Tuesday, October 9, 2012

Nikon D600 autofocus: speed, accuracy, low light, and clustering

10/11/12 UPDATE: Added Nikon D3 shots for comparison
10/22/12 UPDATE: Added shots showing AF speed

Nikon D600 + Sigma 50 1.4 @ f/2.0
Because my subjects often move, one of the most critical components of any camera for me is its autofocus system.  What's the use of a very high resolution sensor with extremely low noise if the shot I capture is blurry - not because of subject movement but because the subject is out of focus?

In this post I will discuss my experience regarding the Nikon D600's autofocus speed and accuracy.  I will also discuss the clustering of autofocus points.

(And even though you may not be in the market for a D600, you may be interested in reading about a couple of alternatives to focus-and-recompose.)


SPEED AND ACCURACY

I tested the Nikon D600's autofocus speed and accuracy using a Sigma 50 1.4.  If you've shot with a 50mm lens at f/1.4 you know the depth of field is razor thin, especially at the usual shooting distances of a full frame camera, i.e., closer than on a crop sensor when maintaining the subject size in relation to the frame.  I tried shooting at very wide apertures, such as f/2 and even f/1.4.

(Before testing the Sigma 50 1.4, I fine-tuned the AF using the moire method described here.)

When I shoot a subject that can move, I usually use AF-C even if the subject is not moving at that particular moment.  I also prefer to set my AF-C to focus priority instead of release priority, meaning the camera will do its best to get accurate focus even if it means that it will take longer to acquire focus.

a.  Subject not moving much.
With this method, I took a few snaps to test the D600's AF.  I found that the D600's AF was fast and accurate enough to capture shots when the subject is not moving much.  This one was at f/1.4 with the AF point on my son's near eye:
The focus was spot-on (click on it to get a high-res version).

This other one was only at f/2.0 but with my son so close to the camera, the DOF was very thin.  The AF point here was on the near eye (his left, camera right).
To me it looks a little bit behind - the far eye seems to be more in focus, but the focus was more than acceptable to me.

Here is a series of shots taken in bursts to give you an idea of the consistency of the autofocus.  These were at f/1.4.  The AF point on these were approximately on the near eye though because of subject movement, it sometimes got outside of that.

Here are the shots.  (Please excuse the garish colors on some of them.  It seems that Picasa is doing something automatically to some or all of the shots.)








As you can see, a few of the shots are not in focus, but the others have acceptable or even spot-on focus.  Considering that these were at f/1.4, I like the Nikon D600's AF performance.

LOW LIGHT
The D600's autofocus is designed to work in low light environments -- as low as -1 exposure value (explained here).  Last week I tried focusing on a subject lit only by a bedside lamp (the EV was around 4), and the camera focused accurately and with no hunting even though I was using a relatively slow lens with an aperture of f/4.5.  A couple of days ago, I was able to focus in an environment that required an exposure of 25600 ISO, f/2.8, 1/60 which is an EV of 1.  Again the camera focused quickly and accurately, with no hunting at all (I was using the Nikkor 24-70 2.8G).  I will post that sample soon.

CLUSTERING OF AF POINTS
The shots above were taken while the AF point was more or less where it needed to be.  The problem for the Nikon D600 is that its 39 autofocus points are clustered tightly in the middle, occupying only about 1/9th of the total frame area.  Essentially it appears that the AF points were carried over from the D7000 with no change, except that now the frame area is twice as large, so the AF coverage is much smaller:
Inline image 1
Nikon D600's 39 AF points*
*From the viewfinder display shown on the Nikon D600 product manual. I removed the surrounding grid lines.
On the other hand, the clustering of AF points is an issue with other Nikon full frame cameras as well, albeit to a lesser extent:

Inline image 2
The Nikon D800 AF points*
*From the product manual, also edited to remove the surrounding grid lines
I overlaid the two AF layouts to show the difference in extra coverage (shown in red).

Inline image 3

On one hand, the extra layer of AF points is not a huge difference.  On the other hand, the extra AF points allow the D4, D800 and other FX cameras (including those of the previous generation) to touch the intersections of the rule of thirds.

RULE OF THIRDS
Here is what the AF points look like when we overlay a grid that follows the rule of thirds.

Inline image 4 

We can see that the extra AF points are just enough to touch the intersections of the rule of thirds, so even though they aren't huge, they can have a disproportionately significant impact for someone whose composition often follows the rule of thirds (like me).

Solution 1: Golden Ratio
One solution is to consider using an alternative compositional aid, such as the golden ratio.  (Indeed, the rule of thirds is arguably just an approximation of the golden ratio.)  A golden ratio grid places the intersections just a tad closer to the middle of the frame:

Inline image 6

The intersections of the golden ratio grid are close enough to be arguably within the D600's AF coverage.

