Testing MethodologyStrobe manufacturers typically publish the guide numbers of their strobes in air, but we wanted to test them in controlled conditions to see how they stacked up against one another. We especially wanted to see how they would perform with diffusers, as this is how we always shoot with our strobes when photographing wide-angle scenes. We also wanted to see how the strobes would perform across an entire frame; the evenness of the light is just as important as the intensity when lighting a wide angle reef scene.
For these tests, we used the same Sekonic Flash Meter in a room with the lights off. All strobes were tested at full power, ISO 100, at a distance of one meter, the same standard at which manufacturers also test their strobes. We wanted to test all manufacturers in the same conditions, measuring the relative differences in brightness between strobes.
While manufacturers quote the guide number at center, we calculated Guide Numbers across what would be a typical wide angle frame; at center, 33cm out, 66cm out, and 1 meter out.
Interpreting These ResultsThe important thing to look at is the relative difference between the strobes, not the numerical value of the guide numbers. The guide numbers on the graph are ƒ-stop numbers, which are a ratio between lens length and opening, and are not an absolute measure of light. Each "stop" is double the amount of light. ƒ32 to ƒ22 is 1 stop of light, as is ƒ22 to ƒ16, and ƒ16 to ƒ11, etc. down the side of the graph. As the guide numbers decrease, the lens allows more light in, and the strobe does not need to emit as much light. As the number increases the lens opening is smaller and therefore the strobe needs to be brighter. All of our testing was with the strobe at full power.
This latest round of testing was prompted by the introduction of the Sea & Sea YS-D2J and Inon Z330 strobe which had a guide number specified from the manufacturer at ƒ32 and ƒ33 respectively, where the previous brightest strobe in our testing was the Sea & Sea YS-250 at ƒ32. We were surprised by the results in how close the guide numbers were comparing the Sea & Sea YS-D2, Inon Z330 and Sea & Sea YS-250. Not surprisingly, the YS-250 has the most even pattern of these top 3 strobes with its circular flash tube as opposed to the straight tubes of the YS-D2 and Inon Z330. When shot with diffusers, these strobes are all within about a 1/4 stop difference in brightness, and get closer to the more even beam patterns of the YS-250. Since most cameras exposure adjustments are in increments of 1/3 stop, you would most likely not adjust the exposure between these strobes.
Inon Z-330 vs. Sea & Sea YS-D2JIn looking at the 2 top current model strobes, their patterns without diffusers are almost identical. The differences are so minuscule, it would be hard to tell a difference in the real world evaluating a photo. With diffusers the Inon Z330 has a slightly less powerful diffuser than the Sea & Sea YS-D2J, leading to a brighter beam in the center, but a quicker fall-off compared to the Sea & Sea YS-D2J. The Sea & Sea YS-D2J has a more even beam pattern with the stronger diffuser, which is more favorable for wide angle scenes, albeit at the sacrifice of a lower power overall.
Sea & Sea YS-01 vs. Sea & Sea YS-03The Sea & Sea YS-01 and Sea & Sea YS-03 offer the same maximum output. The Sea & Sea YS-03 is a TTL only strobe, so it is hard to get accurate max power, but from our testing, it is the exactly the same as the Sea & Sea YS-01.
Out of all the strobes tested, Ikelite came closest to their published guide numbers.