Monday, October 14, 2013

Nikon Autofocus Guide: D600, D610, D7100 and D7000 (Phase Detection)

Give a man a hammer and the whole world looks like a nail. Give him 20 hammers and he won't know where to begin. The human brain isn't naturally good at making quick decisions when given multiple options; it performs much better when the options are limited and. This is why it's so hard to decide on where to go to lunch when you have a large group of people and you ask everybody for a choice of venue. On paper, the Nikon autofocus system can look intimidating to a new user because of the high degree of variability. It does not help that the manner in which the manual describes the AF function is technically correct ("the best kind of correct") but does not give you a sense of how it actually works.

Adjustability is a large strength of camera equipment as you move up the price range. Yes, the image quality improves, but more importantly, the adaptability to any situation improves as well. It's the latter that is more important than the former, as there is no such thing as a good image of a missed opportunity. Fortunately, if you break down the Nikon AF system into principles rather than features, its usability no longer becomes intimidating. Even though there are a multitude of settings, most situations can be reduced to a bi-polar either/or situation, making the decision making easy.

The following is not a comprehensive description of all of the autofocus settings, but rather a guide to understanding the behaviour of the phase detection AF system in action. For the purposes of discussion, the D7100 will be lumped in with the D7000 and D600/D610. Even though the D7100 uses a different sensor unit, it's real-world operation is more or less the same as the other cameras in this group. There are 51-points instead of 39, 11 central cross-type sensors instead of 9, but for the most part, it's the the same menu set-up and operation.

Note: while diagrams like the ones below show you the entirety of the AF array, there is no way to display the entire grid. Depending on your mode, the viewfinder will only show you your active AF point(s). However, the camera can be adjusted to highlight the active AF point red when focus is achieved (menu item a4).


Nikon D7000, Multi-CAM 4800DX 
Nikon D600 and D610 Multi-CAM 4800
Nikon D7100, Advanced Multi-CAM 3500DX

Compared to the other two cameras, the D7100 AF array covers the largest portion of the viewfinder, making it the easiest for composition. The D610 AF array is the same size as the D7000 unit, but because the D600 is full frame, the AF array occupies a smaller portion of the viewfinder. This makes focusing on off-center subjects a bit trickier, but it also means that the D610 can be more precise with center field focusing, and especially for tracking moving subjects with the center portion of the viewfinder.

The duality of operation extends pretty much through the whole design of the AF control. At the heart of it, you have the AF mode selector on the left side of the camera. The most obvious first step is to choose between manual (M) or autofocus (AF). After that, the system falls into the classic Nikon "press and twirl" operation. Hold down the control button and twirl either the front or rear command dials.

Depth vs Width: The Basic Setup of the Controls


The front and rear command dials are divided between two basic divisions: the front dial controls how the AF system deals with focus in terms of width relative to the viewfinder, the rear dial likewise deals with how the system controls depth. So there are two basic decisions that you have to make.

  1. How is the subject moving in terms of depth (distance) relative to me
  2. How much width (precision) do I need to account for?

Back Control Dial (Depth)

Once we move to the front control dial, there is another dichotomy to deal with. In terms of depth within the field of view, is the subject static or moving? The back control dial adjusts the primary behaviour of the AF system, toggling between stationary subjects and moving subjects.

AF-S

Use for stationary objects when you want to have pin-point accuracy of your focus placement. Once focus lock is acquired, that's it. The camera will not re-focus until you let go of the shutter button and re-focus again. With AF-S, you can only set the front command dial to either single-point AF or AUTO-area AF mode. If you are shooting deliberately and specifically, it makes sense to use AF-S with a single-point; letting the camera decide automatically is permissible but counter-productive.

Fixed focus point at hyper focal distance.

For the most part, single-point AF-S will be your default mode if you are shooting landscapes other other still subjects, which for most people is the majority of the time. Also, if the light conditions seem bright enough to give you a fastshutter speed and you have a fast focusing lens, it might be tempting to leave the camera in AF-S, but if there is any possibility of movement, you are always better switching to AF-C. In the example below, the paraglider was relatively stationary on the ground, but with the wind blowing and the dynamic nature of the sport, it would have been a mistake to assume that AF-S would have been enough.

Sill, but potentially erratic moving subject.

In menu item a2, you can select the AF-S release priority, meaning that you can choose whether or not the camera will be in release priority or focus priority. Release priority means that the camera will deploy the shutter when you full-press the shutter button, regardless of whether or not the subject is in focus. It is sometimes jokingly called "focus frustration mode", as it gives you snappy AF speed, but if you didn't focus adequately during the half-press there would be nothing to stop you from taking an out-of-focus picture. Focus priory is the the opposite; even if you full-press the shutter button, the camera won't take a snap until the AF sensor thinks that it has achieved focus. This mode is sometimes jokingly referred to as "subject frustration mode," as the AF operation is slower (but more accurate), allowing for the subject to drift out of composition after focus has been locked.

