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Old Oct 25, 2011, 4:29 PM   #1
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Default Flashgun "light beamer" - DIY project

G'day all

Many years ago I wanted to do lots of night pix of possums in our aussie trees - and with a camera to subject distance of 10m ~ 25m [30ft to 75ft] is simply wasn't possible with the flash gear that I had

So I made up a telescopic sort of light concentrator, made from a cardboard tube + a fresnel lens + the external flash gun I already had. Project cost < $10

Testing the fresnel lens before buying - and it was one of those to help book readers - it seemed to focus at about 200mm [8inches]. When the tube was 200mm long, I got a perfect image of the flash-gun's flash tube onto the trees, so I had to cut 25% of the tube length to fuzzy it up a bit

images
1- here is the completed device ...


2- here is the cut down fresnel lens bit


3- daytime shot of target / test tree > distance camera to tree about 20metres, showing my backyard & clothes drying line too !


4- flash in use - panasonic camera zoom at 2x [about 55mm]


5- flash with extender - typical result - zoom at 12x [about 400mm]


Hope this helps others who feel like spending a rainy night doing something different :-)

Regards, Phil
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Old Oct 25, 2011, 6:39 PM   #2
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Ooooo ... Duck Tape ... I like it.
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Old Oct 28, 2011, 9:44 AM   #3
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Perfect, great idea!

I might have wrapped the entire tube in duct tape just to make it look more manly.
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Old Oct 28, 2011, 4:42 PM   #4
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My problem is in finding and identifying the correct fresnel screen for a specified distance, and also how to know the distance of the screen from the flash. At present I am not having much success.
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Old Oct 28, 2011, 6:55 PM   #5
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G'day grounded

The fresnel lens I bought was a $5 job from a discount store, sold as a reading aid for 'the elderly' of which I am one, having had my 65th birthday !! ... It is meant to be held "about 6" to 8" away from the page"

Holding a torch behind it and moving the torch bulb in & out, I worked out that its focal length was "about 8 inches" - so I started with this measurement

As mentioned earlier - when it was placed 8" in front of the flash, it gave a perfect sharp image of the flash tube onto the trees next door - so the cardboard tube was cut to 6" to make it go out-of-focus but still to give a concentrated burst of light onto the trees

Hope this helps
Regards, Phil
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Old Oct 28, 2011, 7:27 PM   #6
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Since 8" is about 200mm, and the diopter # is 1/focal length (in meters), you should look for approximately a 5 diopter value for the lens, to get an equivalent of what Phil has done.
Try Wikipedia, using the term "collimator", and you should find info that will help you out.

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