Problems with Flashes for Cave Photography on EOS Cameras

The Canon EOS range of digital SLR cameras give superb results underground, but we had real problems getting them to work with off camera flashes. What we were looking for was an affordable way to trigger multiple flashes in sync with the camera shutter, and without a flash mounted directly on the camera from providing any light for the scene (see the bit about fogging in the previous post for why).

Canon's own flashes feature remote triggering with master and slave modes, but there are two major problems with these. Firstly they are expensive (GBP 200 or more per flash, and we need several for many of our pictures). Secondly in practise this system does not work well underground. The cave environment is dark, damp and muddy. These conditions simply eat up light, and the master/slave flash triggering system built into the Canon flashes is just not sensitive enough. It only works with direct line of sight between flashes, and in most cave work we want to hide the flashes. In a studio setting which this system was designed for, there is plenty of reflected light to trigger the remote flashes, and often the master (on camera) flash is simply bounced off a wall, ceiling or reflector to avoid directly illumination of the scene. But underground there is simply not enough reflected light to work except over fairly close distances.

So we need a much more sensitive slave flash trigger. The answer to this is the Firefly slave unit. These were designed for cavers by cavers and work very well underground. Line of sight triggering distances can be as much as 1000m! They work very well round corners too, so flashes can be hidden out of sight and will still fire.

This just leaves the question of which model of flashgun to use. The firefly comes with a hot shoe connector, so any flash with a hot shoe will fit. There is no automatic exposure control, so we need flashes which have a manual mode. Auto sensing flashes will be fooled by reflections from nearby objects (often not visible in the actual photograph) and under expose thinking that they have brightly lit up the scene (instead of having just brightly lit up a bit of nearby rock off camera).

Caves are dark, light eating places, so we need powerful flashes too. This brings us back to the cost problem. We need several powerful flashes which we will use in manual mode. So we will not be taking advantage of any fancy features. So there is no point in spending a lot of money on Canon flashes. I buy the cheapest brightest second hand flashes I can find. Ebay is a great source of these.

That leaves one last problem, how do we get the camera to trigger the remote flashes? Again there is no point in putting an expensive dedicated flash on the camera, as all we need it to do is fire in manual mode to trigger the other flashes. There are two other issues here as well. We do not want the flash on the camera to provide any direct light to the scene due the to fogging problem (see previous post), and we do not want to damage our camera (many cheap flashes have very high voltages on the trigger contacts which can damage modern electronic cameras).

So how do we get the flash to trigger the firefly slaves without directly lighting the scene? There are several tricks you can use. The simplest is to mask the flash with black electrical tape. This allows through the infra-red radiation which the Firefly slaves are sensitive to, but which the camera is not. Other materials can be used the same way. Unexposed, developed slide film is one, but in this digital age is not so easy to come by. We'll look at some alternatives another time.

The voltage problem is solved by using a flash with low voltage trigger contacts (Canon rate their cameras as safe with a maximum hot shoe trigger voltage of 6V). The trick is finding a model which is safe, but which will work with the Firefly slave units. The obvious thing to do is buy a cheaper brand of flash which is compatible with Canon digital SLRs, but this has one big pitfall.
The latest Canon cameras (including all their digital SLR models) feature the E-TTL flash exposure control system, and this does not work with the slave units.

The problem with E-TTL is that it actually fires the flash twice. The two flashes are very close together, so it is not obvious to the eye. The first burst from the flash is used to gauge the exposure settings to use for taking the actual picture. The picture itself is then taken on the second flash. So what happens is that the first burst triggers all the remote flashes attached to the slave units, and these are then all discharged when the second burst is fired from the on camera flash. As the on camera flash is masked to prevent it lighting up the scene, we end up with a black picture! And we do not want the automatic exposure system in the first place because we are not using the on camera flash to light up the scene in the first place.

So the trick is to find a flash which has a low voltage hot shoe trigger, but which does not feature the E-TTL system. Ironically these are the flashes which are often described as not compatible with Canon digital SLRs! So we need to find flashes which are compatible with the later Canon EOS film SLRs cameras, as these only had TTL flash metering, but still only support up to 6V trigger voltage on the hot shoe contacts. These are the flashes which do everything we need, and thankfully they are not so expensive and are often available second hand.

You can alternatively turn off the E-TTL feature on some of the higher end Canon EX flash guns, or you can use the Firefly3 slave unit which can be programmed to ignore the pre flash. However both these options are significantly more expensive, so only make sense if you already own this equipment and want to make it work for you.