Stereo Camera Flash Delay

For the stereo photographer


The SONY mirrorless cameras have gained popularity with stereo photographers. For some, the selection of the camera model for a stereo camera rig is not only based on cost but also the potential separation distance between camera lenses. The A5100 offers closer lens separation compared to similar models. The problem however is that some SONY entry level camera models like the A5100 don’t support external electronic flash.
For Stereo Flash photography, it is imperative to only have a single flash operating. Whilst the A5100 has a popup flash, it was found to cause problems. When a single flash is enabled, only one camera would record the flash exposure.
So, what is the problem using A5100 popup flash?
Put in simple terms, from pressing shutter release to recording the exposure there are variable timing components at play. When only one flash is enabled, the timing differs between cameras. The cameras therefore can fire at different times causing one of the cameras to completely miss the flash.
Simple testing using a single A5100 and its popup flash has revealed some interesting behaviours. In the results of the simple test shown below, the time between pressing shutter and the pre-flash is variable depending on distance to subject. In this sample measurement, the final flash fires after 142mS. Whereas the typical delay time for the shutter to open with the A5100’s flash disabled is about in the 21-22ms region!

Shutter delay, by Mark McAndrew

The relationship between the shutter release in the top trace verses the flash energy that is emitted.
Important Note: The results shown above are derived from the Poormans Oscilloscope
The issues using flash has historically plagued stereo photographers. For some stereo camera rigs, the solution is to have a pre-determined delay before firing the flash. The delay is activated by the shutter release then firing the flash coinciding with both cameras’ shutters opening.
The Stereo Camera Flash Delay circuit was designed to overcome timing inconsistencies as well as provide support for the usage of many types of external electronic flashes.
Building a suitable timing circuit can use many different types of circuit design. Stereo Photography forums have provided many ideas to simulate the best technology for a solution. Fortunately, one forum member, Timo Puhakka, discovered a flash delay circuit originally intended to use an electronic flash triggered by a cameras “flash bulb” contacts This type of synchronisation is known as M-sync. The M sync flash mode closes the contacts a few milliseconds before the shutter is open, to give the flashbulb time to reach peak brightness before exposing the film. The link details for the M-Sync project.

The circuit is based on a microcontroller with a minimal parts count. The circuit incorporates a variable delay controlled by a potentiometer. The variable timing allows customisation for any flash for any camera timing. The microcontroller used is an Atmel ATtiny85. The great thing about this processor is that it is the same size as the very famous 555 timer chip, yet offering a programable outcome. The other advantage of the processor is that it can be powered by 3 volts, which makes it very compatible with the SONY A5100 that also operates with a 3 volt power source. The processor also has a comparatively low power consumption enabling it to be powered by a 3 volt button battery. Programming this microcontroller was achieved by using the popular Arduino IDE software platform with special addon software specifically for ATtiny processors. The cost of the microcontroller was inexpensive. In Australia it cost a modest $6.
Adapting M-sync for stereo camera operation
Despite the suitability of the m-sync project for the stereo camera flash delay, a number of modifications were required to customise the design. The development of the project took place in both Canada and Australia. The initial “instructables” based prototype circuit had a number of problematic interfacing issues for both the shutter release contacts and flash operation. The list below lists both modifications and developments required for stereo operation.

1.       Modifying Circuit to run from 3volts (original M-Sync operated form 6 volts)
2.       Alternate Opto coupler design to fire modern recent Electronic flashes (to overcome lockups with some flash models)
3.       Modifying sketch to trigger on high to low (ground) camera shutter switch (to eliminate complex switches)
4.       Utilising internal pullup resistor in ATtiny85 for shutter release switch port (to reduce parts count)
5.       Adding CPU activity LED (to give user a level of confidence that unit is operating)
6.       Adding transistor driver to enable older Thyristor flashes (using low reverse voltage)
7.       Drawing revised circuits
8.       Timing measurement diagrams (screen shots using Poormans Oscilloscope to confirm the timing of the circuit)
9.       Devising a technique to measure the flash delay time (to confirm absolute delay time)


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