Goofed again

With apologies to Britney Spears, but I did it again. I’d got such plans for my specially-modified diving camera. The short story is that, in an effort to replace the camera’s built-in flash with an external one, I made neither of them work. But the best mistakes are the ones you can learn from, so here’s my sad tale of being beaten with the clue-stick.

Now, the first rule of underwater photography is to minimise the amount of water between you and the subject. What may then come-in at second place is to never use direct lighting. Any light that is near the lens will illuminate the silt in the water. So best practice is to put the light(s) out to the side(s) and shine them in at the subject. The ideal then is the smallest amount of water between lens and subject, with no light passing through it from the camera. The subject is lit, but not the silt between subject and lens.

My underwater camera is a digital compact. It has a small built-in flash. The camera housing is transparent, so the flash light passes through it. There is a bulge on the front of the housing to accommodate the lens. Because the flash is so close to the lens, this bulge throws a shadow to the bottom right of the frame. To reduce this and to reduce the backscatter from silt, Canon provide a large diffuser in front of the flash.

I like to use an additional external flash because it can be aimed-in from the side to reduce backscatter. The flash has a trigger cell, so fires when it sees the camera’s internal flash. I had been struggling to balance the internal and external flashes so I came-up with a cunning plan to mask the internal flash so that it could only act as a trigger. Some expensive housings and flashguns do this with fibre optic cables – the internal flash is led through the fibre optic to the external flash’s sensor. But I don’t have this type of housing or flashgun. Simples (I thought), mask the internal flash so that the external one can see it, but it can’t illuminate the subject. So I made a new diffuser and stuck a bit of Mylar mirror tile on it. The internal flash’s light is bounced back so that the external flash’s trigger can see it.

I tried this setup in a flooded quarry and it seemed to work ok. So off we went on a diving trip. What I had not thought through is that the quarry shots were of fellow divers in open water, and the diving trip would include close-ups of wiggly things on walls and wrecks.

Wth the mirror diffuser – a lot of light goes out to the left and bounces back onto the subject if there is a nearby surface.

What happened is that the mirror bounced a lot of light out to the left. If I was near a surface on my left, it got lit by this flash. The external flashgun saw this and turned off early to get the exposure right. Result – wierd lighting and underexposure. I tried taking the mirrored diffuser off entirely, but this made things worse. The bulge on the housing over the lens now threw a shadow to the right. The pictures were awful.

Internal flash and no diffuser – the lens casts a shadow.

So, plan B. Put the original translucent diffuser back on. Turn down the power of the internal flash so that it underexposes. Tweak the power of the external flash to get the exposure right.

Translucent diffuser and external flash – this is what I intended all along. Yes, there is a shadow at bottom right, but my big fat body was casting that from the window behind me.

So I’ve gone in a circle to come back to what Canon originally built. They do say that the best mistakes are the ones you can learn from. Or, I suppose, that other people can learn from if it’s one of those self-limiting mistakes. Anyway, this has justified another diving trip, as I’ve got to go and try-out my light settings to get the internal/external flash balance right. I know I can do it shooting red cushions and fur fabric in dry comfort supported by tea, but, well, diving.