Half-and-half images are some the most exciting underwater photographs, regularly topping the charts in competitions. They show an impossible view, one we can’t actually see with our own eyes and, more important, they transport viewers underwater.

To create a vivid image, you need the right equipment, the right conditions, the right technique and, most often overlooked, the right subject matter. The equipment is relatively simple: Choose your widest lens and biggest dome port. The wider the lens, the more you can show compared with the line formed by the surface. If you don’t own a wide lens, shoot verticals to give yourself the widest view. A large dome helps control the movement of the waterline and makes it easier to keep both subjects in focus.

As long as it is sunny, strobes aren’t necessary. It’s also important to be mindful of timing. The middle of the day offers the most even exposures between the halves. If the sun goes in, the light and colors will go flat, and the underwater half will be much darker. Lastly, since shallow water is the best place to take over/under shots, there is one final piece of equipment that you’re going to need — your snorkel.

Critical Conditions

A successful split needs reasonably calm water to allow the photographer to manage the surface on the dome. Prime time for shooting over/under shots is when the sun is out and high in the sky. High sun ensures the maximum light penetration into the water and similar exposures on both parts. Setting the exposure will require compromise because the above water subject matter will be always be slightly brighter than underwater. I slightly overexpose over-water and underexpose beneath the surface, knowing I can fine-tune later. It is often hard to judge exposure on the LCD in bright conditions, so be sure to double- check the image’s histogram.

A low sun can be used for more dramatic lighting, incorporating silhouetted scenes above the water and/or beautiful sunsets. A low sun does beg for the use of strobes to light up the underwater section. Concerning positioning, pull strobes down to 5 and 7 o’clock positions relative to the camera.

Complete Focus

Over/under shots tell the story of two worlds, and therefore both need to be in focus for the whole image to be successful. The correct technique is to always focus on the underwater subject, and then close the lens aperture (to a higher f-stop), which gives us the extra depth of field to record the above-water section sharply. All the photos on this page were taken between f/16 and f/22.

It is important to focus accurately; for these types of shots I prefer the precision of thumb focus: a setting where the autofocus does not activate when you press the shutter, only when you push a button on the camera and housing under the right thumb. If you focus above the water, the underwater scene will almost always be blurred — this is one of the most common mistakes in this type of image.

Subject Selection

The final ingredient for success is a winning composition. Over/under shots require subject matter on both sides of the divide, ideally with the two telling the same story. The fall colors of the trees above the salmon tell us about the habitat, that fish have migrated in to spawn, and the time of year. The best shots have a reason for showing the two worlds, and that has to go beyond the photographer wanting to try out the technique. Think about the story you want to share, and convey it through a strong composition.

Alex Mustard is a marine ­biologist who has been a full-time underwater ­photographer and author since 2004. To view more of his work, visit ­amustard.com.

Get more underwater photography tips from Alex Mustard:

The Secret to Converting Images to Black and White

How to Shoot in Current

Taking Photos of Underwater Models