Trap Focus experiment - the theory

This morning I received the final missing piece of equipment I need to do some interesting studio experiments with trap focusing. My aim is to get a macro picture of an ice cube dropping into a glass of water. The piece of equipment I was missing was a wireless shutter release for my Nikon D200, I found a cheap one on eBay (<£10) and ordered it last week and when it arrived this morning I tested it and I’m happy that it works. All I need now is some peace and quiet to work in, oh, and darkness cos I want the pictures under studio flash conditions and the window in my lounge let’s in way too much light during the day, even with the curtains drawn, so I’m twiddling my thumbs till tonight.
For those of you who are unfamiliar with Trap Focusing technique or what it is, I’ll try and explain it a little here.
Trap Focus allows the camera to be pre-focused at a specific point and have the shutter released automatically as soon as a subject passes through it. Provided you can predict the path of the subject, this technique can be very effective. If your a Nikon DSLR user then the technique is quite simple to set up, all you need to do is set the camera to Single servo autofocus mode, Single area AF mode (selecting the area your subject will pass through) and set the cameras AF activation mode to AF-ON only mode (so that focusing is only performed when the AF button is pressed and not when the shutter is depressed) and lastly making sure the focus mode priority setting is set to Focus only…this last setting normally prevents the camera from taking an out of focus shot if the lens has failed to achieve focus, but of course here were preventing the camera from telling the lens to refocus and just wait until it sees something that is in focus. If your using another make of camera then the chances of getting this to work are quite slim, certainly Cannon DSLR’s cannot achieve this and I suspect that a lot of other makes won’t do it either. It’s not that the Nikon cameras are any cleverer than other makes, it’s just that the auto focusing system designed by Nikon has a strange and unintentional feature which leaves the camera in limbo where it can’t tell the lens to try and refocus and yet it won’t release the shutter until it has achieved focus.

So, in my particular case I’ll be setting the camera to use a focus area just above the glass of water and placing a piece of card with some lines drawn on it on top of the glass (giving something for the camera to focus on) pressing the AF-ON button to focus the camera and then removing the piece of card so that the camera has nothing to focus on and then using the wireless shutter release to activate the cameras shutter, then whilst keeping the shutter pressed (the camera won’t take a picture at this point because it will think it’s failed to achieve focus) dropping an ice cube into the glass. As soon as the cube passes through the selected focus area the camera should suddenly wake up and release the shutter and hey presto!

Hmmm…Well that’s the theory anyway, I dare say that it’s going to take a while for me to get it to work just the way I want it to and I fully expect to be posting out of focus images of a strange man, looking a bit bemused holding ice cubes over a glass of water with one hand and a small radio device in the other… oh and the wet patch on his groin… it’s just water… honest! ;O)

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