Underwater photographers are trained to fill the frame. Get close, then get closer. Fill the picture with color, texture and detail. It's good advice, and it has produced countless strong images.

But some of the most memorable underwater photographs do the opposite. A lone manta drifting through endless blue. A frogfish with his lure reaching into the open. A shark swimming across the frame with somewhere to go. These images work not because of what they show, but because of everything they leave out.

That emptiness has a name: negative space. Learning to use it is one of the fastest ways to make your images feel calmer, bolder and more deliberate. The negative space gives the viewer of the image their own creative interpretation: what do they think is on the other side of that negative space? This sense of mystery is of high appeal in your images.

Negative space is the area around and between your subject. The subject itself is the positive space. In underwater photography, negative space is usually open blue water, black background, a plain sandy bottom or a bright surface.

Negative space isn't wasted space. It gives the subject room to breathe, draws the eye straight to it, and sets the mood of the whole image. A small subject surrounded by a lot of empty blue feels very different from the same subject cropped tight. A tightly cropped image filling the frame leaves nothing to the viewer's imagination, while a negative space shot adds that intrigue.

Neither approach is right or wrong. The skill is being able to work both types of images into your portfolio. A portfolio of work should be diverse, and each image should have its own intent. By learning negative space, you will broaden your portfolio rapidly.

Negative space works best when it adds meaning, not just distance. Use it when you want to:

·       Show scale. A whale shark or a wreck feels enormous when there's open water around it. A diver in the frame makes that scale even clearer.

·       Create mood. Wide, empty blue water can feel peaceful, lonely or vast. Dark negative space feels mysterious or dramatic.

·       Simplify a busy scene. Reefs are chaotic. Isolating one subject against open water cuts through the clutter.

·       Emphasize movement. Leaving space in front of a moving subject gives it somewhere to go and makes the image feel dynamic.

·       Make a strong graphic image. Clean shapes, silhouettes and outlines are at their best with nothing competing around them.

It's less effective when the background is murky, busy or distracting, or when the subject itself is the whole story and nothing else is needed.

Good negative space starts with finding a clean background. Underwater, that usually means changing your position, not your settings.

·       Get low and shoot up. Angling toward open water or the surface removes the reef from behind your subject. It's the most reliable way to create clean blue negative space.

·       Use the sand. A plain sandy bottom can be clean negative space for subjects resting on it.

·       Pick your moment. Fish move constantly. Instead of chasing, wait for your subject to swim into clean water.

·       Watch for distractions at the edges. A stray fin, bubbles or a bit of reef in the corner breaks the effect. Check the edges of the frame, not just the subject.

Blue water is the most common form of negative space underwater, and how it looks depends heavily on your settings.

·       Shutter speed controls the blue. With strobes, shutter speed sets how bright the background water is. A faster shutter makes the blue darker and richer. A slower one makes it lighter and brighter.

·       Watch for banding and noise. Large areas of smooth blue show noise and color banding more than busy scenes. Keep your ISO reasonable and shoot RAW.

·       Aim for an even gradient. Blue water usually gets lighter toward the surface. Use that natural gradient rather than fighting it; it adds depth and a sense of direction.

·       Keep it clean. Backscatter is very visible against plain blue. Angle strobes appropriately and light your subject from the edges.

Black backgrounds are a powerful alternative to blue. They create drama, focus attention completely on the subject and hide distracting surroundings.

To get a black background, use a fast shutter speed and small aperture to underexpose the ambient light, then light only the subject with your strobes. Snoots and narrow-beam lighting help keep light off the background. Shooting toward open water rather than toward a reef also helps, because there's nothing behind the subject for your strobes to reach.

Black negative space suits macro subjects, fish portraits and anything with strong color or shape. It's also a good way to rescue a scene with an ugly or busy background.

Once you have a clean background, composition decides whether the image works.

1.     Make the subject small, but not lost. It should be clearly the subject, even if it fills only a small part of the frame. Sharp focus and good lighting on the subject are essential.

2.     Place the subject off-center. Rule-of-thirds placement usually works better than the center, leaving space on one side for the eye to move.

3.     Give movement room. Leave more space in the direction the subject is facing or traveling.

4.     Balance the frame. A large area of empty space should feel intentional. A small subject in one corner can balance a lot of open water.

5.     Try different shapes. Vertical frames suit divers rising toward the surface. Horizontal frames suit animals moving across the scene.

Both wide-angle and macro lenses can use negative space, in different ways.

With wide-angle, negative space usually means open water around large subjects. Fisheye and wide rectilinear lenses capture a lot of background, so finding clean water matters even more.

With macro, the shallow depth of field can turn any background into soft, plain color. Shoot at an angle that puts open water or distant reef behind the subject, and use a wider aperture to blur it further.

On your next dive, give yourself one rule: every image must have a clean background. No reef behind the subject, no clutter at the edges.

You'll take fewer photos. You'll spend more time waiting and moving into position. But you'll start to see the background as part of the image rather than something that happens behind it.

Sometimes the most powerful thing you can put in a photograph is nothing at all.