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Dive Photo Guide


Building and Tuning a Compact TTL Underwater Photo Setup
By Pavel Kolpakov, August 13, 2021 @ 08:00 AM (EST)

A compact, modern TTL-capable system: Seafrogs housing for Sony a6000-series cameras with Retra Flash PRO strobe connected via a fiber-optic cable; inside the housing is a UW Technics TTL converter

While advanced and professional underwater photographers tend to favor manually adjusting strobe power for maximum creative control, many beginner and intermediate shooters prefer the convenience of through-the-lens (TTL) exposure control. TTL metering leaves your camera to evaluate a scene and adjust flash power accordingly, so you—the photographer—can focus on composition and capturing the decisive moment.

If you’re putting together a new camera setup, deciding you want TTL capabilities is the easy part. While companies like Ikelite offer everything you need—housing, TTL converters, and TTL-capable strobes—designed to work together seamlessly, photographers assembling a system using components from different manufacturers may find the prospect daunting. In fact, with some careful thought, this isn’t as challenging as it first appears, and in this article, we look at how easy it is to put together a few different products to build a compact, modern setup that employs the latest tech.

The example system we consider here is based around the Sony a6000–a6600 series, as these mirrorless APS-C-sensor cameras are popular choices for underwater shooters. Our plastic housing is from the Seafrogs “Salted Line” series, which is aimed at both freedivers and scuba divers, and waterproof down to 60 meters (~200 feet). The housing is compatible with optoelectronic converters from third-party manufacturers, and in our example, we use a TTL converter by UW Technics, which is specially designed for the Seafrogs A6xxx series housing.

UW Technics 16033-HSS TTL converter for Seafrogs Sony A6xxx housing: (L–R) hot shoe, main electronic board (with two CR1220 3V lithium batteries), leak sensor, and optical LED module

UW Technics TTL converters can be installed in housings by a variety of manufacturers and offer both optical and electrical strobe triggering, but since the Seafrogs housing in our example features dual optical ports, this dedicated kit (#16033-HSS) includes an appropriate optical module with two high-power LEDs. The converter supports classic underwater strobes by Ikelite, Inon, Sea&Sea, and Subtronic in TTL mode, but it also supports high-speed sync (HSS) for compatible strobes, which is currently limited to the Retra Flash Prime and Retra Flash PRO (as well as the new Retra Flash Prime X and PRO X). The converter kit includes a leak detector with a gold-plated sensor, alarm and indicator light.

In our example, we’ll use a single Retra Flash PRO, since we want to have the option of HSS. In HSS mode, the Retra strobes work at shutter speeds up to 1/8000s, but Sony’s a6000-series cameras have a shutter speed limit of 1/4000s.

The components of our example system ready for installation


Installing the Converter in the Housing

Installing a TTL system is straightforward and can be done in minutes. These were the steps we followed for our example setup, but the procedure will be similar for other setups:

  • Install the optical LED module included with the UW Technics converter kit.
  • Remove the original Seafrogs leak sensor, which is fixed to the bottom of the housing with adhesive tape. Replace it with the gold-plated leak sensor from the UW Technics kit. It comes with similar double-sided adhesive tape.
  • Connect the optical LED module and the leak sensor to the connectors on the UW Technics main electronic board.
  • Install both batteries in the UW Technics main electronic board, positive terminal up.
  • Remove the leak detector electronic module installed in the Seafrogs housing. It is fixed with a drop of glue on the right side of the housing and can be easily dislodged by hand. Install the UW Technics main electronic module with connected leak sensor and optical LED module.

UW Technics converter installed in the Seafrogs housing

Important Considerations on the Use of Fiber-optic Cables

There is a great variety of fiber-optic cables on the underwater gear market, ranging in price and performance. Not all are suitable for precise control of TTL and HSS using LED modules. Most LED control systems, including the TTL/HSS system here, are configured for a very specific type of cable, Type 613 optical fiber.

Cables of this type are made by companies such as Nauticam, Sea&Sea, Inon and HowShot, but all of them are based on an optical core manufactured by the Japanese company Asahi Kasei that has 613 fibers with a very high signal transmission index and low losses in the optical core. With other fiber-optic cables that have a lower transmission index, the user can get incorrect exposures or even misfires.

In addition to using fiber-optic cables of the highest quality, it is better to use single cables rather than dual cables, since the maximum intensity of the LED is in the center of the beam and the width of the radiation pattern of the LEDs is quite narrow, only 8–15 degrees. On this housing, as with others, the optical ports are such that it is permissible to use dual cables, but it should be kept in mind that single optical cables always offer optimum accuracy and reliability.

Setting Up the Converter

The UW Technics converter is preconfigured for a variety of common strobe models made by Ikelite, Inon, Sea&Sea, Retra and Subtronic. Select the correct protocol by turning the miniature rotary switch on main electronic board (using a small screwdriver) to the numbered position appropriate for your strobe, as follows:

  1. Fixed manual mode (no TTL protocol)
  2. Inon Z-240
  3. Sea&Sea YS-D1
  4. Sea&Sea YS-D2 (China)
  5. Sea&Sea YS-250, Subtronic Pro-270
  1. Ikelite DS-161, DS-160
  2. Inon Z-330
  3. Retra PRO
  4. Subtronic Pro-160
  5. Inon S-2000

In our example, we set to position “7” for usage with the Retra PRO strobe (including HSS functionality).

