Pointing the strobe at the correct spot so that it eliminates backscatter, while still putting light on your subject, is the trick to using a strobe/flash successfully.
The backscatter section gave you a background on the cause of backscatter and how to eliminate it. This section addresses the specifics of strobe aiming a little more in depth.
There are several forces at work when you try to take a picture. Let’s look at the air/water interface. (What you see isn’t quite where you think it is). The experience of reaching for the anchor line and finding that it’s just out of reach is an example of the trick that our mask and its air/water interface plays on us. So, the fish that you want to take a picture of isn’t quite where you thought either. The fish is a little further away than you thought. The terms we’ll use are apparent subject versus real subject (see the illustration). (Please note that the illustration is not to scale). If the apparent subject/fish is three feet away, then the real subject/fish is four feet away. If the apparent subject is 12 inches away, then the real one is 16 inches away. (For the math inclined: It’s Measured Distance x 0.75 = Apparent Distance. Or, going the other way, Apparent Distance x 1.33 = Measured Distance.) So you can see that you’ll need to aim the strobe a bit farther away, behind the apparent subject/fish. Aim your strobe about one third or more behind the apparent subject that you see.
Your strobe is not a spot light. The flash/strobe you use underwater is more like a floodlight. Different strobes have different amounts of coverage. Check your owner’s manual to see how wide the coverage of your strobe is. Some can be as wide as a hundred degrees or more with a diffuser. You’ll want to try to feather/turn the strobe outward, so that you’re using the inside edge of the strobe beam (see illustration). The more the strobe is turned toward the camera, the more water between the lens and the subject gets lit up, producing backscatter. You want to light the subject, while illuminating as little of the water between the subject and camera as possible.
The closer the strobe is placed to the camera, the more likely it is to produce backscatter. As previously noted, if light from the strobe goes directly out from and directly back to the camera, it creates large amounts of backscatter. Move the strobe out away from the camera to create side or top lighting of the subject. Top lighting tends to look slightly more natural than light coming from other directions. Try to have the light fall just in front of the real subject. A point to remember: If you are pointing the strobe by holding it in your hand, don’t let yourself subconsciously point the strobe toward the apparent subject. It’s a natural tendency to do so.
Aiming/modeling lights can help achieve correct strobe placement…that is if they don’t scare your subject. Aim the light behind the subject. How much depends on how far away the subject is, generally about a third more than the distance to the subject. Don’t get confused by the apparent versus real subject issue while using the aiming light. The aiming light isn’t affected by air/water interface issues. See the “X” on the illustration.
When you change strobes or lenses, the angles of coverage of each piece of equipment can change where you need to place the strobe to achieve proper placement. A very common mistake is to let the strobe come too far forward while using very wide-angle lenses. This mistake shows itself by the huge flare/burnout area in the corner of the picture where the strobe was. If your new strobe has a wider angle of coverage than your previous strobe, you’ll need to feather/turn it outward even more than the older, more narrow angle strobe to achieve the proper use of the inside edge of the light.
For those of you who have digital cameras, you’ll be able to adjust as you progress from shot to shot. The learning curve for the folks using film is a little longer, because you can’t see your mistakes until you get the film back.
Remember these tips:
- The strobe is not a spot light, it’s wide like a floodlight.
- Use the inside edge of the strobe light.
- Point the strobe behind the real subject.
- Move the strobe out away from the camera.
- Take time to look at the mistakes and learn from them.