Some Simple Truths About Strobe Intensity and Coverage


Guide Numbers, Angle of Coverage, Watts, Kelvin Temperature - OH MY!

Yes, strobe manufactures love throwing out all these impressive numbers that sound essential to choosing a strobe that will do exactly what you want. But are they really helpful? Well... sort of. Unfortunately there is no official standard way of measuring much of this and it is almost guaranteed that no two manufacturers are doing things the same way. And to whatever extent there is fudging taking place, the consumer can rest assured it will usually be to make things sound more impressive. 

So we are going to do a quick skim of the complicated part and then get on with the simple and interesting comparison

Guide Numbers (GN) are supposed to represent how much light a strobe puts out at a set distance at full power - usually measured without a diffuser in the center of the light beam. But there is not a standard distance. Some strobes are also almost unusable without a diffuser and when measured in water, the guide number will always be much less than in air.

Angle of coverage is also measured by wildly different standards and Watts refers to the power available, but not necessarily to the light output.

So what are we to do as consumers and underwater photography enthusiast? Do we need to break out the slide rules and calculators? Hardly - the idea of it makes our brain hurt already. Perhaps the simplest approach is to set up a tripod and just see how all the strobes compare in the "real world" - or at least a facsimile of it. Not perfect perhaps (yes, shooting in water will tweak the results a bit), but a lot better than just running with the jargon from the manufacturers as gospel. So call this a seat of the pants estimate for what you can expect from most of the popular current strobes.

As you scroll through some of the images below, the biggest take-away might not be how big the difference in light intensity and coverage is, but rather how small some of the differences are in the leading strobes and how misleading some of the specifications can be.

Along those lines, here is a quick note about the color temperature of strobes. The light emitted by a strobe underwater will be a different temperature (color) for every inch of distance the subject is from the the strobe. At 6" from the subject, the light from the strobe will be fairly close to its manufactured origin. At 4-6 feet from the subject, the light will have gotten much cooler because of losing red, orange and some yellow spectrums. Some manufacturers will choose a warmer Kelvin temperature to make up for this loss. One UW, Retra and Ikelite are examples of manufacturers that do this. The down side is close up macro shooting might be a bit too warm for some, while wide angle can benefit. Inon makes their strobes closer to daylight spectrum output, so very natural for close up, and a bit cooler for distance work. Inon also makes warming filters for wide angle work and thus perhaps offering an option for both types of shooters. Sea & Sea generally threads between these two philosophies of light temperature.

Strobe Coverage Image Comparisons

So lets take a look how this plays out in our seat-of-the-pants test. All these images were shot with DSLR and fisheye lens at f22, 1/200 and ISO 100 at the same distance for the strobe and camera. No processing at all of the image.


 This exposure, above, was made with one of the most high-end and highest quality strobes available: the OneUW 160X. Of all the strobes tested, it was clearly the most powerful and best coverage - but it lists a Guide Number of only 20 which is the lowest GN of any of the tested strobes. You would expect it to have the highest GN - clearly One UW does not test the way other strobe manufacturers might. So take all these numbers with a bucket of salt when doing your strobe research.

 The above exposure was taken with an Inon Z-330 V2 with a new better diffusion pattern to the dome that allows it to be used without a diffuser for maximum intensity. This is the least expensive strobe in the group, but the power output is still impressive and so it proves to be a great value. The light fall off is a bit more noticeable at the edges than the One UW and the color is closer to daylight. For maximum coverage a diffuser can be added to the Z-330 and the color temperature can be selected when doing this. Coverage and intensity with a diffused Z-330 is below.

Retra is a boutique strobe manufacturer that has gained popularity and they have taken the approach of soft warm light without a diffuser and their coverage and intensity can be seen below.

The Retra Pro-X (their newest) is not quite as powerful as a OneUW or an Inon Z-330 V2, but they have a pleasing smooth coverage with very little light fall off that many photographers like.

Sea & Sea has two strobes on the market with a similar design. The older YS-D2J and the newer, more expensive, supposedly more powerful, YS-D3 II. While most contemporary strobes have gone to a domed front port in front of the flash tubes to maximize coverage underwater, Sea & Sea still uses a relatively flat port in front of the flash and relies heavily on translucent diffusers to spread the light at the cost of intensity. 

Below is a YS-D2J with no diffuser.

Because of the single flash tube design of the Sea & Sea, the light pattern is very ovoid with harsh light fall off when used without a diffuser making it impractical to be used this way. Diffusers should always be in place.

Below, the YS-D2J shows a more pleasing coverage at the cost of intensity by utilizing a diffuser.

The YS-D3 is even more harsh and unlikely to be used without a diffuser, so look below to see how the the YS-D3 with diffuser measures up to the YS-D2 with a diffuser. 

If you can't tell much difference, you are not alone - we couldn't either. Interestingly, the color temp moved a little closer to Inon.

Our last test subject is Ikelite's venerable DS-160 strobe. This much loved strobe is now almost a 14 year old design that has had some battery updates along the way. It is still a fast recycling strobe with a soft warm circular flash tube design, but its performance is showing its age to a degree. Without a diffuser, coverage looks like this:

With a translucent diffuser, the results look weaker as seen below.

