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Topic: Camera strobe modification (Read 8 times) previous topic - next topic

focalist

Jan 23, 2011, 05:29 pm Last Edit: Jan 23, 2011, 05:38 pm by focalist Reason: 1
Recently, I've been using the Arduino to do some high-speed camera strobe triggering to accomplish stop-motion photography.  Standard project done a thousand times- electret microphone, sound triggered.  Typically, when using an Airsoft gun to shoot Christmas Tree bulbs, I use a 75ms delay from sense of the sound to firing of the strobe.  The shutter is otherwise open.

The problem I'm running into is that the flash duration from my Interfit 150 studio head is way too long, even dialed back to minimum power.  I'm getting flash durations of around 1/2000sec, where I'd like to be dealing with flash durations more in the range of 1/10,000 or less, to freeze as much motion as possible.  As the flash duration IS the exposure time, 1/2000 sec is pretty slow when dealing with exploding glass.

So, I've done research and have thought of several solutions (short of glacialwanderer's spark-gap flash).

One is modification of a cheapo flash (like from a cheapo disposable camera).  Inherently the overall power is lower of course, but it's typically a shorter duration flash to get that.  As the trigger on these is high voltage, I'll probably use a 5v relay (they have 20msec switch time, which I can take off the strobe delay).  I don't think I've got a transistor or SCR handy in that HV range, but a PCB-mount mini-relay should suffice.

In reading, one method is to replace the flash storage capacitor with a smaller one, resulting in lower energy, which is shorter flash.  I may try this if i have a cap that's in the right range.  If I don't, I was thinking about being a real idiot:

In the flash, an oscillator is fed through a transformer to charge a HV cap.  Instead of reducing the cap, couldn't I instead place a bleeder resistor across the cap, and feed the transformer PWM?  In this way I could control the charge level of the storage capacitor by PWM.  Sounds good on paper until I start working the WATTAGE of that bleeder, I'd need a very high wattage (high voltage at high current storage) resistor, wouldn't I?

Thoughts?

Last but not least, also looking at trying to use a couple of power led's I've got around.. led's react in the nanosecond range...  I know the actual power is low compared to xenon- orders of magnitude, in fact- but proximity to the subject (inches) may compensate for that- nothing to lose in trying....
When the testing is complete there will be... cake.

Udo Klein

If you reduce the charge level (at constant capacity) you will reduce the voltage. Thus your flash might not trigger anymore.

I think this won't work. What might work (but I never tried it) could be to interrupt the current to the flash tube. No clue if this is easy. As I said I never tried it.

Udo
Check out my experiments http://blog.blinkenlight.net

focalist

#2
Jan 23, 2011, 07:27 pm Last Edit: Jan 23, 2011, 07:36 pm by focalist Reason: 1
Well, I've been looking at Sam Wasserman's strobe pages and links frome there, one of which details the modification of a kodak max disposable camera flash, by replacement of the storage cap- from 160uf to a 5uf (chosen because the author had it in junk box), resulting in a power reduction (duration reduction) to 1/36th the original.  He goes on to make a repeating strobe, but for the purposes I need, simply the reduction in flash duration is all that is needed.  For most photography, you'd like a longer flash duration.  For this, the power isn't as important as the speed.

Excellent discussion of abuse of strobes in general:http://repairfaq.cis.upenn.edu/sam/strbfaq.htm#strbnsc

Assuming we are talking typical cheapo, that's a 1ms strobe.. which already is faster (lower power) than the studio flash.  If the result is real- 36 times faster - that's a VERY usable interval.

For stop-motion, you don't need a ton of light- but you need that light to come on and off VERY fast.

What you've suggested is called a quenching circuit, and it's what is used in high-end strobes costing thousands... the damping circuit to interrupt the arc takes HUGE energy dissapation, methinks..
When the testing is complete there will be... cake.

mmcp42

I would think high power LEDs is the way to go
you have to watch colour temp as they are very monochromatic
(can you be slightly monochromatic?)

the big plus is you shoud be able to fire them rapidly and often with very little circuitry
there are only 10 types of people
them that understands binary
and them that doesn't

focalist

#4
Jan 23, 2011, 07:44 pm Last Edit: Jan 23, 2011, 08:11 pm by focalist Reason: 1
Well, I now have a reason to rip apart something I've been using for less nefarious purposes... Home depot sells a 10w CREE LED light- warm white, 2700K, CRI in the high 80's.  I got one and have been using it as a great little portable modeling light.  Coupled with a pair of Interfit 150's and some creative reflector work, it makes for a cheap (under $350) and very usable portable portrait "studio" setup.  I recently posted some shots taken with the CREE LED light.

However, there is a significant delay between applying power and the led actually striking.. enough that it's noticable.  I'm sure that's a function of the driver circuit.

I *should* be able to place some type of control circuit in the works, and be able to use it... yet another hack to try ;)

Here's a shot from that series.  No strobes were used for this shot, it was "Rembrandt" lit 45 degrees off subject on both sides with 75w of halogen at about three feet through silk softboxes for fill.. but the main direct lighting is that 10w CREE.  It's quite visible in the shadow tones and uplighting.. the halogens are at her eye level.  If anything, it's a bit too deep in rendering of the reds...  I am not displeased with the rendering, it works really well if used in concert with more standard light sources.

When the testing is complete there will be... cake.

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