Canon 5D Mk 4 Auto Focus Performance

Canon 5D Mk 4 Auto Focus

2ppi 400x400 Canon 5D Mk 4 Auto Focus PerformanceLike Nikon, Canon never do me any favors!

But I do feel that I must say to the world that it ought to give this camera a bit of a break.  It’s had a good mauling in a lot of places, usually by idiots and no-nothings, who keep comparing it to its big brother the 1DX Mk2 – a camera not without its very own set of unique foibles!

The Canon 5D Mk 4 is NOT designed to be a “poor mans” 1DX Mk2.

It’s hardly what I’d call ‘cheap’ in the old purchase price department for starters, so ‘poor mans’ and ‘budget’ are not terms I can easily associate with it.

There are lot’s of things I need to delve into further on this camera to give you guys a fuller picture of the cameras overall performance – most of which is going to involve Calumet or Canon lending me more lenses.

But I can say that I’ve formulated a solid opinion on the Canon 5D Mk 4 Auto Focus performance, and it’s turned out to be a lot better than I’d first imagined.

These are the style of shot that really tells you if your auto focus is working and up to the job:

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“Joey” 1/3200th, f7.1, ISO5000

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“Joey” 1/3200th, f7.1, ISO5000

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“Joey” 1/3200th, f7.1, ISO4000

But before you can start producing the shots you have to go through the tedious bit of testing the AF first.  It was while testing the overall sharpness and accuracy of the AF system that I came across a little problem.

When photographing the old ‘brick wall’ static target I found the system was front focusing by around 40 centimeters at 30 meters.  If I added +4 on the AF micro adjustment (using the 500mm don’t forget) then everything was razor sharp.

This didn’t seem right in my eyes – I’ve never felt the need to use micro adjust on Canon gear to achieve sharp focus on a static target – perhaps I’ve just been lucky!

But after testing this body with another 500mm L IS II, and Calumets lens on 3 other bodies, all tests revealed the same necessary +4 adjustment.

MG 0202 Edit Canon 5D Mk 4 Auto Focus Performance

The difference is quite marked!

  • Bare in mind that all these ‘static tests’ MUST be done with the aperture WIDE OPEN (in this case f4).
  • I always use the high ISO capabilities of a camera to the maximum, which allows me the luxury of shooting at between f6 and f8 to maximise DoF and use a high enough shutter speed to stop the action.  Manual exposure with Auto-ISO is my usual method of shooting with long glass.  A noisy image that is razor-sharp will ALWAYS out-sell a low noise image full of motion blur!
  • At f8 hardly any of the ‘poor sharpness’ (above left) is visible in the image because DoF is doubled from 40cms to over 80cms behind the plane of focus at this distance.
  • If I was to swap out to a shorter lens then the required amount of micro adjustment would be less, and with a longer lens MORE!

However, when we come to photograph the likes of ‘Joey’ we have a BIG problem!

Adding positive micro-adjustment values is basically like adding BACK FOCUS – you are telling the system to focus BEHIND where it perceives sharp focus to be – in other words ‘focus further away’.

So with a head on closing target/subject the resulting AI Servo sequence of frames will all be back focused.  The camera will be focusing behind a subject that’s getting closer – it’ll never work!

What we need is the system to move the plane of focus AHEAD of the subject, so that when the shutter opens for the next frame, the subject and plane of focus are hopefully in the same place.  This is how PREDICTIVE AF works, and cameras like the 1DX Mks 1 & 2/Nikon D4/4S truly excel at it.

Dialing in an opposite value of -4, and using AF Case 4 settings with Zone AF for the AI Servo sequences of little Joey yielded good results, but the level of consistency was still below what I thought was possible.

And it certainly got even less consistent when I changed to Point Expansion or Point Surround AF modes.

But now I’ve settled on a custom setup that is NOT obtainable on any of the fixed AF cases; TS & ADT both at +2 together with -3 AF micro adjustment:

1D9A4149 Canon 5D Mk 4 Auto Focus Performance

‘Morgan’ 1/4000th f8 ISO4000 & -3 AF micro adjustment

I’ve just uploaded a new video to my YouTube channel where I discuss the performance of the Canon 5D Mk 4 Auto Focus system, and go through A LOT of full resolution images.

