Retra Prime Underwater Strobe

Retra Prime underwater strobe, Hands on review in underwater journal
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.


Back in April 2020, I put together a three-part roundup of new strobes for Scuba Diver’s Destinations magazine’s website. In the second installment “New Strobes Part 2 – Compact, But Mighty”, I talked a bit about Retra’s new Prime and Pro model underwater flash systems.

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.

Underwater Journal Retra Prime review
All dressed up and ready to dive. My Nikon D850 system with two Retra Prime underwater strobes setup for some blackwater macro photography.

Since last May, I have been working almost exclusively with a pair of Retra’s new Prime model flash guns. For a full year before receiving my first two Retra Primes I had been shooting with Inon’s newer Z-330 model strobes, which gave me with a good baseline to draw on, as 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, the improvement 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.

Underwater Journal Retra Prime review
Shooting at a depth of 145 feet can get tricky when trying to get a proper balance between frontal strobe lighting and background ambient light levels. For this exposure camera settings on my D850 were at 1/125 sec. with the ISO set at 580. Aperture on my Nikonos R-UW 13mm fisheye lens at f/6.7, Retra Prime strobe power setting in manual at slightly less than half power.

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 Here

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.

Underwater Journal Retra Prime review
The controls for mode and power placement are not only simple in their layout but also easy to manipulate even with thick gloves.

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.

All Retra Prime and Pro model underwater flashes feature both a double O-ring seal on their battery doors, along with a water intrustion warning system.

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.

The Retra Prime’s LED indicator light position in center will display which shooting mode the strobe has been set on. In the image above, the magenta illumination (left) signifies that the strobe is set on it manual, whereas the cyan color (right) confirms the strobe is in TTL mode.

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.

When HSS mode in U1 is enabled, the ready light will show a steady green illumination. Should none the modes like Smart SL, HSS, Modeling (Pilot) light adjustment, and Low power SL be activated, the ready will simply stay off.

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
Underwater Journal Retra Prime review
A tough lighting situation when you want to maintain the appearance of the ambient lit depths which give the water wonderful bright blue appearance while at the same time balance the light on your intended subject in the foreground complete with plenty of bright reflective elements meshed equally large dark objects in the back ground. For underwater lighting to handle this best, illumination from the strobes not only broad and relatively even, but also with a level of warmth to bring out the color.

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.

Underwater Journal Retra Prime review
I don’t shoot macro as near often as I do wide angle, but when I do, I want it to look right. During a blackwater dive, I found this juvenile flying fish hanging out just below the surface. Camera settings: shutter speed at 1/180 sec. with ISO on 280, and the aperture on my 60mm macro lens set at f/22 and. For lighting, both of my Retra Primes where placed in manual mode with their power output set at 12 to be at half power.

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.

Underwater Journal Retra Prime review
There are few things that find as challenging to light up effectively underwater than fish with bright silvery scales like the groups of snook that I came across inside a wreck. Once again, subjects highly reflective sides that will in most instances bounce the light from my strobes as effectively as if they were made from shards from a broken mirror. Oh, I did mention they were also lurking in a very dark, shadowy corner inside the wreck? Viva soft, warm lighting! Camera settings on my D850 were at 1/90 sec. with the ISO set at 400. Aperture on my Nikon 18-25mm f/3.5-4.6 lens was set at f/8. Strobes set in manual mode at quarter power.

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.

Underwater Journal Retra Prime review
Large lemon shark shot over light sandy bottom at a depth of 90 feet. Camera settings: shutter speed at 1/180 sec. with aperture set at f/9.5 and on ISO 400. For lighting, both of my Retra Primes where placed in manual mode with their power output set a little below half power (basically between 6+ and 12+).

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.

Underwater Journal Retra Prime review
Portrait of a sandbar shark up close out in open water a depth of 40 feet. Like the lemon shark, the camera and strobe settings (shutter speed 1/180 sec., aperture f/9.5, ISO 400, etc.) were near identical rendering very pleasing results.

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.

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 does offer an optional add on battery holder for four additional AA’s called the Supercharger. This should double the flash capacity and (provided their specs are true), do so with a recycle time that is cut more than in half under my current shooting habitats. Until then, I will keep shooting as I have been, and enjoy the results.

After all, it is about quality, not quantity.