AF-C

Use for objects that are moving distance wise relative to you (forward and back). I want to emphasize that by-itself, AF-C is about tracking distance; it is only when you pair it with an area mode (more about that latter) that the camera tracks side-to-side motion. What this means is that the best way to help the camera track distance relative to you is to keep the active AF-point(s) steady on the subject. If you pan erratically, you are not helping the camera to do its job. AF-C is similar to the VR function; the more steady you are, the better both will work. In menu item a1, you can select release priority as you could for AF-S.
 

AF-A

An automatic mode that is primarily AF-S, but will switch to AF-C if the camera thinks that movement has occurred. Primarily intended for situations when you have twitchy subjects that won't stay still, but for the most part, not recommended. The amount of motion required for the camera will change AF behaviour can is somewhat unpredictable, leading to misplaced focus. It's not that it's a bad system, but it can be frustrating when you get the occasional non-intuitive result. If there is any chance of movement (kids, pets, sports, etc.), you are better off with AF-C. It would be a stretch to say that the behaviour of the AF-A mode is unpredictable as it is the default setting on the camera, but it's certainly not the favourite mode for most of the experienced shooters out there.

Menu item a3, combined with AF-A, often gives newer owners fits of focus frustration. When focusing on one object, this setting adjusts the amount of time that will elapse before it switches focus to an object that inadvertently comes between you and the subject. Theoretically, this makes the focus system more predictable, but in practice, it seems to produce unpredictable results on occasion, especially when paired with AF-A. When more control is handed over to the camera, the results can be somewhat unintuitive. It's highly recommend turning a3 to "Off."

Front Control Dial (Width)


After you've set the primary behaviour of the system with the back control dial, the front control dial adjusts the precision of the AF system; is it looking at a smaller area or a much larger one. If it's the first, your chances of achieving an in-focus shot go up. If you choose a wider area mode, your chances of capturing a fast movie subject in the frame are improved, but you sacrifice some probability in capturing said subject in perfect focus. Things get more interesting when you start matching the AF-C with the different area modes. As mentioned before, AF-S by it's very nature only allows for single-point or auto-area. I don't recommend using AF-A with the area modes because it takes control out of your hands and makes the system's operation somewhat unpredictable.

AUTO

The camera will pick the AF-point for you. For thoughtful photography, not recommened, but it's actually not bad as it the system uses a degree of pattern recognition (ie, face detection) that works most of the the time. Use this mode if you are handing the camera off to somebody who is not familiar with DSLR's. Also known as the "waiter mode" when you give your camera to an inexperienced bystander to take a group photo.

Single

Limits AF to only one point. The most precise way to to focus. Technically, single point is not a dynamic AF-mode, as it cannot track side-to-side because no other AF-points active to give the camera motion information. Single-point and AF-C are best for situations when you want the greatest precision and the subject is moving directly towards you. Likewise, this setting is not ideal for subjects moving horizontally or vertically across the frame.

9-Point

Like single-point, but uses the surrounding 8-points to gather additional focus information. Note that even though there are 9 AF-points being used, the center point of the group is still the primary; the peripheral points are there to help the camera maintain continuous focus with the central point. If the subject motion becomes too erratic, the peripheral points can take over and becomes the new primary point.

Note: The dynamic area modes often confuse people because the selected AF point does not move in the viewfinder. Nonetheless, the system is calculating focus within the 9-block area of selected points, but the viewfinder will only highlight the initial AF point.

Below is an example of 9-point AF-C. As anybody who has owned pet rabbits knows, even when they are not moving, they are "moving", meaning that you want some element of motion tracking, but you want the motion tracking to be precise and specific.

AF-C, 9-Point on Nikon D7000

Some people prefer using 9-point and AF-C instead of 1-point AF-S; when shooting relatively still subjects, this setup still gives you a measure of precision, but can account for slight movement, like when you accidentally lean in or out after acquiring focus or if your subject fidgets out of place. Some go even further with this setup, and move the focus function to the AF-On button on the back of the camera instead of the default of half-pressing the shutter button. This decouples the focus from actuation, meaning that focus is activated only when AF-On is depressed. In this way, you can turn 9-point AF-C into a sort of enhanced version of AF-S single point.

21-Point

Like 9-Point, but the camera is looking at a larger area in which to acquire focus. Remember that more is not always better. The more points you choose, the more the camera has to "think" before it locks focus, which lengthens the focus acquisition speed, even if imperceptibly in most cases.

39-Point (51-Point on D7100)

All of the AF points are active. It's the least precise method of focusing, but it may be your only choice if you are trying to capture an erratically moving subject. Remember, more points are not better; if you are asking the camera to be more sensitive, then it has less brain power to be as specific. A common question that arises, then, is which which area mode works best when? The usual answer is that you use a larger area mode as the subject motion becomes more erratic, but there's a better answer. 39-point is best for when the subject is moving erratically and fills up a relatively small portion of the frame. That is to say, in these instances, the camera is relatively steady, but the subject is moving. 9-point and 21-point are good for panning on objects that fill up proportionally more of the frame; in other words, object is moving, and camera is moving as well.

