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Thread: Trap focus - does it really work

  1. #1
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    Default Trap focus - does it really work

    A while back I mentioned using trap focusing on this website. I think it was before the big meltdown so the original post is probably lost. Anyway, I havenít had a chance to really try it out till now.

    For those who donít know, trap focusing is where you keep autofocus switched on but disable the association between the autofocus and the camera shutter, fix the camera focus at a certain distance, hold the shutter button down continuously, and wait till the moving subject arrives at the preset distance at which point the camera takes the photo itself. On a Nikon, this is achieved by setting the AR-L/AF-L menu option to AF-ON (as opposed to the default AE/AF Lock), and tweaking a few other minor settings. The focus distance is thus set by pressing the AE-L/AF-L button, and remains constant until you deliberately change it again. You then simply hold the shutter down continuously, move the camera around as you please and the shutter will trip whenever the camera is able to get good focus on something. Naturally this will only occur when the subject is at exactly that preset distance.

    Why would you do this ? Well, in theory the response time of even the clunkiest camera electronics is many times faster then a humanís entirely pedestrian shutter finger. In short, if a moving subjectís trajectory is even mildly predictable then you should get sharper photos of moving objects using trap focus.

    Did it work ? A couple of days ago I went down to the Parramatta flying fox colony. This spot is well worth a visit. Bad weather (for bats) in the north of NSW has pumped up the numbers from the usual 5-6,000 to 17,000, and you can stand on the rock platforms in the river and observe everything. Even in daylight there are bats in the air almost constantly Ė perfect for flight shots. And the trap focus worked producing lots of keepers versus none at all using conventional shutter techniques. However I should point out that I doubt the difference would always be this clear cut, as in these light conditions it was difficult to get shutter speeds up and my ageing camera doesnít really tolerate ISOs above 400.

    Is it a good way to take photos though ? Yes and no. Though flight shots were almost invariably sharper using trap focus, the problem arises when you want to take a photo of a bat that is not at the critical distance. Perhaps a bat does something fascinating just beyond the critical focus distance (I like to catch them when they swoop down to drink in the river) Ė you cant easily convert the camera back to conventional shutter mode to take a quick photo. I guess whether it is worth using depends upon how predictable and how repeatable the subjectís trajectory is.

    Anyway, its another trick to store away so I hope people find it useful.
    Fairly typical photo taken with trap focus is attached.
    Cheers
    Arron
    www.patonga.me : botany, wildlife, fishing, check it out

  2. #2
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    Default Re: Trap focus - does it really work

    An interesting idea which I, for one, had never heard of before. I assume us Canon users can do it somehow as well? What are the benefits of this trap focusing as opposed to AI servo, particularly when dealing with (birds) in flight? I think the idea of AI is to predict the path of the subject and thus make the adjustments accordingly with the focus. Of course, it is not right 100% of the time, but pretty decent on some of the recent cameras.
    Also, say for instance a bird is moving very fast (lets assume a swift or something). Wouldn't that mean that by the time the shutter was fired the swift would have already moved out of the focus point?
    Regardless, an interesting idea, thanks for bringing it up.
    Tobias Hayashi
    Canberra, Australia
    www.flickr.com/photos/callocephalon

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    Default Re: Trap focus - does it really work

    Hi Tobias,

    I tried trap focus versus AF servo mode. Trap focus was FAR better, in fact I didnt get a single decent shot with servo mode. I understand the theory behind AF servo but it just didnt work under these conditions.

    However, I had better caution that these results were limited to a particular bunch of conditions, notably:
    1. I was using a fairly outmoded camera (Nikon D80). I really dont know how its servo mode compares to modern cameras.
    2. It was not a particularly bright light. Had it been bright, and had I been able to get the shutter speed up to 2 or 3000/sec then the difference may not have mattered.
    3. Bats are fairly lumbering critters. Swifts arent. There were swallows around and I couldnt get decent photos of them with any method.

    I wasnt suggesting that trap focusing was a panacea for flight shots - just another string in the bow for when it suits to use it.

    cheers
    Arron
    www.patonga.me : botany, wildlife, fishing, check it out

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    Default Re: Trap focus - does it really work

    Yeah, sure. I wasn't implying that you would try it on swifts. And swallows are all but impossible, I agree I think that for me using AI servo will work most of the time. The IDMKIV does a great job, and I think the 7D is very similar in terms of AF capabilities. I imagine the Nikon D300S wouldn't be bad either.
    Certainly an interesting idea, and one to be aware of. I've got to go and dig up a way to set it up now, just in case!
    Tobias Hayashi
    Canberra, Australia
    www.flickr.com/photos/callocephalon

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    Default Re: Trap focus - does it really work

    Actually, I looked at this in a reference book on the D80/D200 cameras last night, and the conclusion I came to is that there is no reason I can see why predictive autofocus (PA) wouldnt be working when using trap focus. Unfortunately, I cant be sure because there is nothing in these cameras or in the EXIF data that tells you whether it was on or not. Further its turned on automatically when it detects movement, and there is no way to turn it off.

    What makes it hard is that most people seem to misunderstand how PA works, and seem to be of the belief that when you are focus tracking a flying bird then the camera is taking samples and calculating the bird's path as the autofocus engine works to keep up with the bird's progress. This isnt true, the sampling and associated predictive activity only begin after you fully depress the shutter and the mirror begins to swing up (not sure which of those two is the trigger but the book reads like its the latter). It continues through that microsecond till shortly before the shutter opens during which time it must sample several times, apply the algorithm and nudge the autofocus motor a tiny amount. Note the key thing here is that focus has already been set before the sampling begins so if the focus was not perfect to begin with then PA will not save you, it will simply shift from one position of poor focus to another, which is what was happening during my trials when using normal shutter mode.

    The other piece of information the camera needs is whether the subject is moving or not, and the book reads as though in some shooting modes the camera makes this decision during the sampling period. Therefore all the information and triggers needed to invoke PA are present when trap focusing and as it turns on automatically and cant be turned off by the user then I doubt the camera can choose not to use it either. I think only a Nikon engineer would know.

    cheers
    Arron
    www.patonga.me : botany, wildlife, fishing, check it out

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    Default Re: Trap focus - does it really work

    This is really interesting thanks Arron. Never heard of this before. Like Tobias I reckon the AI servo on the 1DmkIV is pretty good so it would be interesting to see how the two compare. I think i would be frustrated by the scenario you described with something interesting suddenly happening out of the focus range etc.
    Can i ask where the colony is and where you think the best spot is? I live at Nth Rocks so I would love to go and have a look!
    That shot you have in the other thread with the water is fantastic!
    Cheers, Dave

    www.davidstowe.com.au WORKSHOPS

    Aus Life List IOC= 688 - Carpentarian Grasswren
    "So God created every winged bird according to its kind. And God saw that it was good." Genesis 1:21


  7. #7
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    Default Re: Trap focus - does it really work

    David, its in Parramatta Park. Just park in the Parramatta Leagues Club car park and walk down to the river from there.

    Or just put 33 48 15.02s 150 59 57.22e into Google Earth.

    Arron
    www.patonga.me : botany, wildlife, fishing, check it out

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    Default Re: Trap focus - does it really work

    Thanks Arron!!
    I'll have to try to get down there for sure - less than 5 minutes from home
    Cheers, Dave

    www.davidstowe.com.au WORKSHOPS

    Aus Life List IOC= 688 - Carpentarian Grasswren
    "So God created every winged bird according to its kind. And God saw that it was good." Genesis 1:21


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