Retra Prime Underwater Flash 

An in-depth, hands on review detailing what Retra UWT’s new Prime underwater flash system, a surprisingly worthy tool for wide angle photographers in its ability to provide a beam spread of 130 degrees without the need of an add on diffuser.  

The standout feature of these new flash systems is a complete departure from the more conventional straight flash tube design to that of a fully unbroken circular flash tube with an LED modeling light in the center. Retra touts this feature as a world-first for underwater flashes, delivering a color temperature in the 4900° Kelvin range and a more even and wider coverage natively to 130 degrees without a diffuser.

Back in April of 2020, I received delivery of my new Retra Prime underwater strobes. 

From that point on, I began working almost exclusively over my set of Inon Z-330 model strobes. In having an already great deal of experience with the Z-330’s I had a baseline to draw on. 

As a point of reference, the Z-330’s provide coverage of 110 degrees and color temperatures of 5500° K without the benefit of a diffuser. What I didn’t like about the Z-330s, although their Guide Number of 33 is very close to the same brightness of the Retra Primes at full power, is that the Z-330’s dual flash tube configuration yielded very noticeable hotspots whenever I shot wide-angle without a diffuser in place. When used with a diffuser and losing more than half a stop in the process. Plus, the improvement over reducing those hotspots was only nominal. 

Prime vs. Pro

You may ask why the Prime and not the Pro version? The primary difference between the Prime and Pro is flash output. The Prime has a max output of 100-Watt seconds (Ws) at full power with a recycle time of 2 seconds, whereas the Pro is 50% brighter at 150 Ws with a Recycle time of 3 seconds. 

The tradeoff for that extra power is a smaller number of flashes per charge. The Prime is rated to provide 200 flashes at full power with four fully charged Eneloop Pro 2450 mAh AA batteries, while the Pro delivers 150 full-power flashes.  For more detail on this subject see the comparison on Retra’s website –

I felt I didn’t need the additional power the Pros provide as I found with my former Z-330s that I very seldom shot any above ¾ power. For me, what mattered more was recycle time and the greater number of flashes obtained from the same four AA batteries. In addition, what I saved on the price for the Primes versus the Pros (at the time I purchased them) is enough to buy a set of Retra Supercharger Extended Battery Holders when they become available, which will effectively double my battery capacity. 

Nice, But How Nice are They?

The first take away for both the Retra Prime and Pro is the high quality. Each housing is milled on a CNC machine from a solid block of aluminum and given a flat silver tone with a satin finish. While the feel is certainly very solid, the secondary purpose to using aluminum is that a metal housing serves as a heat sink to reduce the chances of overheating. 

Other than the difference in flash output, other aspects of the Prime and Pro are identical. Retra stuck with the same housing dimensions (130mm L x 102.5mm W) as used in the original Retra flash gun so that all the original accessories – LSD, diffusers and reduction rings, etc. still work with both Pro and Prime models. The one change Retra did make, which I thoroughly welcomed, is that the battery door now features a double O-ring seal rather than a single O-ring seal. Retra also added a water intrusion indicator inside the battery compartment in all their strobes, but I’d just as soon not have water enter in the first place.

This is something I had already experienced with one of their former models. Yes, I had a pair of the Original Retra Flashguns in 2017; one of them suffered a small leak into the battery compartment with rather disastrous results. Now that the new Retra includes a double O-ring seal that event is far, far less likely to be repeated.

There are two control dials: one dial for adjusting power output between 13 different settings, and the second serving as mode dial that includes Batt Test, Manual (ON), TTL, two custom settings marked as U1 and U2, plus a SOS emergency light function.  Both knobs are raised high enough to make them easy to grab and differentiate by touch, with gloves and even at night.

Rather than include an electronic Sea & Sea 5-pin or Ikelite bulkhead, Retra took a simpler route by configuring their strobes with a single optical port on the back. Should your housing only feature electronic bulkheads, Retra offers an add-on E-Opto converter so that your housing, with electronic cables, can interface with the strobe’s optical trigger ports.

Between the dials is an LED indicator light that displays in various colors to indicate current battery status (green for full, yellow for half, red for low), as well as mode (i.e. magenta for manual, cyan for TTL, etc.). A small push button in the center activates the LED modeling light and can also be used for firing the strobe independently from the camera. Being able to trigger by hand with the camera mounted on a tripod and set for a long exposure is certainly a handy feature for opportunities like cave painting or creative lighting inside a wreck.

If as little as 2 ml of water manages to get inside the battery compartment, the integrated leak detector will initiate the LED indicator light, which will begin flashing between red and blue, plus the modeling (Pilot) light will be activated. If this happens, Retra advises to stop what you are doing and remove the batteries as soon as possible followed with cleaning the battery contacts with a little soap and fresh water. A notable feature is that the electronics in the strobe are sealed away from the battery compartment to mitigate further damage to the strobe.

The U1 and U2 settings on the mode dial of both the Prime and Pro feature a first in underwater flash functions; Retra has included Bluetooth connectivity via a smartphone app. Using the Retra UWT App, the user can check the status (batt status, number flashes activated, and current firmware version) of each individual strobe. The app also includes a user manual for their strobes to confirm what does what. The coolest feature of the app is that you can assign a number of advanced settings like Smart SL, HSS, Modeling (Pilot) light adjustment, and Low power SL to the U1 and U2 modes on each individual strobe. 