3D-Tracking 

This is where all of the camera's brain power is in use. Once focus has been acquired, the camera will not only track for/aft movement, but will track your subject moving side-to-side. Unlike the other area modes, you will see the highlighted AF box move about in the view finder. This mode works differently than the other AF-modes. Phase detection autofocus compares the light rights coming from different angles, and then calculates how far the lens has to be adjusted to bright the light rays into focus. 3D-tracking also looks at colour and brightness information (from the exposure meter in the viewfinder housing, as well as from the PD sensors, which are colour sensitive). Because of this, 3D-tracking works best when the subject has a degree of colour contrast with the background.

39-point vs 3D-Tracking

The question then: when you do you use 23-point or 39/51-point area modes versus 3D-tracking? Note once again that the selected AF box doesn't move with the non-3D-tracking area modes: that should tell you something about their operation. In other words, the area modes are best used when if you can keep the subject steady on your initially selected AF box, and if you are quick about releasing the shutter. If you have a subject that is not moving too quickly, and which you want to track for a longer period of time before snapping, then 3D-tracking will work.  Here's an example of a shot done with 3-D tracking on the Nikon D7000:


I was following the seagull for about 2-3 seconds as it was coming towards me, so the amount of side-to-side motion was not obsessively erratic. 3-D tracking was able to pick out the contrast of the bird's darker markings against the blue of the sky, though it was a bit of a lottery on which part of the bird the camera was focusing on, either the head, body or wings.

It's best to play around with the 3D tracking setting to get a sense of what it can and cannot do. Try locking on to a target, like the eyes of your pet dog. Then move the camera around. If you don't move too violently, the AF points will the stay on the eyes, but if you move faster than that, the AF point will start drifting, moving around the face instead of staying put.
 

Additional Resource


  • For a tutorial on how to calibrate your autofocus with the moirĂ© fringe method, click here. This method is easier to use than the conventional way of judging focusing using scales and rulers.
  • For a neat trick in getting the outer AF points to focus better in tricky lighting situations, click here.

15 comments:

  1. genius post, very intuitive.
    claified much of my questions and doubts.
    thanx.

    ReplyDelete
  2. For the last few weeks my D7100 struggling to focus has been driving me crazy! This guide is simple, sensible & spot on. Thank you!!

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  3. Just got my 7100, upgrade from a D80 ... huge change. Have been trying to get a handle on the AF system ... this helped a lot. Need to now get out and experiment.
    Your blog is now in my favs...
    Thanks
    DenS

    ReplyDelete
    Replies
    1. Thanks! I felt the same way when I upgraded from the D80 to the D7000, but the AF system became easier to understand for me once its operation was "chunked" into the divisions that I tried to lay out. Enjoy your new camera!

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  4. Extremely helpful post!
    Nikon, please put this post in your user manuals. It will make your customers very happy!
    Thanks,
    Don

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  5. Thanks so much...I've had my D7000 for over 2 years...with no real clue of how the auto focus worked. Great informative article.

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  6. Great article in clarifying the AF system on my D7100 and how to use the two command dials-back and front in addressing the different selections.

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  7. Great post. Makes you wonder why Nikon cannot write such an intuitive document with their resources.

    ReplyDelete
  8. Excellent article, thanks! Word missing? in this section????

    21-Point

    Like 9-Point, but the camera is looking at a larger area in which to acquire focus. Remember that more is not always better. The more points you choose, the more the camera has to "think" before it locks focus, which [lengthens???] the focus acquisition speed.

    ReplyDelete
    Replies
    1. Thanks. "Lengthen would have been the word."

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  9. Great article! Finally, an AF explanation that makes sense!

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  10. Thank you, sir! I almost fell off my chair when I read "Frustration Mode." I got my D7100 2 months ago (upgrade from an old Konica-Minolta DSLR) and am still on the fence as to whether it's my new camera-buddy or an extremely costly paperweight. I'm still on the fence but this helped a lot! At least now my Focusing Malady has a NAME!

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    Replies
    1. When I upgraded to the 7xxx series, it took me a couple of years to really explore everything. The cameras are now really good at the basics, but there's a lot to learn and improve with technique-wise if you really want to dive into it.

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  11. Excellent explanation about auto focus system mode and area mode. Even much much much better and having a sense than the original manual of D7000. Thank U. Only 2 questions....Do I keep pressing the shutter release button half way while in AF-C single point, 9 point, 39 point and 3D all the way until I take the shot? Secondly, can I pan the cam with AF_C single,9,39points or keep it still(no cam movements)

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    Replies
    1. 1.) Yes, while 1/2-pressing the focus button, you must keep it in that position until full press. If you lift your finger, the camera will stop tracking focus.
      2.) Can only pan in 9/39/51 pts if you keep your initial focus point on the subject. If your focus point moves off of the subject, the chances are very high that you will have lost focus will have to start again. The mult-point modes are not side-to-side tracking modes; they try to anticipate erratically moving subjects. If you want to do a focus-recompose, then use the 3D-tracking mode.

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