Sharp focus is a key element in excellent underwater photos. In fact, sharp focus is one of the foundations of all photography and a main criteria in advancing images into final judging rounds of photo contests.
Underwater camera autofocus systems are extremely accurate, fast and reliable these days, however you still need a strong understanding of how to use the system. This will allow you to coach the camera into achieving sharp focus in challenging situations while also selecting the best autofocus settings to capture the creative photo you envision.
My 14-minute tutorial below explains everything you need to know about autofocus for underwater photography.
Autofocus for Underwater Photographers
How Does Autofocus Work
Autofocus systems today use phase detection technology. This can get very complex with dedicated autofocus sensors and processors, dual sensor pixels, and other powerful technology, however the basic concept is fairly simply. Here is how it works.
- Light enters the camera through the lens.
- The light is reflected through a prism into two spots on the autofocus sensor.
- The camera measures these offset images, calculates how much the lens needs to adjust focus in order to crisply overlay these two images over one another (essentially looking like one image), and then moves the lens accordingly.
This process occurs extremely quickly and is much faster than older contrast detection systems that relied on racking the lens back and forth (lens hunting) in a search for sharp focus.
Autofocus Drive Modes
There are two primary autofocus drive modes we use in underwater photography: single shot and continuous.
Single Shot Autofocus
Single shot autofocus is activated when you engage the autofocus system (half depressing the shutter button). The camera achieves focus, the focus box turns green or blue, and you can then capture the image. You’re not able to capture the image until the camera has achieved focus, which is helpful in ensuring your images are sharp.
Single shot autofocus is ideal for underwater macro and wide-angle subjects that don’t move too much around the frame.
Continuous autofocus will maintain focus on the subject as long as you have the autofocus system engaged. This is different from single shot, which essentially turns off once it achieves focus, since continuous AF holds focus on the subject. This makes it ideal for fast action and subjects moving around the frame.
Unlike single AF, your camera will always allow you to capture the image when using continuous AF regardless of whether the subject is in sharp focus. Today’s autofocus systems are really good at maintaining focus, however there is the chance you could capture images that aren’t totally sharp. Most pro photographers accept this small sacrifice because they want to capture the image right when they push the shutter.
One PRO TIP for using continuous autofocus is to try a technique called Back Button Focus. Learn more in my article and tutorial video linked below.
Video: Back Button Focus
Subject Tracking Autofocus
Most cameras today offer a variety of subject tracking autofocus modes. These may or may not work for you underwater. It really depends on what you’re shooting and how.
For example, person tracking does an excellent job tracking your dive buddy and is a great choice for simple diver shots. But if you’re composing a reefscape with your dive buddy in the background, you will want to focus on foreground but the camera will still focus on the diver. In this situation you would want to turn off person tracking.
Eye tracking is another autofocus mode that is gaining popularity with underwater photographers. It works great for macro with subjects like fish, where the eye is easy to distinguish, however you may have trouble with other subjects, fish in wide-angle shots, etc.
I always recommend testing these tracking settings yourself to see if they’re successful and reliable in helping you. If not, keep them off.
Autofocus Area Selection
Autofocus area selection is an important setting for underwater photography.
In some fast action situations you may choose to use automatic autofocus area, letting the camera choose where it believes the subject is for focus. While this is easy, the camera may not always be able to identify the subject, even with subject tracking active.
Most underwater photographers select a manual focus point, allowing them to move this point around the frame to lock on the subject. This manual control accommodates any sort of composition since you can position critical details like the subject’s eye in any area of the frame.
Compact camera shooters can select a small box as a manual focus point, while interchangeable lens camera shooters will have a variety of manual points. These start as a pinpoint and expand into larger area groups. The larger the group, the more probability of locking focus in that area.
Generally I prefer spot focus for macro, supermacro and some wide-angle, while using a wider cross-type autofocus area for wide-angle, especially with fast subjects entering the frame suddenly.
If you read this far and didn’t yet watch the video tutorial, you’ll find much more detailed info there, including a follow-up video with Pro Tips for autofocus underwater.
Tips for Autofocus Underwater
I produced a follow up video on autofocus in order to share some tips and answer some of your questions. The video below discusses how to coach your camera to achieve sharp focus in tough situations underwater, minimum focus and working distance, photographing sharks, and AF for split-shots.
5 Tips for Shooting Fast Action