AI servo AF question

[John Chardine]

I shoot Canon but the answer may come from anywhere.

A couple of weeks ago I was photographing Black-legged Kittiwakes coming towards me in full, flapping flight with my 50D and 500/4 on a Mongoose head. BG was brown cliffs a long way away. I'm not sure of the range but I was picking up the birds a fair distance away and attempting to track them in. Initial focus acquisition on the centre square seemed to work quickly, then I would track the bird in so far and would inevitably lose the subject and then try to reacquire- this is where the problems begun. I would keep my finger on the shutter release, half pressed, through the whole process of initial acquisition, loss of reacquisition and attempted re-attainment, but the reacquisition would not happen. The camera would not re-focus even when I had the centre square dead on the bird, let alone when one of the other squares was over the subject- something Canon says the AI Servo AF should do (that is transfer control to one of the other sensors).

Am I doing something wrong here? Should I hold off the shutter release after loss and then reacquire by pressing again? Are there particular settings I should make to help the process?

Any help would be much appreciated.

[Tell Dickinson]

Hi John, same setup as you, if I lose focus I normally take my finger of the shutter button and reapply to start over again as I find that works for me. I will also have switched the limit switch on the 500 f4 if the subject will always be more than 10 metres away.

Tell

[Grace Scalzo]

Same problem as you, John with the same set up. Looking forward to the development of this thread.

[Mike Milicia]

The following is taken from :
http://cpn.canon-europe.com/content/...iii_af_tips.do

"When tracking subjects against a background with little detail, a bird in flight, for example, if the AF point moves from the subject to the background,
the camera can begin to ‘hunt’ for a subject to focus on. Setting C.Fn III-5-1 will stop the camera hunting – and risking a completely
out-of-focus situation while the lens searches for a subject to focus on – so when the AF point re-finds the subject, it can re-acquire focus faster."

I have found that setting C.Fn III-5 to 1 can be helpful in situations like you encountered at Cape St. Mary's.
It works well with birds flying parallel to the camera. If the bird is flying directly toward you, it should
still be helpful as long as you get the sensor back on the bird fairly quickly. The down side is that if you
suddenly want to focus on a subject much closer or further away than where focus was last acquired,
you may need to provide some manual assist to acquire initial focus since the "hunting" is disabled.
In fact, I find that it's wise to keep your hand on the manual focus ring at all times when using this function,
just in case. Also, be sure to set it back to 0 when it is no longer needed.

[Fabs Forns]

Tracking a subject coming towards you seems to be tougher on cameras regardless on the make. It is easier for them to predict lateral movement. My suggestion is to press the shutter when you feel acquisition is right and forget the idea of a burst of sharp images coming towards you.

[Alfred Forns]

Hi John

That is the most difficult situation for the AF !!! Coming at you I was trying for the same images at the Cape !!!

What I tried doing is pre focus in the area you want to capture the bird ... then watch in viewfinder but don't press the shutter for AF till the bird is fairly in focus .. then acquire and shoot !!!

btw I did my best at the cape with shorter lenses since the Kittiwakes would fly out of nest and back and giving a chances for banking and top views !!!

[John Chardine]

Makes sense Fabs and Alfred. Movement at any angle towards you can be decomposed into two vectors- one coming straight at you and the other at 90°. The one coming straight at you is the one the AF has to work against. If almost all the movement is towards you then that vector is much larger than the other and the AF has more trouble keeping up. Lateral movement would have a small associated vector towards you and the AF could keep up. BTW I noticed that gannets were a lot easier to track and I thought at the time it had to be due to their size, but it was probably due to their slower flight speeds.

I'll try the setting Mike. Thanks!

OK Alfred, how does this AI Servo AF work with a F1 car or motorcycle rounding a bend at 150 mph???!

[Alfred Forns]

... smooth as silk :) Speed and line are both predictable and easy to track :)

[Tim Dodd]

I'm not sure whether the specs for the 50D AF differ from the 40D but here is an extract from the 40D White Paper....

Compared to the EOS 30D, the EOS 40D’s focusing calculation speed is 1.3 times faster, and the AF data-processing time is shorter. Predictive AF can track a subject approaching at 50 kph/31 mph up to about 26.2 ft./8m away with an EF 300mm f/2.8L IS USM lens.

It is not only the subject's closing speed that is important but also how far away it is. For a subject such as a race car, perhaps 100m or 200m away, a change in distance of 10m would require only a miniscule movement of the lens focus. Relatively speaking the demand on the AF system is not that great. As the car gets closer the focusing demands would increase as the %age distance closed each second would rise and rise.

e.g. a subject travelling at 10m/s at 100m away will only close the distance by 10% in the first second. At 400mm and f/8 the DOF at that distance would drop from 19m to 15m, which gives some margin for error. Once the subject is 20m away, in the next second it would close the remaining distance by 50% and the DOF would drop from 75cm to 18cm. That's a whole new ballgame. In the final second it would close the remaining distance in full. As I'm sure you know, there is a much large degree of rotation of the focus mechanism to cover the range between, say 20m and MFD compared to the degree of rotation to refocus between 100m and 90m. I doubt many people photograph racing cars approaching head on at 150mph from 20m away :)

This is why people have no difficult photographing jets at 500mph at public displays. Apart from the much larger distances involved, and much larger DOFs that follow, a passing jet doing 500mph will actually have a closing speed of 0mph at the point it passes closest to you. In fact, rather than trying to track it with AI Servo you could probably do just as well with One Shot AF, or simply manual prefocusing, as it gets to the closest point to you. A 400mm lens at f/8 focused on a subject 200m away has a DOF of 78m. At a subject distance of 300m the DOF soars to 185m. That's quite a bit of room for error.