Check that the communication protocol between the camera and TTL converter is working. The camera should recognize a compatible TTL device is attached to the hot shoe by showing two flash symbols on the service screen.

TTL for blackwater photography? No problem


Initial Camera Settings

  • First, try shooting with the classic shutter type, that is, select “Mechanical Shutter” in the camera menu. Electronic first curtain shutter can also be used, in which case “e-Front Curtain Shutter” should be selected.
  • Select the desired exposure mode (“M”, “P”, “A”, “S”) according to your shooting experience and preference. Most underwater photographers prefer “M” mode, so they can set aperture and shutter speed manually. When shooting with traditional underwater strobes, the camera will automatically limit shutter speeds faster than the sync speed of the camera. For the Sony a6000-series cameras, this is 1/160s. As mentioned, faster shutter speeds (HSS) are available with Retra strobes.
  • Set “Flash Exposure Compensation” to “0 EV” as an initial setting.
  • Set ISO sensitivity. For the most accurate TTL operation, it is recommended to select an ISO between 200 and 500.
  • Set the desired metering mode (“Multi”, “Spot”, “Center”) appropriate for the subject or scene. Choosing the correct metering mode is key for proper TTL exposure. The wrong metering mode could result in significant overexposure or underexposure.

Choose a simple subject and take time to learn how camera and flash settings affect the resulting image


Camera Menu Settings for Underwater Strobe Control

  • Selecting the flash mode
  • In the “Flash Mode” menu, set the synchronization type: “Fill-flash”, “Slow” sync, or “Rear curtain” sync, depending on the shooting task. The most common setting is “Fill-flash”.
  • Setting flash exposure compensation
  • In the “Flash Comp.” menu, dial in the flash exposure compensation (–3 EV to +3 EV) appropriate for the subject and shooting conditions to obtain the correct exposure.
  • Shooting in manual flash exposure mode
  • For our example setup, there are three possible ways to switch to manual flash exposure mode:
    1. Control via the camera menu
    2. Use manual mode on the strobe
    3. Set the TTL converter to manual mode using rotary switch position “0”
  • Most shooters will prefer the first method since this allows you to switch to manual flash exposure mode during a dive. However, Sony’s camera menu does not have a manual flash mode setting. The UW Technics converter gets around this by making use of the “WL” mode, which is normally used for wireless flash. Thus, the “WL” setting “ON” switches the converter to manual flash mode (without pre-flash).
  • Depending on the camera firmware, if there’s no “WL” option in the Flash Mode menu, the “Wireless Flash” setting can also be found in the flash section (page 11) of Camera Settings 1. The “Wireless Flash” setting “ON” overrides the Flash Mode menu setting and the TTL system will operate in manual mode, while the “OFF” setting means the system is in TTL mode.
  • When switching to manual flash mode by this method, it’s possible to manually adjust flash power via the camera exposure compensation dial, where –3 EV to +3 EV corresponds to flash powerds 1/64 to 1/1. (Note that the strobe must be in S-TTL mode for this to work.)

Top-left: “Flash Mode” menu; top-right: “Flash Comp.” menu; bottom-left: using the “WL” setting to control manual flash mode; bottom-right: the “Wireless Flash” setting can also be found in the Camera Settings 1 menu—on page 11 of 14!  


High-Speed Synchronization with Retra Strobes

With the arrival of the Retra Flash, there has been an increased interest among the underwater photography community in high-speed synchronization (HSS) and HSS-enabled underwater strobes. Higher shutter speeds can be used when shooting near the water surface, when shooting in bright sunlight and against the sun, when shooting fast-moving objects, and for creative photography with different depths of field, among other applications.

With Sony a6000-series cameras, HSS shutter speeds are available in the range 1/200s to 1/4000s, though Retra strobes are capable of syncing up to 1/8000s. Retra HSS is manual only, not TTL, so in HSS mode, flash power is manually adjusted via the control on the strobe. Thus, in order to use HSS, you need to switch the converter to manual flash exposure mode (via the camera menu as described earlier). On the Retra Flash Prime and Flash PRO, you will first need to use Retra’s smartphone or tablet app to assign the HSS function to the “U1” or “U2” position on the strobe’s main control dial. The latest Flash Prime X and Flash Pro X have a dedicated “HSS” position on the dial.

Amazing sunbursts with HSS – Top-left: f/11, 1/2500s, ISO 200; top-right: f/13, 1/2500s, ISO 500; bottom-left: f/13, 1/4000s, ISO 400; bottom-right: f/11, 1/5000s, ISO 200. Images were shot on a Nikon D850 with 24–70mm lens, Nauticam WACP-1, and dual Retra Flash PRO strobes   


Final Thoughts

With basic know-how, a little time, and some simple DIY skills, anyone can put together a modern TTL-capable underwater photography setup using a housing, converter and strobes from different manufacturers. Depending on your camera model, you might have to deal with some usage quirks—in our case, Sony’s notorious menu diving—but your new system should give you the ability to enjoy accurate and reliable automatic flash exposure, use exposure compensation controls to refine flash power, and switch between manual and TTL on the fly. With some practice, you’ll be worrying less about your strobe exposure and instead putting all your creative skills to use in capturing the beauty of the underwater world.

Add TTL strobe exposure to your rig and leave flash power in your camera’s capable hands

Pavel Kolpakov is an inventor and developer of electronics for many brands of underwater photography gear. His company, UW Technics, is based in Moscow, Russia.


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