Auto Focus Work Out

My recent summer trip to Flatanger in Norway, and to the famous “Eagle Man of Norway” Ole Martin Dahle, proved, as ever, a severe test of the auto focus capabilities of the gear!

We had 4 guys on the trip, 3 Nikon and 1 Canon, and White-tailed Eagles doing more than 40mph and turning on a dime is one of the hardest tests for auto focus tracking and lock on that you can imagine – especially when it’s all done hand held from a boat that’s rolling around in the sea swell.

We had a conglomeration of D4’s, D800E’s and 200-400 f4’s, with a smattering of 300mm and 400mm f2.8’s – and then there was Mohamed with his solitary 1Dx and 300 f2.8.

And our target:

Just to set the scene with regard to the technical side of things; birds fly into the wind given the choice, and the sun is wherever it decides to be! So the boat driver – Ole – always needs to position the boat so that “wind and sunlight” are coming from pretty much the same direction, otherwise the birds are not front-lit and cast their own shadows across themselves. In other words the images look like crap!

Some birds come towards the boat, take the fish and then turn away; some will do their approach parallel to the boat; and gits like Brutus will fly low and fast straight at you, pick the fish and then turn straight for the boat and climb.

But no matter how they choose to approach the camera boat all the birds pick the fish and go back to where they’ve come from.

Ole has intimate knowledge of these birds as individuals, and so has a damn good idea of what they will do as they come to the boat.  This enables him to manoeuvre the boat for the best shots, and this skill is what you pay for.

Perhaps by now you’ve got the general feel for the situation – a boat that’s subject to wave motion and which might suddenly go backwards 10 yards through its own wake – not the steadiest of camera platforms!

Couple that with trying to make the auto focus lock on and track the bird, and maintain a modicum of composition – it’s just damned hard work.

Photographing anything that’s moving is hard work; moving erratically is even harder; and hand holding on an oscillating camera platform makes the job beyond hard.  This style of shooting will NEVER yield vast rafts of sharp sequential images – anyone who tells you different is an outright liar. Christ, even licensed FIA F1 ‘togs are on “easy street” by comparison.

Auto focus cannot be set up perfectly for this sort of situation, but understanding it is a MUST if you want to maximise the opportunity.

Auto Focus Choices

There are 3 main things that control the effectiveness of auto focus and AF tracking:

AF Area Mode

AF Tracking Lock-on interval

Frame Rate

(Bare in mind I’m talking Nikon here, but sorting Mohameds’ 1Dx out showed my that Canon AF is pretty much the same).

Now I dealt with the latter in a previous post HERE and so we need to concentrate here on AF area modes in the main.

Let’s look at what we have to work with on a Nikon body – in this case a D4:

Firstly, the AF sensor layout.

All 51 focus sensors, and there approximate layout in relation to the image frame:

Just the Cross-type Sensors:

The Linear-type Sensors:

Single Area AF

9 Point Dynamic Area AF:

21 Point Dynamic Area AF:

51 Point Dynamic Area AF:

As a stills photographer you are using what’s called Phase Detection auto focus (that’ll be another blog post topic!) but it still relies on a mix of contrast,luminosity and colour to work out what it should be concentrating on in the frame.

Consider the following 2 images, A & B:

Auto focus is dumb; just plain stupid, left to its own devices.  It, like me (yep, me dumb too!) favours lighter things with a higher degree of contrast.  The lighter something is then the brighter and more saturated it colour is, and this in turn gives it higher localised contrast.

Auto focus will be happier locking on to and tracking Eagle B than Eagle A.

In A, the AF will want to switch to the lighter, more contrasty water behind the bird – unless of course you “hobble it” and stop it from doing so…

And you stop it by BLINDING IT – in other words use LESS active auto focus points!

“If it ain’t got ’em it can’t switch to ’em!”

If all the AF points in use are on the important part of the subject (the EYE in this case) then there’s little or no chance of the auto focus switching to somewhere you don’t want it to go to.

In a perfect world we’d all be using Single Area AF on a tripod and panning away quite happily keeping that single sensor on the targets eye……………oh I wish!!!!!!

51 point AF is out for this sort of work – with what I’ve just written you should now easily understand why.

So we are down to either the 9 point or 21 point Dynamic Areas.

It all comes down to two things:

• How steady you can keep the camera.
• How big in the frame the birds are – in other words, subject distance.

But accuracy of auto focus will always be improved by using the least number of sensors you can get away with.

 

Image A. is at 240mm and a subject distance of 15 meters, and Image B. is at 360mm and a subject distance of 29 meters.  Both images were shot using 21 point Dynamic Area AF, 1/2000th @ f7 and 1600ISO.

On the upper detail image there’s one, perhaps two of the 21 sensors that are NOT on the subject.

On the second image there are at least 9 sensors out of the 21 in the group that are NOT on the bird.

If the bird in image A. had been 29 meters away I’ll guarantee it would have been out of focus – why?