For example, if I were using a camera like the Sony RX100, I would need the Smart SL mode enabled to cancel the TTL pre-flash. When activated, the Smart SL function detects the number of (1 – 10) pre-flashes emitted by the camera to effectively ignore the TTL light during the flash cycle thereby allowing the flash to serve in a manual exposure mode.  

The most interesting setting is the HSS (High Speed Sync) mode. With HSS — provided both your camera and housing LED circuitry is capable of transmitting the right signal to the strobe — your Retra Flash will be able to sync with shutter speeds well past the camera’s own maximum flash sync speed. The downside, if you are like me and shooting with a Nikon or Canon DSLR, is that your housing will require the proper TTL circuit board to permit this capability. At present, only one company, UW Technics, offers such a TTL Converter that will work with my Nikon D850 and Nauticam housing. Considering that the D850 can synchronize natively to any underwater strobe up to 250/sec., not having that particular circuit board is not a deal breaker. But, should I make the investment, I will be sure to share the results with you.

Where it Counts

I said it once, I’ll say it again. It doesn’t matter what camera, lens, or housing you might use, if you don’t have light, you won’t have an image. Ambient light is well and good if you are shooting close to the surface or if the water clarity is sufficient to adequately penetrate the depths. But having the right artificial lights in the form of an underwater flash system is essential for all other underwater photographic purposes. 

Almost every strobe currently available can meet your needs for macro photography. But for wide-angle images, if you’re critical as I am, good is not going to be good enough. While I often indulge myself in macro photography, including blackwater, I am predominantly a wide-angle shooter. When lighting up a reef scene or a wreck, coverage from that strobe needs to be both broad (100 to 140 degrees) and powerful enough to effectively light the scene. 

Where things start to get real is when the focus is on larger marine like sea turtles and sharks, or schools of fish; those with highly reflective sides are the worst. In those situations, the quality of your lighting becomes more critical.

First, the temperature range should be relatively warm. Most strobes provide a color temp range between 4400K and 5600K. A good baseline to keep in mind is that color temperatures over 5000K lean towards the cool range (often with a bluish tint), while temperature ranges below 5000K moves into the warm range. Second, in addition to the beam pattern needing to be wide, it should be relatively free of hot spots, which was precisely the problem with my previous Z-330’s.  

Although the base illumination capability of the Z-330’s at full power was comparable to the Retra Primes at full power, the Z-330’s T-configured dual flash tube yielded very noticeable hotspots in the center of the spread. Even with the placement of a diffuser, which brought the color temps down from 5500K to 5400K, and losing close to a stop in the process, the hot spots were still not fully eliminated. This trait would always make itself known any time I had something really reflective like a fish with white, or worse yet silver, scales. These images often came out looking like shards from a broken mirror.

With the Retra Prime, the quality and consistency of the light on subjects I was seeing in post when viewed on the 5K screen of my 27-inch iMac was far more pleasing. The lighting from the Prime’s fully circular flash tube was impressively even and noticeably less contrasty across the full 130-degree width of the beam, even without the aid of a detachable diffuser. Adding to that the flash tube’s color temperature being in the 4900° Kelvin range rendered a warmer white to the lighter color regions on subjects like sharks. 

Touching back on why I chose the Prime over the Pro, the main reason was mileage instead of horsepower. To provide some insight to my reasoning, back in the day when most of us were shooting with Fujichrome film with a rated ISO of 100, the most desirable strobes were the big gun models that offered 150 to 250 Ws on full power. With today’s digital systems we are no longer constrained to one ISO setting during a dive. In fact, there is almost no excuse for not upping your camera’s ISO setting from 100 or 200 ISO to 800 ISO or even 1000-plus. For me, the whole concern about noise in the image is often far overrated. 

Considering that even with the Prime’s more moderate max output of 100 Ws, you might expect there will be limitations from a power source comprised of only four AA batteries. In general, the number of shots I would take over the course of two consecutive dives typically varies between 250 and 450. I cannot provide a more precise count as shooting situations and subject matter (which applies to both wide-angle or macro) in the real world will always present a constant set of variables. However, I should add that in all those situations, shooting was not cut short because one or both strobes failed to recycle for another shot by the end of the second dive.

The Need for More Power

While I do not see the current battery capacity as a deal breaker, I wish Retra had designed their newer strobes from the get-go to accept either a more powerful drop in lithium-ion battery pack, or at least configured a battery chamber capable of holding eight AA’s instead of four. 

Retra Prime strobe with the Supercharger installed.
Retra Prime strobe with the Supercharger installed.

Instead, Retra choose a rather goofy looking but effective workaround by way of an add on battery pack. Called the Supercharger, this option is essentially a separate housing for four additional AA’s that replaces the strobe’s standard battery door when used. While I am not a fan of this approach as it sticks out like a sore thumb, I must admit it does double the flash capacity of that strobe, as well as give it a noticeable bump recycle time when shooting at higher than half power. A really nice advantage when I am doing blackwater photography or out on a three-dive day excursion. Otherwise, until Retra comes up a better approach, I will continue shooting as I have been, and enjoy the results. 

After all, it is about quality, not quantity. 

For more info Retra UWT products –