Note, some of you may get bored and think I examine too many images – shame on you! There are 4 sequences, and each one represents around 4 seconds in real time and are a ‘buffers worth’ of shots.  So all those boring shots took less than 20 seconds to acquire –  I have to show all the shots in a sequence to illustrate the level of consistency, and I show 4 sequences to prove none of them are a fluke – I DO THE JOB RIGHT – unlike some other reviewers!

You can view it at full size by clicking the YouTube icon bottom right once you press ‘play’, but be warned it’s 36 minutes long!

I’m not finished with this camera just yet I don’t think; I must admit that I do quite like it!

Is the Canon 5D Mk 4 Auto Focus capable of better performance than that of the venerable old 5D Mk3 ?  Yes, it is.

Is the image quality better than the 5D Mk3 – oh you betcha it is, by a country mile and just like the 1DX Mk2 advantage over the 1DX.

Are the G/T algorithms (ISO), sensor and ADC output better – from what I can see that’s a ‘yes’ too; but then I’ve not done any dynamic range testing yet – kinda hard when the only lens you’ve got is a 500mm!

I’m getting used to the ‘touchy-feely’ screen now, but the fixed 7fps HS frame rate pisses me off a bit, I’d like to be able to drop it to 6 or 5 to the AF system even further.

Take my advice and don’t be impressed with the ‘Duel Pixel Raw’ feature – it’s CRAP. It does absolutely bugger-all apart from slow the camera down and produce massive files that are not worth the time or effort.  And you can only ‘work’ them in that clunky DPP software which is a total abomination to try and use!

And old UHS1 SD card tech? The camera would be better with a CF slot in conjunction with a CFast2 slot ‘a la’ 1DX Mk2 – in my opinion of course.

1D9A4186 600x400 Canon 5D Mk 4 Auto Focus Performance

Great Tit. Canon 5DMkIV, Canon 500mm f4 L IS II, ISO 10,000 +4 AF Micro adjustment

logo simple Canon 5D Mk 4 Auto Focus Performance

Canon 5D Mk 4 from £115.96 per month – Click HERE

D5 from Nikon – Latest News

The Nikon D5 – more news & musings

D5b D5 from Nikon   Latest News

Well, the grapevine is saying that Nikon Europe will only be supplying the Dual XQD-slot version – not a bad thing in my opinion as I really like the speed increase of XQD over traditional CF.

Rumour also has it that the 30 second 4K UHD video record limit of 3 minutes is going to be increased to match the D500 30 minute capabilities.  There is some speculation as to whether this will be a “straight 30 minutes” or a 10x 3 minute sequential file recording method (WOW…..I can see that option going down like a lead balloon!).

Nikon Rumours have got their hands on a rather interesting 17 page Nikon Support internal  “CONFIDENTIAL” document about the Nikon D5. marked for limited distribution, with the instruction that the information can only be given to customers on a one to one basis.  This is a MUST READ folks:

READ IT HERE

There is also one for the D500 HERE

On the AF side of the new D5 equation, with its 153 AF points, don’t forget:

  • Only 55 of those are “selectable” – that’s just 4 more than the D4/4S
  • The total AF frame area coverage is only marginally larger on the D5 than it is on the D4/4S

So there are 98 AF points that the camera has full control of AND YOU DON’T – let’s hope they are singing off the same hymn sheet as the camera operator ALL THE TIME.

In my opinion Nikon have basically tried to re-invent the wheel somewhat with this vast number of AF points.  Canons 61 point Reticular AF unit is a damnable good standard which Nikon could have added to simply by increasing the FX frame area coverage with an extra 10 to 20 AF points.

But NOOOOOO…………..Nikon couldn’t possibly think of doing something quite so logical.

To be brutally honest, there’s a chance that Nikon have failed in the D5 to conceive a camera that meets the full requirements of the photographers it’s theoretically aimed at – the pro photograher; especially when you consider the D5 price point.

Instead, it looks to me as if they may have concieved the right thing, then added to it in order to make the camera appeal to that unique bracket of consumer – the one with more money than sense!

Let me qualify somewhat –  a proper “pro” knows what they are doing, knows their kit inside out, thinks on their feet, and can make settings decissions ‘on the fly’ virtually without thinking about them.

Give him or her a camera with 98 AF sensors that they can’t control – and the first thing they are going to look for is some way of turning the things OFF; just like they do with VR!

