center) then 14 or 15 cross type ( info is a bit sketchy here but i think they need f6.3 or better) so my guess would be to get good quick consistant AF on the outer points you would need f5.6 or better.

mikew said:

Which lens are you using as the larger spread of focus points may not help if its a f6.3 lens,you may be better to stay with the ones close to the center.

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I use three lenses: Sigma 10-20 f3.5, Sigma 18-200 f3.5-6.3, and Sigma 100-400 f5-6.3

You could be ok on the outer points but be aware they may not like f6.3 so not always be as quick.

mikew said:

You could be ok on the outer points but be aware they may not like f6.3 so not always be as quick.

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Thanks for the tip Mike. The Sigma 100-400 is newly acquired, so I'll see how it performs (quite impressed with the OS so far).

Gewitty said:

Thanks for the tip Mike. The Sigma 100-400 is newly acquired, so I'll see how it performs (quite impressed with the OS so far).

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I have got focus lock on the outer points with the Sigma 100-400 on Swallows in flight but was with the D500.

I use back button focus and almost always use single point. On occasion, I will use d9 or higher. For a birds/animals in the open, d9 helps to keep on it. For a bird in the tree, d9 often gets confused.

As far as only seeing the main focus point in the viewfinder when using dynamic modes, if you turn the playback option on, you can see the focus point that was actually used when you review the images. Maybe you know, but worth a mention.

Menu A3 is something to be aware of too. Its basically a delay before autofocus corrects. If its off, you will get quicker response to changes. Especially if you are holding the back button down and looking for your subject. If its on, you will have a slight delay before focus updates. That delay can be helpful on small subjects if you catch focus and momentarily fall off target. Focus won't instantly jump to something behind your subject. No 'best way' for menu a3, just something to be aware of. Surprisingly, if you push the ? button on the d7200 while in menu A3, you get a fairly long explanation.

When using a slow lens, one that has a maximum aperture that is small, such as the f/6.3, AF performance will be degraded when using any but the center focus point. So tracking will be unreliable further from the center because that center AF point is the most sensitive one.
Using slow lenses requires tracking moving object by panning to keep the center point on the subject. Sometimes, in good light it will work using outer focus points but it will not be consistent.  That is one of the benefits of fast lenses, with wide maximum apertures, because even when stopped done to, say f/8, the metering and focusing it done before the aperture is closed down for the exposure. That f/2.8 fast zoom meters and focuses at that wide open f/2.8 and the instant you click the shutter, the aperture closes down to the exposure aperture.

The 51 point and 11 point tracking have different processing times so the 51 point should be used when slower moving subjects are needed to be tracked over a wider portion of the AF sensor. The 11 point tracking is best for fast moving erratic movement because fewer focal points are being evaluated so the AF processor can update faster. For sports where you pan with the movement, the 11 point will be better.
The tracking does not show the FP on the monitor because the  starting point is the only one the  camera knows about, until the subject moves out from under the selected point. If it showed you every transition point, you could not pan or track with FPs lighting up randomly, quickly on the VF. The last thing you need with following action is too much data to distract you. It does not help you to know the FP shifted 3 times in 1/10 of a second between press of the AF-On button and shutter release.

One more consideration when using the focus and recompose technique, be aware that with shallow depth of field...wide open fast lenses....Field Curvature will almost assure out of focus images if recomposing away from the point used for initial focus lock. The arc of infocus distance stays pretty much constant so by moving the angle off the center axis to the subject, you are swings that arc of constant plane radius so suddenly your subject is farther away than the focal distance locked. When doing center focus and recompose consider using a deeper depth of field by stopping down a bit so the subject stays within that depth of field. Another factor in recomposing is lens distortion is greatest at portions of the lens further from the center access. A lot of design work when into reducing that and why so many lens elements of exotic design/material is needed, but there is still some on most lenses and that distortion we see as distorted aspect ratios of length and height but it also impact distance of the focal plane, another good reason to provide a bit more depth of field when using lock and recompose technique.