Strobes, Settings, and the Inverse Square Law

[Ryan Neely]

I've been a photographer on land for decades (mostly art stuff, but there was a year or so when I shot professionally). I'm currently still mastering my scuba skills, so I'm not ready to take the plunge into underwater photography quite yet. However, that hasn't stopped me from reading about and researching underwater photography.

Yesterday I came across an article discussing strobe placement for macro shots. The author was discussing the use of two strobes on his rig and, essentially, using one strobe to illuminate objects in the background by feathering it away from the subject and pushing it forward so as not to create backscatter.

Immediately, I wondered if the inverse square law functioned differently when light was passing through water. I mean, it makes sense that one could achieve a black background when lighting a macro subject with all the light from both strobes. There would be so much light on the subject the camera's aperture would need to be quite small to compensate to the point that it wouldn't be able to pick up any light reflecting off objects in the background. Perhaps inverse square is more like inverse cube, or something.

My next reaction was ... why isn't the photographer using a slower shutter speed? Does shooting underwater affect the fact that the camera's aperture is the setting affected by strobes? Perhaps the shutter was already very slow, but that's doubtful with so much light coming off the strobes.

I'm trying to understand what I could be missing here. If I wanted to shoot a macro subject using a lot of light from dual strobes but not achieve a black background, wouldn't it be simpler to shoot at a slower shutter speed to allow more of the ambient light to enter the camera?

I'm confused.

 

[tursiops]

The inverse square law still applies, more or less....it is about a point source and a perfect medium, and a strobe is not a point source (nor is it in air!) and the water is not a perfect medium, especially if it is crappy water. But it is nowhere nearly as bad as inverse cube.

But if ALL you want is a non-black background, yes, slow your shutter down....but as you point out, you may be at a very small aperture and also at a low ISO, so a slow enough shutter might not be hand holdable. Also, he may have been trying to paint only part of the background with some light, and not all of it.

 

[MaxBottomtime]

Backscatter is the bane of all underwater photographers. Unless you're diving in clear water, you will fight backscatter, especially with macro shots. When I want a blurred background I will open the aperture. This Cuthona divae was shot at f16.

For most of my macro shots, I try to shoot upwards with a water background and use a lower f stop to achieve a black background. I used f25 for this Antiopella fusca.

My favorite strobe position for macro using two strobes is to have them pointed directly at the sides of the housing. In murky water, which I dive in most of the time this creates a backscatter problem. Other than shooting ambient light, I have never been able to avoid backscatter no matter where my strobes are.

Brent Durand has some excellent tutorial videos on YouTube.

 

[Chris Ross]

The issue with slowing down the shutter speed to admit ambient light is that the light is blue-green while the strobe is putting out 4500-5500K daylight depending on strobe model. With strobes you have the inverse square light fall off compounded by rapid selective absorption of red light then yellow etc. The strobe light needs to go out and back again - placing a remote strobe could be done but is limited by absorption on the way back to the camera. If you let in too much daylight it will also impact the colour of your main subject. For macro the object is 100% strobe illumination - it's mainly with wide angle you might be considering utilizing BG light.

You can reduce backscatter but rarely eliminate it, so you will be needing to so some cloning - the best solution is to get close and when you think you are close enough get closer - this means less volume of water to contain particles, so this means using relatively short macro lenses or for wide angle, close focusing ultra wide lenses.

 

[Ryan Neely]

Right. I hadn't considered the desire of painting a portion of the background with light. Nor had I considered white balance imbalance between foreground and background (I typically find this a pleasant effect that helps the viewer identify the subject as having been filmed under water).

Thanks for the help, folks! I have been enlightened.