Backscatter

What Are All Those Ugly White Spots in My Picture?

Backscatter… the nemesis of every underwater photographer, is a potential problem even in clear water with high quality photo gear.

Backscatter is caused by the reflection of strobe light off particles (crud and sand) in the water back into the lens of the camera. These reflections cause bright highlights (white spots) to appear in your image. Depending on how close and large the particles are, how wide your lens is, and what angle and how close your strobe or strobes are to the camera, determines how many, how large, and where the white spots are on your pictures.

Obviously, if the particles are close, they appear larger than if they are farther away. And if the particles are big, the backscatter looks bigger than if the crud in the water was smaller. Even a school of reflective baitfish can be so highly illuminated as to look like large backscatter.

The wider your lens, generally, the smaller and more numerous the backscatter spots are. Wide-angle lenses tend to make things look smaller and further away than they really are. As a result of the wide-angle lens’ “wideness,” strobe positioning is critical to avoiding backscatter. Macro lenses and framers usually have a lot less problem with backscatter because they are so close to the subject that there isn’t as much space between the subject and lens for crud to collect. No matter what lens you have, the closer you are to the subject, the less potential you have for backscatter — less space and crud between you and the subject to light up. Macro lenses also avoid backscatter by their narrow angle of view. If the backscatter is outside the lens’ view, then it doesn’t get seen.

The more closely aligned the strobe and camera lens are, the higher your likelihood of creating backscatter spots on your images. Point and shoot cameras are an example of a strobe and camera lens with close alignment. The strobe and the camera lens are right next to each other and pointing in the same exact direction. As a result, the light from the strobe goes straight out and bounces straight back, just like a mirror, into the camera. It also bounces off the crud in the water between the camera and the subject. On more advanced camera/housing systems that have the strobe or strobes mounted on separate moveable arms, the strobes can be positioned away from the camera and at various angles so that the direct light from the strobe bounces the reflection from backscatter away from the camera/lens. The crud is still there, it’s just not highlighted or illuminated from the camera’s view angle.

So, what can you do to avoid this nemesis of the underwater world? For the diver with a point and shoot camera, your options are more limited than the diver with the more advanced system. You can’t move the strobe to a better position, so you might consider turning off the strobe, to eliminate backscatter, if your subject is far enough away that the flash is not going to do much anyway. A natural light picture is better than a natural light picture with backscatter. A point that some folks don’t think about is “don’t stir up more backscatter.” In other words, watch your buoyancy and bubbles. Don’t kick up the bottom and stay out from under the overhang where your bubbles knock silt off the overhead. If the silt does get kicked up, consider moving to another area.

Figure 1
Figure 1

For the folks with moveable strobes, strobe positioning is the key to reducing or eliminating backscatter. First, remember that a strobe is like a floodlight, not a spotlight. The little spot your aiming at isn’t the only thing that’s going to get lit up. Think about the tricks that the “air/water interface” play on us. The subject is farther away than it looks. Aim the strobe slightly behind where you “see” the subject. If you have a spotting light, you may be tempted to point the strobe directly at the subject. Instead, think about flaring it out some, to work with the inside edges of the strobe light if you can. Angle the strobe so it comes down across the subject rather than straight at the subject. See figure 1. Remember, lighting up the subject while lighting up less of the water in between you and the subject means less backscatter and cleaner looking water.

It’s a balancing act that takes practice. Digital cameras have brought the ability to instantly see the mistakes and readjust the shot to improve it. That’s a great learning tool, as long as you put a little thought into the process, and just don’t delete without thinking about what you could do to improve the picture. So, get out there and dive, dive, dive and shoot, shoot, shoot and have fun while you’re learning and improving your skills.