Only a handful of manufacturers worldwide produce flashguns designed and built for this purpose, and one of these is Japanese specialist Sea & Sea. It’s been around for as long as I can remember and has recently released a new model, the YS-D2.

Through The Lens (TTL)
Modern cameras come in all shapes, sizes and designs. Point-and-shoot, zoom, compact, bridge, mirrorless, cropped or full-frame sensor digital SLR – all have one thing in common, and that’s the ability to fire either an on-board or camera-specific off-board flashgun.
Each type of camera and each maker uses different protocols to assess the amount of artificial light needed to create an accurate exposure. This is primarily done by firing a series of pre-flashes, which are reflected off the subject and received by a camera sensor.
This protocol is commonly referred to as “through the lens metering” or TTL.
To confuse the issue there are several versions: Advanced (A-TTL), evaluative (E-TTL), and a protocol used by underwater strobe-makers called Optical Synch TTL (S-TTL).
Of course, these flashguns give the user the ability to set the output manually (M) as well.
The YS-D2 is compatible with virtually every make and model of digital camera. It deals with camera flash modes using A-TTL, E-TTL and S-TTL, plus what the maker calls “oddball flash protocols” by employing a custom mode. It is also compatible with cameras with manual pre-flash and manual non-flash modes.

The Design
The YS-D2 has a power output measured as an on-land guide number of GN32 at ISO 100 without a diffuser; this drops to GN24 and GN20, depending on which diffuser is fitted.
It delivers white light at a colour temperature of 5600° Kelvin through an 80° beam angle (without diffuser) or 100° (with 100 diffuser) and 120° (with 120 diffuser).
It also has a built-in LED focus light with two output settings and is powered by four AA batteries, which can be rechargeable ni-mh versions. The battery type and capacity will determine the flash recycle time and number of flashes (Sea & Sea claims 200 flashes with a recycle time of 1.5sec if using rechargeable 2400mAh ni-mh batteries).
The YS-D2 can be synched to camera housings with either a fibre-optic cable or electronic five-pin Nikonos/Sea & Sea sync cord. It can also be “daisy-chained” to a second slave strobe using a fibre-optic cable.
It comes with two diffusers (120° and 100°), both YS and 25mm ball-mounting options and red filter inserts for the focusing light.
Congratulations if you managed to read through the last few paragraphs of geek-speak without falling asleep, because in fact the YS-D2 is the simplest all-round underwater strobe I’ve ever used.
Sea & Sea has given the user only two dials and one button. The clever bits are the backlit, colour-coded modes, and there are only four from which to choose.
The first click on the magnetic mode dial is orange, for cameras with a manual pre-flash. Next is green for cameras without a pre-flash, then dark blue for Sea & Sea’s DS-TTL mode for cameras using S-TTL and, with a little button-jiggling, light blue for its DS-TTL II mode.
This is said to be subtler and more accurate than DS-TTL when used with Optical YS converters designed for DSLR housings (this removes the camera’s flash from the procedure and uses data directly from the hot-shoe).
The second (right-hand) magnetic dial is used to adjust power output from the strobe, and can reduce or increase the exposure value by +/- 2.0 stops in any of the TTL modes. It doubles up to adjust light output manually in 11 increments from GN1 to GN32 while in Manual mode.
A central push-button switches on the LED focus light at full power, with a further push to switch it to half-power, another short push to switch it off or a long push to switch from DS-TTL (dark blue) to DS-TTLII (light blue) mode.

In Use
I integrated two YS-D2 flashguns into my DSLR housing-and-arm system using the supplied ball mounts, connected them to the camera’s pop-up flash with fibre-optic cables, then set them to Manual.
I marvelled at the brightly lit green colour displays on the rear control panel, which took all the confusion out of working out the mode in which I was shooting, and let me view the output/EV compensation settings easily.
I then set the output dial to its middle setting of GN5.6 and started to flail away with the camera, adjusting the flash output by clicking the right-hand dial up or down as needed. This quickly became second nature, and no different in use than my existing flashguns.
Under water I always shoot both camera and strobes manually, and in the past none of the TTL modes with which I’ve experimented has provided a successful outcome.
So it was with trepidation that I switched to the light blue DS-TTLII mode and changed the camera’s flash setting to TTL to shoot that most difficult of subjects, a school of silver-flanked fish. This would be a true test of how well Sea & Sea had developed its TTL technology.
I let the camera determine the flash output and was pleased to see that it had worked brilliantly. All the fish were perfectly exposed over a series of shots, leading me to believe that Sea & Sea’s TTL protocol had come of age.
It was so good that I shot wide-angle with a fisheye for eight dives straight using only the DS-TTLII mode without any hiccups. I could also easily fine-tune the output, reviewing the shots on the camera’s rear screen and using the EV compensation dial before shooting again.
YS-D2 flashguns are a bit bigger than a coffee-mug and could be tucked in tight to the sides of my smaller (170mm) dome port to get much-needed light onto the face of very close subjects when shooting close-focus wide angle.
I reverted to Manual and set the output to its maximum (GN32) to shoot a small juvenile Mexican hogfish admiring its reflection in my port glass. This tested the strobe’s maximum output, as it needed to overcome the stopped-down lens aperture. Again it worked perfectly.
I managed around 230 shots on each set of batteries shooting at various power levels, but the recycle time limited me to single shots instead of quick bursts. A nice feature was the audible beep to let me know that the flash had recycled and was ready to fire again.

The YS-D2 is a true all-rounder. It works in most underwater scenarios and with any camera or housing. It can be synced to a camera’s pop-up flash, hot-shoe or TTL converter with electric or fibre-optic cables, and used with any TTL protocol.
The backlit display with colour-coded modes and its sheer simplicity in use are genius. Add to this the size and weight and it could be a winner for travelling photographers.
This is also the only strobe that has allowed me to get consistent results from my own camera’s TTL mode under water – something I thought would never happen.

PRICE: £580
SIZE: 89 x 133 x 116mm
WEIGHT: 623g (without batteries)
MAX OUTPUT: GN32 (ISO 100 land)
COVERAGE: 80° without diffuser. 100°, 120° with diffusers
BATTERIES: 4 x AA 6V. Ni-mh, 4.8V
TARGET LIGHT: LED, two outputs
DIALS: Sealed, magnetic
FLASHES: Ni-mh 200. Alkaline 150
RECYCLE TIME: Alkaline 3sec. Ni-mh 1.5sec