Friday, September 18, 2009

External shutter

One problem with using a position detector such as a photo beam to automatically capture a moving subject is the shutter lag - typically between 50 and 100ms for modern DSLRs. For a large or slow object this may be no big deal but smaller objects photographed up close may have moved out of the picture (or at least out of focus) in this time.

For an object that is moving predictable (e.g. falling under gravity) it may be practical to simply point the camera at where the object will be 100ms later - so long as the shutter lag is repeatable. Alternatively, you can hold the shutter open and use the detector to trigger a flash instead - but this requires working in the dark.

For insects in flight, neither is practical, and it seems the only route to success is to shorten the shutter lag. You might think that using mirror lockup or live-view there would be no reason for a long shutter lag on a modern DSLR but unfortunately it seems that there is still quite a lag. Another route - suggested by the excellent and inspiring work by Fotoopa at - is to attach an external shutter in front of the lens - and this is the route I started playing with.

A couple of years ago, I bought an Ilex electronic shutter on eBay. Having worked out that it needed a 3ms pulse at 192v to open it (and then 24v to keep it open), and looked at the cost of controller boards that delivered such strange requirements, I set it aside for a while. It has been sitting on my shelf mocking me ever since.

Yesterday I decided to have another shot at it. The flash circuit from a disposable camera is perfect for delivering pulses at high voltage, it seemed to me, so I set about disassembling one (caution - the capacitor in a flash gun can hold a charge of 300V and may retain it for a long time after the battery is removed. DO NOT TOUCH any contacts - or the circuit board - without first ensuring that it is discharged using a screwdriver across the capacitor terminals).

I measured the voltage generated in the capacitor at about 300v - a bit more than the 192 I was after. The shutter's solenoid resistance is 130 ohms, and the capacitor was 120 uF, suggesting it would discharge in about 15 milliseconds. 300v for 15 ms didn't sound THAT much more than 192v for 4 milliseconds so I decided to try it anyway.

Success (of a sort) in that the shutter snapped open nicely, then shut again a little while later. However there was a disconcerting puff of smoke...

Fortunately it seems to have survived, but I think I need to dial down the voltage a bit and reduce the time (probably by adding a series resistor and reducing the capacitance, respectively).

No comments: