David Gibson

"How to play the flute. Well you blow here and move your fingers up and down here"
[ Monty Python's Flying Circus, 1972 ]

How to make a very big flashgun. Well you connect lots of little flashguns together. That's about it really, but perhaps some words of explanation would be useful.

Electronic flashguns are not, generally, as powerful as flash bulbs (see table), and they can be prone to problems because the high humidity (or outright wetness) of caves affects the high-voltage, high-impedance electronics. Flashbulbs are expensive, and the connections are prone to failure due to cave mud. In the last few years flashguns have started to rival flashbulbs, and a £30 unit is now of a similar power to an AG1/AG3 flashbulb. The problem is that they are still not cave proof, and they are more expensive to replace when they break. A flashbulb still has the edge on an electronic flashgun when it comes to photographing moving water - the very short duration of an electronic flash "freezes" a cascade into a few drips.

An alternative to a big flash gun is to wire up several small ones. The advantage here is that each individual flash-gun is cheap and does not cost much to replace. In addition, if it develops a fault underground you will still be left with most of your light source. Small flashguns can be obtained on the second-hand market, or you could try asking for damaged stock at a camera shop. I managed to find an electrical junk shop which was selling, as manufacturer's surplus stock, the insides of flashguns at £2 each. Using four identical flashguns will double the effective guide-number. The table shows, though, that you need eight "Sunpaks" to equal one PF1B.

If you accept that each flashgun will still have its own battery then you only need to connect the trigger contacts together in parallel. However, you would need to make sure that you get the polarity correct on each terminal, and you need to stop current flow between the units. There two solutions: you can either use an opto-isolator or relay at each input; or you connect all the inputs through bridges as shown in Fig.1 so that the polarity of the connection becomes unimportant. The bridges should be capable of withstanding 400V p.i.v. and a current of a few amps. There are numerous devices to choose from, or you could use individual diodes.

You may decide that you want to common the battery terminals too. This makes things complicated because the flash contacts are often "floating" with respect to both battery terminals. The only solution in this circumstance is to use a relay or opto-isolator at each trigger contact.

A third option is to build a slave-flash unit into each flashgun. This gets round the problem of making connections to the trigger contacts and makes a more versatile system.

This works on the basis that a very low current is available at the terminals of the flashgun. The bridge allows for the fact that the polarity is not known. The current charges up the capacitor, clamped by the zener diode. When the photo-darlington conducts the SCR shorts out the terminals; now the bridge allows for possible bi-directional current flow during the resonant discharge of the trigger circuit.

You could wire the innards of several guns into one box with a transparent lid, or you could simply mount a row of flashguns onto a long flash-bar. The flash (fig 1) and slave (fig 2) circuits are both suitable for use with flashbulbs as well as electronic flashes.

Comparison of flashguns

The table has been prepared from the collection of flashguns and flashbulbs that I have lying around. The guide-number (GN) is quoted for a film speed of 100ASA. Guide-number is numerical-aperture x distance of flash to subject so doubling the guide-number corresponds to a fourfold increase in light energy. This energy is often measured in beam-candlepower-seconds (BCPS). The relationship between GN and BCPS is (and I don't want to get too bogged down in technical detail here) dependent on the definition of film speed and on the requirements of exposure latitude, contrast ratio and so on.

Using the relationship BCPS ÷ 220xGN²/(ASA-film-speed) allows the relative energy of the devices to be tabulated.

(For those who may be interested, the energy in BCPS is the product of energy [lumens] x time / solid-beam-angle [steradians]. A flash tube has an efficiency of about 50 lumen/watt (ie. 50 lumen-second/joule), and a GN of 32m @ 100ASA is usually about 50 to 75J of electrical energy (depending on reflector & beam angle). That is the starting point if you are designing your own flashgun).

Flashbulb manufacturers appear notorious for wild attempts at conversion between metres and feet, and from one film speed to another. The tolerances given in the table are included to highlight the conversion errors, though none are significant, given the wide variation in photographic subjects and the latitude of the film. Some of the difference in guide-number of the flashbulbs can be attributed to different methods of measurement or different reflectors. The available energy of the flash obviously depends strongly on the efficiency and beam-angle of the reflector used. It also depends on the reflectance of an "average" room. Outdoors, or in a room with dark walls, the guide-number is lower. In caves it can vary widely.