Shallow depth of Field
One way to make your images stand out from the crowd is to shoot images with shallow depth of field. Our eyes naturally move to the area of sharpest focus, so by isolating your subject against a beautifully blurred background it directs your viewer to the important part of the image. Your subject won’t have to compete against a busy background that is equally as sharp. For underwater photographers this is going to be most easily achieved using macro lenses. The super wide lenses we typically use underwater for wide angle have a lot of DOF, even wide open. There are a number of factors that affect depth of field but some of the more important ones to consider are sensor size, lens focal length, subject distance and aperture. Larger sensors, like those found in full frame cameras, have less DOF than the smaller sensors found in most mirrorless and compacts. Longer focal length lenses have less DOF than shorter focal lengths. Moving closer to your subject or anything that adds magnification, like adding a SMC or CMC, will give you less DOF. Wide apertures or f–numbers like f/2.8 have shallow DOF while narrow stops like f/22 have deep DOF.
Macro lens (preferably a longer macro lens like a 100mm)
Manual camera exposure.
Starting aperture around f/11 (compacts f/4)
Starting shutter speed (1/125)
Starting ISO 100
Manual or TTL strobes
Strobe placement (There is no one size fits all for strobe placement but since we are shooting macro, keep your strobes close to your macro port. Experiment with different angles and power ratios)
Getting the Shot
Look for subjects that have a little distance between the subject and background. When shooting with higher magnifications the background can be closer. You can even use the subject itself to create the background.
Set your camera to manual and adjust aperture, shutter speed and ISO.
If shooting manual strobes set to a lower power setting. (You will be letting in a lot of light with the wide apertures)
Take a shot.
Check focus, with very shallow DOF it may be impossible to to have both eyes or both rhinophores of a nudibranch or similar subject in focus. Typically you want to focus on the eye that is closest to the camera. Because the plane of focus is perpendicular to the lens, how you angle the camera will have a big impact on what is and is not in focus. An enhanced viewfinder is a nice accessory to have for these type of images.
Make adjustments to your aperture and strobe power until you get the desired results.
Be aware of objects that may still be visible in the foreground of the frame. Because we are working with very narrow depth of field, if these objects are closer to the lens than the subject, they will also be out of focus. Out of focus foregrounds can be distracting, but used intentionally you can create some interesting images.
By Todd Winner