• Lack of good directional light.
• Poor subject contrast and illumination.
• Brighter, higher contrast background.
• More sensors “Off Target”.

And the auto focus hasn’t wanted to wander to the background on image B. because there’s nothing there for it to favour over the main subject.

How Dynamic Area AF Works

9 point DA auto focus uses the single AF point that you select, but activates the 8 points surrounding it.  If you, or the subject, or both, move so that the single point you selected comes “off target” then one of those 8 surrounding points will “cover” the error and maintain focus lock and tracking until you get back on target.

In 9 point DA, auto focus ALL the sensors activated are “cross type” sensors, assuming you use a sensor on the vertical center line of the AF grid.

In 21 point DA, auto focus is still centered on the single sensor you select, but now the surrounding 20 are activated. But at least 6 of these sensors will be linear, not cross type sensors.

Auto Focus Senor Types – Cross and Linear (line).

This is going to be immensely paraphrased!

AF sensors need to see edge detail in order to work. A linear sensor can work more effectively when the edge it’s looking at is perpendicular to it.

The more an edge is parallel to said line sensor then the harder time it has in discerning when said edge is sharp or not.

But if we add 2 line sensors together at right angles to each other, then an edge that is parallel to one line is perpendicular to the other – so edge detection is greatly enhanced.

In an ideal scenario 9 point Dynamic Area AF, centered in the middle of the view finder and kept on the eagles head would be the ideal way to go, but with the other circumstances of:

• Moving camera platform
• Potential closeness of subject (sub 15 meters possible)

then 9 point DA might be a wee bit tight on both counts, and 21 point makes more sense from a tracking and shooting perspective.

But it leads to an initial problem with the auto focus acquiring the target in the first place.  You have to pick these eagles up quite a way out, and if one is coming low to the water then there is possibly too much in the frame to act as a distraction to the auto focus unit itself; though this isn’t quite such an issue if the bird is high in the sky.

So my recommendation for any form of bird-in-flight photography is to start out at 9 point DA and see how you get on!

There is always the AF Tracking Lock On feature that you can deploy in order to “hobble” the AF unit from switching  to subjects closer to or further away, but if I’m honest I find this the most sticky and difficult aspect of the Nikon system to get a precise handle on.  It does exactly “what it says on the tin” but it’s the “when” and “how much by” bits that have me slightly guessing.

Sometimes I put it on long and it basically waits for perhaps 4 or 5 seconds before it tries to switch focus, while at other times it does so in less than half the time.  Sometimes I feel it actually diminishes the effectiveness of the “predictive” side of the auto focus tracking unit.

But if I turn it off when hand holding the camera for flight shots then everything turns to crap – so I turn it back on again!

Again, my base recommendations for this are SHORT to NORMAL and see how things go.

One thing that can have a considerable impact on the way you perceive your auto focus effectiveness is how you have your AF release priority set up (CS a1).

There are 4 options:

• Release
• Focus+Release
• Release+Focus
• Focus

By default this is set to FOCUS.  With the default setting, it’s theoretically impossible to take a soft shot.  But in practice that’s not so simple, and I’ve taken many a soft shot when the D4 “thinks” things are sharp; though in the main, that seems to have been cured the minute we got trap focus back with the latest firmware upgrade.

Release means the camera will take shots irrespective of focus being acquired or not.  I NEVER use this option.

Focus+Release means that the first frame will only be taken once focus is acquired, and subsequent frames will be taken irrespective of focus.  This is one of my preferred options when everything is unstable – that first frame hopefully sets up the auto focus and AF tracking and so everything SHOULD keep the subsequent frames sharp – please note the use of the word “should”!

Both the above release priority modes do NOT slow the frame rate.

Release+Focus – works the opposite way to Focus+Release – it does slow the frame rate down giving the mirror more down-time and so the auto focus system has more time to work.  This is my other preferred option, the one I use when the “action” may not be as repeatable.

Focus – This is the option I deploy when shooting from a tripod or when the action is not quite so fast-paced.  Again, this option slows the frame rate.

The Back Button Auto Focus Option

I always use the back button for auto focus activation.  There are plenty of arguments for doing this, but I just feel it’s darn right more efficient than having AF activation on the shutter button.  Just don’t forget to turn AF/Shutter ON to OFF in the menu, otherwise you are just wasting time and effort!

Conclusion

A lot of folk feel that their auto focus is flawed; but more often it is they and their setup choices which are flawed.

There is no blanket panacea or magic bullet setting for your AF system – as with everything else you have to constantly evaluate the light around you, anticipate the shot and make the necessary changes to setup – otherwise it’s going to be a sad day.

But knowing how your gear works and how it reacts under different scenarios is the “meat and two veg” of good photography.  Couple that with shot anticipation and the proper corrective measures and it’s off home for tea and medals!

But above all, remember to have a laugh – you’re a long time dead……..

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