Monday, May 6, 2013

Geek Time .....

A long time ago I invested in the development of a gadget called Triggertrap.  At the time I was using an intervalometer to do some time-lapse photography and I also had bought a remote trigger to operate the camera from a distance such as in a theatre.  With these thoughts buzzing in my head I read this investment opportunity where the device could trigger my camera or flash.  The trigger source could be sound, light change or laser.  It would also boast time-lapse capability, trigger a camera that had infra red remote control capability and the triggering thresholds could be adjusted as could the delay in triggering and retriggering.
Well, time passed.  So did the deadlines for delivery.  But it did arrive.  I had a brief look at it and, being busy, put it away for another time.

A couple of weeks ago I was passing by a shop and saw that it had air soft pistols for sale.  I remember wanting a simpler version of these when I was a boy.  The used lead pellets, were a one-shot deal and not very accurate.  I know because one or two friends of mine had one.  Today, it's a whole new scene.  The BB guns are replicas of the real thing.  So much so that the Gardai will not concede there is a difference as far as they are concerned.  The words "gun", "triggertrap" and "balloons" went through my mind so I bought a gun - a Taurus PT 92.  Why that one?  'Cos it had my initials in the name.  Yes I am that spontaneous and reasoning at times.  The gun will hold 26 BB projectiles that are 6mm in diameter.

Two BBs (or not 2 bb)

Last week I did some homework to see if I could possibly photograph a BB projectile bursting a balloon.  There were several variables.  Some I hadn't even considered.
The first was the speed of the BB.  The box had the figure of "407 fps" written on it.  That's very exact so I naturally immediately mistrusted it!  Preferring to work in metric I converted it and it came out at 124053 mm/sec.
I figured the only way to freeze that kind of motion was with flash.

Most people think that flash will freeze any motion but it won't simply because of the way it works.  A lot people think that reducing a speedlite's power is the equivalent of reducing its light output but it's not.  It's reducing the length of time the speedlite is putting out light.  Full power is the same level of light for a longer time.  Minimum power is the same light level for a shorter time.  So to get the BB to be frozen I needed a real short time.

Off to the Internet and I found this guy (Andy Gock) who had measured the flash duration of a number of models at different settings.  If I accept his findings as true then there was a surprise to find that the more expensive and well-known brands turned out NOT to be the ones with the shortest duration at minimum power.  In fact, the Yongnuo units I bought from China had the shortest times at 1/23041 sec.


Now that I had (probably) the fastest (shortest) flash times, the payoff for that was low light.  But I had another gizmo - a bracket that holds three speedlites so that would help.

So variables ........
I would pull the trigger.  There would be a bang and the BB would theoretically be travelling at 124053 mm/sec.  The sound from the bang would be travelling at (roughly) 340 m/sec or 340000 mm/sec so the sound would reach my triggertrap much quicker than the BB.
Working on the basis that a ballon and the triggertrap would be about 3 metres from the gun I worked out (roughly) that the sound would reach the balloon/triggertrap in about 0.0088 sec or 9 ms.  The BB would reach the same area in 0.024 sec or 24 ms.

                              Sound =====================>
        Gun bang =>                                                              Balloon / Triggertrap
                                   BB ======>
So theoretically I would need to build in a flash trigger delay of 15 ms to allow the BB to reach the balloon when the flash went off.

Well that was the theory.  Practical tests blew that out of the water!
I set up a board with some targets to see if it was close.  I used a series of small rectangles made of foam core and hinged to a board so that if the BB lost power going through the first or second target it could still knock over the next target.  It became clear that the foam core was too strong because the BBs were embedded in some of them and bounced off others so didn't necessarily travel in a straight line.  At one stage I just put up a  group of 3 targets together and shot them.  You can see from the photograph that the flash has gone off after the BB has hit 'cos they're in the air!

This particular BB has me puzzled.  It looks like it has bounced off the first target on the right but there is a 'comet tail' behind it that suggests it is travelling down from left to right.

 The BB is nowhere to be seen but the timing and placement are about right.

I spent 3 1/2 hours setting up my test case and the only thing I had to show for it at the end was that the gun at about 3 metres was about right, I had a collection of used BBs and bits of foam core all over the floor.  I called it a day.

Next day I was joined by Javier Leite who was interested in what I was doing and offered to help.
We set up a series of targets made from photo paper at roughly where the balloon and triggertrap would be and set about trying to find where the BB would be at a specific delay set on the triggertrap.  Once we had established where the BB would be when the flash went off we replaced the targets with the balloon on a stool.

After a few shots we found out some more.  Some balloons were fighting back!  I had asked Javier to bring safety glasses for himself and I was wearing glasses with plastic lenses.  We needed them.  The power of the gun is limited to less than a Joule so it won't break skin but it will hurt and probably could injure one's eyesight.  You can see from the picture below that the BB is rebounding from the balloon and heading back towards me!  I shot myself a few times and the BBs went in several different directions quite a few times.

They fight back you know.  You can see the depression on the right hand side where the BB has pushed into the balloon and will soon be heading back in my direction.

With trial and error we built in a delay into the Triggertrap to set off the flash 3 ms after the bang was detected - a long way from the 15 ms I had thought would be needed.  We also figured the gun needed to be at full pressure before each shot so charging it became a ritual each time.  Even so, even keeping as much as we could as constant as we could we still found some shots were taken after the balloon was burst and others we could see the BB at least a metre away.  That could be down to tiny variations in the BB, the firing mechanism, the Triggertrap's reaction time, we have no idea.
But we did get some shots and, on the plus side, it kept me off the streets where I'd only be stealing cars or mugging little old ladies.  ;-)

 This was our first success.  Sad the things you get excited about ....

 This is where I had the ambient light (used to allow me aim the gun) a little too bright and you can see the balloon before and after shooting.

 One of those shots that we couldn't predict.  The balloon is burst and the BB is on its way out.

 Another BB exit shot.

 This was interesting.  The BB was trapped inside the balloon.
The BB has burst the balloon and is on its way through.

 The setup.
A - shooting position.
B - 3 speedlites set at 1/128 power
C - Triggertrap
D - Camera position
E - Board to absorb BBs.


Next time ....
I'm hoping to set up something more spectacular so it's a picture rather than a record of a balloon bursting no matter how fascinating that is to see.  And then the Triggertrap has other triggering methods such as the laser sensor which will trigger when sensed or broken.  That could need a rig .....

Andy Gock's flash duration measurements tables.
Speed of sound: Wikipedia  and Google

1 comment:

  1. I will admit that I don't read your blog as often as I probably should, but this really really caught my attention. Something I would really love to try now. Especially since I could set my camera on 14fps. That would make for some interesting captures.