Solution 2: Crop
An alternative solution is to take advantage of the D600's 24mp resolution by shooting to crop.  If you don't need to make large prints, it might be feasible if you absolutely need to position the AF point at the rule of thirds intersection for a particular shot.  Here is an example where after cropping the AF points cover a couple of the intersections of the rule of thirds:
Gray area represents the area being cropped out
However, this solution might be ok from time to time but is probably not going to be satisfactory on an everyday basis.  Who would want to lose the use of a portion of their sensor?  Besides impacting your lens' effective focal length, this solution also has implications for noise (becomes more noticeable) and depth of field (may become shallower at the same viewing size).

Solution 3: Focus and recompose
Another solution is of course to focus and recompose.   This solution tends to work only for a slow-moving or stationary subject because when there is a very shallow depth of field, usually the subject moves out of focus by the time I recompose.  The D600 is a little better because you can choose an AF point that's a little closer to the subject, but the short delay is sometimes enough to get the subject out of focus.

ALTERNATIVES TO FOCUS-AND-RECOMPOSE
Because focus-and-recompose has its own problems, sometimes I use these alternatives:

Alternative 1: Equidistant Target

In this test shot, my wife's eyes were outside of the AF area:

I therefore focused on her mouth which is about the same distance from the camera as her eyes.  Focusing on her mouth therefore got her eyes in focus as well.


Alternative 2: Auto Area AF

Another alternative is to use Auto Area AF (I generally pair it with AF-C).  In this mode, the camera chooses the focal point.  I hadn't tried this before because I was reluctant to give up control over where the camera would focus.  However, when I tried it, it seemed that for some reason, the camera would focus on the right spot, sometimes even when the camera appears to choose the "wrong" focus point.

In this shot, I manually chose the focal point over my daughter's eye.

f/2.0
To me, the focus is pretty good considering that my daughter was being swung up and down.  Here is the same scene, but this time I used Auto Area AF:


Pinpoint precision (focusing on the near eye)!  Again, this was with our daughter being randomly swung up and down.

Now you may be wondering, how does Auto Area AF offer an alternative to focus-and-recompose?  Well, take a look at these shots:



In both of them, the eyes are well outside the D600's AF coverage, yet the D600 chose AF points that resulted in the eyes being in focus anyway (!)

Here are some bursts that show how the Auto Area AF performs with AF-C.  These shots were taken by my wife who doesn't know anything about photography.  Needless to say she doesn't try to confirm the location of the AF points.  Note that the eyes are sometimes outside the AF coverage yet they are still in focus.

Burst 1:



Burst 2:



Burst 3:






As you can see, Auto Area AF isn't perfect but it gets the focus right a surprising amount of the time.  When I reviewed the Nikon D3, I noted that the autofocus wasn't as 'magical' as Ken Rockwell claimed it was.  This one however, is eerily close (and perhaps I was too hasty to discount the D3's auto area AF last time).

Note: One issue with these alternatives is that they can throw your exposure off if you are in matrix metering mode because Nikon's matrix metering will give more weight to the AF point.  In the real world, I haven't found this to be a problem.

FAST-MOVING SUBJECTS
Sometimes subjects were moving fast enough that there was no time for me to select the AF point and position them over the subject.  Instead I had to use Auto Area AF and AF-C and hope for the best.  Here are a couple of shots with this method:

Nikkor 28-105 3.5-4.5D




Nikkor 24-70 2.8G:

Of course this method doesn't work all the time but the hit rate is surprising.  It doesn't usually result in critical focus (with some exceptions like the 24-70 shot above) but the majority of the shots show at least "acceptable" focus (which for me means usable at web viewing sizes or prints of 8x12 inches).

COMPARISON WITH NIKON D3
To give you an idea of how well the D600's autofocus system performs relative to another FX camera, below are some shots with the Nikon D3 that are somewhat similar - the lens is the same, the aperture is also wide, there is a similar amount of activity, and the shutter speed I used was comparable in one of the series.  However, one notable difference is that I used AF-S then whereas I use AF-C now.  Anyway FWIW:

Sample set 1 (f/1.4, 1/800 to 1/1250):





Sample set 2 (f/1.8, 1/200 to 1/400):






MORE SAMPLES
Here are more test shots.  They're just snapshots but they show how well the Nikon D600 can focus in real world conditions.

f/2.0

f/1.4

f/1.8
f/2.0

f/2.0
To conclude, the D600's autofocus is fast and accurate, even in challenging conditions such as low light, fast-moving subjects and shooting with a very shallow depth of field.  Even though the AF points are tightly clustered, there are alternatives that have worked well for me.

RELATED FOLLOW-UP POST HERE: http://betterfamilyphotos.blogspot.com/2012/11/nikon-d600-autofocus-with-fast-subjects.html

RELATED POSTS:
Nikon D600 Hands-On Review
A Miracle Happened Today (Canon 5DIII autofocus)