update: here are the pictures from dives in cozumel, the 60 degree angle of this strobe really only makes it useful for macro shots, gallery link below all pictures were taken with the setup detailed below
Prepping for a dive vacation and the last time I took pictures underwater was with a 35mm Nikonos V. These can be had with a strobe for ~$300 used on eBay. I decided to pass on film preferring digital's ability to instantly review exposures and make corrections. I have a Nikon D200 digital SLR that I really enjoy. Unfortunately the cheapest housing available, from ikelite, runs $1400 with a port. Since the D200 has been out of production several years I tried to find a used housing on the cheap. All the sellers I found (craigslist, eBay and forums) were within 10% of retail, above in some cases. Maybe after these used housing collect dust for a few more years I'll be able to grab one, doesn't make sense to pay $1300 for a used housing to put a now ~$300 body into. I'd sooner put a newer full frame d700 in a new housing. That leaves me with a housed point and shoot. Here is the setup I ended up with.
Configuration
- Canon A720IS 8mp
- Canon WP-DC6 Underwater housing
- Intova ISS 2000 Slave Strobe
- Fiber optic strobe sync cable
The A720IS can be had for ~$100 used, the WP-DC6 is actually designed for the A710 but fits the A720, only slight modifications are required to enable the camera/playback switch and zoom dial. The housing will run you $100-$150 if you get it new or are able to find it used. The Intova strobe with the mounting bracket and arm is $135 new, fiber cable is $15. At almost $400 this setup approaches the cost of SeaLife and Sea&Sea basic digital setups that start at $600 without a strobe.
Canon A720 IS
Canon's A series provides full manual control. The A720 has 35mm-210mm zoom lens (35mm equivalent) at f2.8-f8 that includes optical image stabilization.
I prefer to shoot in raw. As an amateur photographer I make lots of mistakes and don't mind spending time tweaking a good shot afterwards. Luckily there is a 3rd party firmware for most Canons that unlocks a lot of features. This firmware, found at CHDK, can be copied to an SD card and loaded on demand, no need to permanently modify your (in my case borrowed) camera. In addition to saving raw images it enables flash sync above the stock 1/500, up to 1/50,000+. This is possible because the A720 IS has an electronic shutter in addition to a mechanical shutter. These high sync speeds will be useful for photographs that are shot up towards the surface with the sun visible. By setting a fast enough shutter, the sun can be properly exposed while still allowing the strobe to fill in near subjects. Underwaterphotographyguide.com covers this in more detail here
Intova ISS 2000 Slave Strobe
This strobe is cheap. The only other competitors I'm aware of are the Sea&Sea YS-17 and the Bonica Neon-XP, both more expensive. The strobe has two optical sensors on the front rim of the housing. When either of these sensors detects a flash it triggers the strobe. The A720 in manual mode fires only one flash, there is no preflash. The Intova strobe has a pre-flash setting that forces it to ignore 0-4 flashes before firing, with the A720 in manual this is set to 0. Having the external strobe will reduce backscatter but the strobe on the camera housing needs to be visible in order to trigger the external strobe. The fiber optic cable gets around the problem by attaching in front of the camera's built-in strobe and transmitting the light directly into the external strobe's sensor. This setup is more reliable in my experiments, maybe 1/4 shots without the fiber cable would fail to trigger the external strobe. By piping the light direct to the strobe you can also duct tape over the flash to completely block the camera's flash. The strobe also includes 4 power settings, 10% 25% 50% 100%. With fresh NiMH 2700mAh batteries I was getting ~8second recycle time.
Test shots
I was interested in the range of the strobe underwater. I took two test shots at the far wall 20' away. Both shots were at base ISO 80, 35mm zoom 1/500 shutter. The first shot is at f5.0 and the second at f2.8.
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 F5.0 Above |  F2.8 Below |
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Here is a 1:1 resolution crop of each image
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Here is a shot at a wall 4' away. 35mm 1/500 F2.8 ISO80.
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