It is interesting to note that both Ikelite DS-160 and the OneUW 160X are 160 Watt designs - there are just a number of design years between them. Ikelite is expected to be releasing a new strobe in the near future which should be exciting to test.

Thumbnails for comparison

Strobe images on the grid are as follows:

Retra ProX   -----  Inon Z-330 w/o Diffuser -----  Inon Z-330 w/ Diffuser

Sea & Sea YS-D2J w/o Diffuser  -----  YS-D2J w/ Diffuser -----  Sea & Sea YS-D3 w/ Diffuser

OneUW 160X  -----  Ikelite DS-160 w/o Diffuser  ----  Ikelite DS-160 w/ Diffuser


So what do we make of all this?

It is safe to say that when it comes to intensity and coverage, any of these strobes can get the job done and make nice images. And with a few exceptions, the lighting performance is not radically different. But this is only one criteria for choosing a strobe. It is important to take the whole picture into account. So consider some of the following.

Recycle Time

This is the amount of time it takes a strobe capacitor to "recycle" after firing and be ready for the next shot. Many things effect this performance including size and power of the capacitor, quality and charge state of the batteries and the strobe circuitry itself.

For some who shoot a lot of action (think shark feeds and blue water pelagics), this feature is critical, so this is an approximate guide to where the strobes land, all things being equal.

OneUW and Ikelite use proprietary NiMh batteries that are more expensive, but will offer generally faster recycle times. Rechargeable AA batteries have improved, but offer a set limitation to the amount of current they can flow to recycle a capacitor.

OneUW 160X has the best recycle speed to power output of any strobe we tested.

Ikelite DS-160 is also fast recycle, but with a lower output ratio. 

Inon Z-330 has surprisingly fast recycle times for an AA battery powered strobe aided by extremely short flash duration (about 1/1000 of a second at full power) and the ability to fire before the capacitor is fully charged.

The Sea & Sea YS-D3 is very nearly as fast as the Inon, but at slightly lower effective power due to diffusing.

Retras have consistently been the slowest recycling strobes across their model range and tend to be very battery charge sensitive. The new Retra Pro-X has improved on this a bit, but for anyone interested in high speed work, the Retra Supercharger should be a serious consideration. The Supercharger is a novel approach to add an additional 4 batteries (Total of 8 AA) for each strobe which definitely improves performance, though increasing weight and size.

Flash Tube Design

There are two basic designs of flash tubes. Long circular flash tubes and shorter straight flash tubes. Circular flash tubes are known for there soft even lighting that can sometimes be used without diffusers and still be soft. If there is a downside to this long tube it is that it takes a relatively long time to ignite the entire tube. If you shoot at a shutter speed higher than 1/200, at full power, with an Ikelite strobe, the strobe cannot complete the firing process before the shutter closes and you will not get a full power exposure.

The shorter a flash tube, the shorter the flash duration is at full power. Inon uses the shortest flash tubes and achieves full power in less than 1/1000 of a second. This can be helpful for shooters who use high sync speeds.

One UW, Retra and Ikelite use circular flash tubes.

Sea & Sea uses a single medium length flash tube in YS-D2J while it uses two medium length tubes in the YS-D3.

Inon uses two short flash tubes.

All these options work, they just reflect some of the different characters of each strobe.


The Inon Z-330 and Sea& Sea YS-D2J are the smallest strobes in this group, With the Retra Pro-X and YS-D3 slightly larger (the Supercharger for the Retra adds a good 3" to the length). The OneUW is the largest strobe, though only slightly larger than the Ikelite. All of these strobes are small compared to their equivalents of 10-20 years ago.


This can be tricky to confirm as any given photographer could have had terrible experience with one brand and perfect experience with another brand - while others experience the exact opposite. 

Smaller manufacturers also have fewer strobes out in the field to judge by. OneUW and Retra are by far the smallest manufacturers in this group. OneUW has been making strobes much longer than Retra and has generally had a very reliable reputation. Retra has only been making strobes about 3 years now and while early models had some issues, they seem to have improved reliability.

Of the larger manufacturers, Ikelite has been making strobes longer than anyone and has evolved a very reliable strobe. Inon and Sea & Sea have also been around more than two decades. Inons having generally proved to be quite reliable and long lived over the years. Sea and Sea has had mixed results with their strobes. Some older strobes were extremely reliable, while the YS-D1/YS-D2/ and YS-D2J have had well documented reliability issues. So far, reports on the YS-D3 seem to suggest improved reliability.


Conclusions are best left to each individual based on their needs. Most of these manufacturers offer lower powered and smaller strobe options that we haven't covered as they are not trying to be the "best" of their category - that doesn't mean they are not worthy of consideration, they may be a perfect fit for you. But these are the popular top-of-the-line strobes that people have questions about. Hopefully this article has provided a little more info to help in your decision making process. But if you have questions about these or any other strobes, we are happy to help, so give us a call or write us a note and we can talk lighting as it applies to your particular rig.