I would imagine that shooting BIF at 20m is quite a bit more difficult that shooting race cars at 150MPH and 100m distance, unless you're talking of big, slow flappers like swans, herons etc.. I certainly think I'd have more success shooting Formula 1 than a kingfisher whizzing by against a busy background, but since I've tried neither I can't be certain. The unfortunate truth is that I'd probably be dreadful at both.

[Roger Clark]

John,
My experience is similar to yours with XXD series cameras, although I have not tried a 50D for BIF. With XXD Canon's I could often be heard muttering focus d*m it, focus! But then I got a 1D2 and have on many occasions slipped off the subject with while tracking against a complicated background and re-acquired focus tracking on the subject in a fraction of a second while standing next to guys muttering muttering about focus on their XXD cameras.

But even XXD cameras fail at some rate. Tim illustrated that quite well. To be a little more specific, the focus tracking is the rate inside the camera at the focal plane, e.g. how fast is the focus point moving in mm/second.

Somewhere I started a spreadsheet looking at the focus speed versus distance and assuming different lenses but with the idea that in each setup you fill the frame with the subject to a given size. I was wondering if that translated to an optimum focal length for BIF (or other action photography). Assuming you could get as close to the subject as you wanted would different lenses give you an advantage? For example, is 200 mm better than 500 mm assuming when you had the 200 mm on, that the bird was closer by 2/5 so it appeared the same size in the viewfinder. It turns out that the focus point rate of change is the same, so no lens gives an advantage in this regard (assuming all lenses had the same focus speed which is not the case). But as you get to shorter focal lengths, as the subject wizzes by you, the turn rates are higher with a shorter lens than with the bird further away using a longer lens. That makes it harder to follow. Of course it gets more difficult to follow the action because of the small field of view in spite of the fact that the turn rate is lower as the focal length increases, so at some point you miss the subject more. But that gets into individual abilities rather than camera technical specs so is different for everyone.

Anyway, here is my article on autofocus tracking in case it helps:
http://www.clarkvision.com/photoinfo...with.autofocus

It is the performance in re-acquisition of moving targets against a complex background that has kept me wary of the XXD cameras and with the AF focus problem with the 1DIII, I stayed with the 1D2 until the 1D4 comes out (hopefully soon).

Roger

[Robert Amoruso]

I shoot Canon but the answer may come from anywhere.

this is where the problems begun. I would keep my finger on the shutter release, half pressed, through the whole process of initial acquisition, loss of reacquisition and attempted re-attainment, but the reacquisition would not happen. .

John,

This is your biggest problem. I would recommend that you acquire focus further out as you are doing using the center AF and then take the finger off the shutter. As the focus starts to go soft, bump the shutter to reacquire, repeat. As you get to your optimum shooting location, attempt only a few images focusing between each.

[Tim Dodd]

John,

This is your biggest problem. I would recommend that you acquire focus further out as you are doing using the center AF and then take the finger off the shutter. As the focus starts to go soft, bump the shutter to reacquire, repeat. As you get to your optimum shooting location, attempt only a few images focusing between each.

Robert, if you don't mind me saying, that might not be a good technique if using an IS lens, as there is a risk that the IS might spin down if you let things drift for more than a second or two. You'll then need another 1/2-1 second for the IS to spin up again, which might be too late for your decisive moment. If you are tracking a bird flying across your path rather than towards you, you may hardly need to AF at all, but having Mode 2 IS running throughout the panning movement might help improve keeper rate.

I prefer to separate focus from metering and associate focusing with the AF-On button only. That way I can bump focus as you propose, but I can retain a half press on the shutter throughout and thus keep the IS engaged. In fact, when I spot a likely target I half press the shutter button as I bring the camera up to my eye. IS is then already up and running by the time I locate the subject in the viewfinder. I also find that the image is nice and stable, which makes it a more pleasant experience all round.

[Robert Amoruso]

Robert, if you don't mind me saying, that might not be a good technique if using an IS lens, as there is a risk that the IS might spin down if you let things drift for more than a second or two. You'll then need another 1/2-1 second for the IS to spin up again, which might be too late for your decisive moment. If you are tracking a bird flying across your path rather than towards you, you may hardly need to AF at all, but having Mode 2 IS running throughout the panning movement might help improve keeper rate.

I prefer to separate focus from metering and associate focusing with the AF-On button only. That way I can bump focus as you propose, but I can retain a half press on the shutter throughout and thus keep the IS engaged. In fact, when I spot a likely target I half press the shutter button as I bring the camera up to my eye. IS is then already up and running by the time I locate the subject in the viewfinder. I also find that the image is nice and stable, which makes it a more pleasant experience all round.

Good point Tim that I neglected to mention. I will bump focus before IS shuts down.

I find the AF activate using AF-On or * button fatiguing so prefer this way though I will do it both. I always have IS=2 on for any lens designed for its use on a tripod and certainly with handheld. I see you have better dexterity then I. As I have tried your method and usually end up making images I don't want.

[Tim Dodd]

Hah! My results speak for themselves. I did not say I was any good :) I'm afraid that for me there is still a big gap between theory and practice.

[Alfred Forns]

HI Tim

I had a chance to use the 40D along with the 50D and found a difference in my AF with the 50D being faster !!!

I'm usually a center point user but have been relying more on all points for the 50D with good results, still in the testing stage.... if is a must will revert to single.

[John Chardine]

This has been a great discussion and I've learned a lot. Many thanks. Will try to put it into practice next week in Newfoundland (again!).

[Alfred Forns]

Wish I was there with you John !! Hope you get some clear weather to shoot and a hug for Tony !!

[John Chardine]

Will do! At Gull Island mainly this time putting loggers on kittiwakes. Will go to Cape for the day.