Alley in Pisa, Italy

by Lars Kotthoff

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I came across the term trap focus on the autofocus page of wikipedia, and it seems to be a very handy feature that triggers the shutter release when a subject moves into the focal plane. This feature seems to be built into only a small number of cameras though (possibly the ones with high FPS typically used for sports).

The wikipedia page also linked to an article called almost trap focus for cameras that don't support the feature natively. However, the steps described in the article seem to be more suited for macro or close-up photography, and that too using a FTM lens.

So, is there any way to trap focus on DSLRs without the feature, and without using specialized equipment?

As a reference point, I have the Canon EOS 550D with the 18-55mm kit lens.

Note: This feature could probably be enabled on P&S cameras using CHDK scripts (something along the lines of a motion detection script), but I haven't come across any solutions for DSLRs.

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I can't see this being possible on a compact/P&S camera because they only have contrast-detect autofocus. Contrast-detect autofocus has no ability to know whether something is "in focus" or not, and it needs to wind the autofocus motor back and forth to see it getting "sharper" or "blurrier" to do any detection. It will only know when something was in focus once it's gone too far and it starts getting more blurry again. Then it winds the focus motor back the other way until it's back there. Obviously this means it cannot have an AF confirm indicator, or trap focus for that matter. –  thomasrutter Mar 10 '11 at 6:58
    
There are several solutions listed in the references immediately after the Trap Focus section of the Wikipedia article you refer to. I have just tried the Nikon one with my D5000 and it works reasonably well. –  ElendilTheTall Mar 10 '11 at 10:16
    
@ElendilTheTall: The techniques seem to be primarily for the Nikon & Pentax cameras. Haven't seen a proper one for Canon yet. –  ab.aditya Mar 10 '11 at 11:06
    
The Nikon technique only makes use of AF-S and using the AE-L button to autofocus. I thought these were fairly universal features. I'm sure Canons have AF-S; do they not have the ability to autofocus with a button other than the shutter button? –  ElendilTheTall Mar 10 '11 at 11:25
    
@thomasrutter you can implement a version of trap focus without a phase detect AF system if you have a good blur metric and set a threshold at which the camera fires. This sounds like an interesting CHDK project. –  Matt Grum Mar 10 '11 at 14:04
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up vote 3 down vote accepted

Magic Lantern, a freeware firmware hack, is available for your camera and it supports Trap Focus. It's inside the Focus menu.

http://magiclantern.wikia.com/

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Thanks for the link. Looks very useful and similar to CHDK (for Canon P&S). I'll try it out and share the results. A note for the others - Magic Lantern is currently available only for the 5DMkII & 550D with beta support of 500D & 7D work in progress. –  ab.aditya Mar 11 '11 at 4:49
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