But turning them off is not an option, and the majority of Nikon pro users I’ve spoken to are of the same frame of mind as me – we are suspicious.  Yes the Multi-CAM3500 AF system wasn’t perfect and was in desperate need of improvement – but bloody hell Nikon, did it have to be quite so damn radical!

It’s all very well Nikon showing cool action jpgs on their website and promo material – but these are meaningless.  All the shots could have been taken on a D3 for all we know – yes, sharp action photographs were possible back then too.

What they don’t show you is a high speed sequence of 30 or 40+ full resolution images shot at 10 fps – only THAT would actually prove that the new fangled AF system actually does all that it’s hyped up to do.

On the flip-side, as I mentioned in my previous D5 related post, at least Nikon have given us access to the one thing AF-wise that was missing; an equivalent to Canon Accel/Decel tracking – but they could give that to D4/4S owners with a simple firmware upgrade – yeah they could, ‘cos it’s already there in the form of preset differences between 9 & 21 point Dynamic Area AF.  But they don’t tell you that!

At the end of my previous post I said I wanted to get my hands on a D5 now – and I still do.  But I never said anything about paying for it upfront or sight-unseen.  I only buy ‘stuff’ that I KNOW works; and I only tell you guys ‘n gals about equipment once I KNOW how well it does its job.

Until I’ve given the D5 a thorough work-out I’m just going to advise a bit of buyer-caution though – the beast might be brilliant, but then again it might not, once you peel back the hype and look at the nitty-gritty.  And £5200 is a big sum to gamble with.

Come on Nikon, have the courage of your own convictions and send a D5 to your Uncle Andy – let him prove his suspicions wrong; let him set out to prove the D5 isn’t all it’s cracked up to be – and fail miserably!

 

 

 

Autofocus Guide for Long Lens Bird in Flight Photography

GX2R2055 Edit 21 Autofocus Guide for Long Lens Bird in Flight Photography

My Autofocus Guide for Bird in Flight Photography is finished and available for download in my online store – here, priced £29.00

The download is in the no-frills .pdf format.

Screen Shot 2016 12 10 at 12.20.12 352x400 Autofocus Guide for Long Lens Bird in Flight Photography

This is my ‘real world’ guide to Canon & Nikon Autofocus which is specifically aimed at photographers using long lenses for Bird in Flight photography.

I sell my full resolution wildlife and natural history images every day via the various global image libraries to which I am a contributor.  The largest percentage of these sales are Birds in Flight.

Image libraries demand tack sharp, full resolution uploads from their contributors;  even marginal sharpness will result in an image being rejected by Quality Control.

D4D7980 Edit Autofocus Guide for Long Lens Bird in Flight Photography

A large male White-tailed eagle, locally known as “Brutus”, carrying a very large Coalfish.

In this guide, I take you right back to the basics of subject speed and distance; and how this impacts on our choice of camera body and lens focal length and working aperture.

D4S6788 Edit Edit Autofocus Guide for Long Lens Bird in Flight Photography

A Red Kite in a fast dive against a blue sky.

You’ll learn how phase detection autofocus works – only by really understanding how your autofocus system works, and what its control settings actually do, will you truly be able to control it in the way you need to for the particular task at hand.

I give you exposure and autofocus control settings for both Canon and Nikon, based on ‘real world’ full resolution images – settings that actually work, and do the job you expect them to do.

We also discover the various ‘tips ‘n tricks’ we need to know to help the autofocus system do the job we are asking it to do.

We also look at the short-comings of both the Nikon and Canon systems, and how to work around them in order to produce tack sharp images of birds in flight – HAND HELD – forget that tripod; you can’t move fast enough with one!

Photographers who have read this guide as it was being written have called it ‘the definitive guide’.  I’m not going to be so big-headed as to promote it as such myself, but I will say that it’s taken a while to produce, is pretty darn thorough, and I have the shots to prove it!

Available for purchase in my online store right now.

Please Note: This document relates to LONG LENS continuous auto focus tracking in Nikon AFC and Canon AI Servo modes in conjunction with continuous shooting modes on Nikon D4/4S and Canon 1Dxbodies with v2 firmware or higher, and is primarily related to capturing Birds in Flight and other fast-paced wildlife action photography.
Canon 5DMk3/7DMk2 users will also find this guide very useful, as will non-wildlife shooters.

Auto Focus Work Out

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.

The Guys 752x900 Auto Focus Work Out

The Guys – yours truly, Malcolm Clayton and Paul Atkins; and Mohamed El Ashkar (all the way from Cairo!) and our Cambridge “Don” – all trips should have one – Jamie Gundry. Photo by Ole Martin Dahle.

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:

D4D7980 900x599 Auto Focus Work Out

Say “Hello” to “Brutus” – an eagle who lives up to his name for sure – a total brute, especially to a boat full of daft photographers! CLICK for larger view.

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:

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All 51 of the Nikon Multi Cam 3500 FX focus sensors – both cross and linear sensors depicted.

Just the Cross-type Sensors:

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The 15 Cross type focus sensors on the Nikon Multi-Cam 3500 FX unit.

The Linear-type Sensors:

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The 36 Linear type focus sensors on the Nikon Multi-Cam 3500 FX unit.

Single Area AF

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Single Area, or single point AF.

9 Point Dynamic Area AF:

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9 Point Dynamic Area AF

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9 Point DA AF as displayed in the viewfinder (drop shadows added in Photoshop behind the dots to aid visibility in this article).

21 Point Dynamic Area AF:

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21 Point Dynamic Area AF

AF 21pointDVF 900x599 Auto Focus Work Out

21 Point DA AF as displayed in the viewfinder (drop shadows added in Photoshop behind the dots to aid visibility in this article).

51 Point Dynamic Area AF:

All51AFpointsVF 900x599 Auto Focus Work Out

51 Point DA AF as displayed in the viewfinder (drop shadows added in Photoshop behind the dots to aid visibility in this article).

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:

D4D8114 900x599 Auto Focus Work Out

A. Dark Subject and Light Background.
Subject itself is low contrast, background water is higher contrast. Subject is at 15 meters, Focal Length is 240mm

D4D7971 900x599 Auto Focus Work Out

B. Light Subject against a Dark Background.
Subject now has a slightly higher contrast, and background is lower contrast. Subject 29 meters, Focal length 360mm

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.

AF 21pointDVFclose1 900x900 Auto Focus Work Out

21 point AF, 15 meters and 240mm focal length.

AF 21pointDVFclose21 900x900 Auto Focus Work Out

21 point AF, 29 meters and 360mm focal length.

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……..

D4D5661 Edit 900x599 Auto Focus Work Out

“GIMME SOME, YOU MEAN BARSTARD!”

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This blog really does need your support. All the information I put on these pages I do freely, but it does involve costs in both time and money.

If you find this post useful and informative please could you help by making a small donation – it would really help me out a lot – whatever you can afford would be gratefully received.

Your donation will help offset the costs of running this blog and so help me to bring you lots more useful and informative content.

Many thanks in advance.

 

Auto Focus & Shooting Speed

Auto Focus & Shooting Speed

 

Firstly, an apology to my blog followers for the weird blog post notification this morning – I had one of those “senior moments” where I confused the Preview button with Publish – DOH!

There is truly no hope………..!  But let’s get on….

The effectiveness of auto focus and its ability to track and follow a moving subject IS INFLUENCED by frame rate.

Why is this I here you ask.

Well, it’s simple, and logical if you think about it – where are your AF sensors?

They’re in the bottom of your cameras mirror box.

Most folk thing that the mirror just sits there, reflecting at 45 degrees all the light that comes through the lens up to the focus screen and viewfinder.  The fact that the mirror is still DOWN when they are using the auto focus leads most people into thinking the AF sensor array is elsewhere – that’s if they can be bothered to think about it in the first place.

 

So how does the AF array SEE the scene?

Because the center area of the main mirror is only SEMI silvered, and in reality light from the lens does actually pass through it.

 

D4R4484a 600x400 Auto Focus & Shooting Speed

Main mirror of a Nikon D2Xs in the down position.

 

Now I don’t recommend you jam a ball point pen under your own main mirror, but in the next image:

 

D4R4484b1 600x400 Auto Focus & Shooting Speed

Main mirror of a Nikon D2Xs lifted so you can see the secondary mirror.

 

Now there’s a really good diagram of the mechanics at http://www.reikan.co.uk/ – makers of FoCal software, and I’ll perhaps get my goolies cut of for linking to it, but here it is:

 

af04 Auto Focus & Shooting Speed

This image belongs to Reikan

 

As you can now hopefully understand, light passes through the mirror and is reflected downwards by the secondary mirror into the AF sensor array.

As long as the mirror is DOWN the auto focus sensor array can see – and so do its job.

Unless the MAIN mirror is fully down, the secondary mirror is not in the correct position to send light to the auto focus sensor array – SO GUESS WHAT – that’s right, your AF ain’t working; or at least it’s just guessing.

So how do we go about giving the main mirror more “down time”?  Simply by slowing the frame rate down is how!

When I’m shooting wildlife using a continuous auto focus mode then I tend to shot at  5 frames per second in Continuous LOW (Nikon-speak) and have the Continuous HIGH setting in reserve set for 9 frames per second.

 

The Scenario Forces Auto Focus Settings Choices

From a photography perspective we are mainly concerned with subjects CROSSING or subjects CLOSING our camera position.

Once focus is acquired on a CROSSING subject (one that’s not changing its distance from the camera) then I might elect to use a faster frame rate as mirror-down-time isn’t so critical.

But subjects that are either CLOSING or CROSSING & CLOSING are far more common; and head on CLOSING subjects are the ones that give our auto focus systems the hardest workout – and show the system failures and short-comings the most.

Consider the focus scale on any lens you happen to have handy – as you focus closer to you the scale divisions get further apart; in other words the lens focus unit has to move further to change from say 10 meters to 5 meters than it does to move from 15 meters to 10 meters – it’s a non-linear scale of change.

So the closer a subject comes to your camera position the greater is the need for the auto focus sensors to see the subject AND react to its changed position – and yes, by the time it’s acquired focus and is ready to take the next frame the subject is now even closer – and things get very messy!

That’s why high grade dSLR auto focus systems have ‘predictive algorithms’ built into them.

Also. the amount of light on the scene AND the contrast between subject and background ALL effect the ability of the auto focus to do its job.  Even though most pro-summer and all pro body systems use phase detection auto focus, contrast between the subject to be tracked and its background does impact the efficiency of the overall system.

A swan against a dark background is a lot easier on the auto focus system than a panther in the jungle or a white-tailed eagle against a towering granite cliff in Norway, but the AF system in most cameras is perfectly capable of acquiring, locking on and tracking any of the above subjects.

So as a basic rule of thumb the more CLOSING a subject is then the LOWER your frame rate needs to be if you are looking for a sharp sequence of shots.  Conversely the more CROSSING a subject is then the higher the frame rate can be and you might still get away with it.

 

Points to Clarify

The mechanical actions of an exposure are:

  1. Mirror lifts
  2. Front shutter curtain falls
  3. Rear shutter curtain falls
  4. Mirror falls closed (down)

Here’s the thing; the individual time taken for each of these actions is the same ALL the time – irrespective of whether the shutter speed is 1/8000th sec or 8 sec; it’s the gap in between 2. & 3. that makes the difference.

And it’s the ONLY thing shutter-related we’ve got any control over.

So one full exposure takes t1 + t2 + shutter speed + t3 +t4, and the gap between t4 and the repeat of t1 on the next frame is what gives us our mirror down time between shots for any given frame rate.  So it’s this time gap between t4 and the repeat of t1 that we lengthen by dropping the shooting speed frame rate.

There’s another problem with using 10 or 11 frames per second with Nikon D3/D4 bodies.

10 fps on a D3 LOCKS the exposure to the values/settings of the first frame in the burst.

11 fps on a D3 LOCKS both exposure AND auto focus to the values/settings of the first frame in the burst.

11 fps on a D4 LOCKS both exposure AND auto focus* to those of the first frame in the burst – and it’s one heck of a burst to shoot where all the shots can be out of focus (and badly exposed) except the first one!

*Page 112 of the D4 manual says that at 11fps the second and subsequent shots in a burst may not be in focus or exposed correctly.

That’s Nikon-speak for “If you are photographing a statue or a parked car ALL your shots will be sharp and exposed the same; but don’t try shooting anything that’s getting closer to the camera, and don’t try shooting things where the frame exposure value changes”.

 

There’s a really cool video of 11 fps slowed right down with 5000fps slo-mo  HERE  but for Christ’ sake turn your volume down because the ST is some Marlene Dietrich wannabe!

 

So if you want to shoot action sequences that are sharp from the first frame to the last then remember – DON’T be greedy – SLOW DOWN!

 

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