Mastering Underwater Autofocus with Alex Mustard and Matthew Sullivan

In a recent episode of The Underwater Photography Show. We answered a fantastic question from @gregthomas6400, a diver who is new to underwater photography. Greg asked about our autofocus techniques, specifically whether we use AFS, AFC, or 3D tracking, and how much of his topside wildlife photography skills apply to shooting underwater. This is a simple question, but one that lacks a simple answer.

The truth is, there isn't one right way to do autofocus. In fact, if you're overthinking autofocus at the moment of shooting, you're probably wasting your time and missing the photo. Autofocus was designed to make your life as a photographer easier, freeing you up to think about the creative aspects of your shot—like composition, lighting, and what you’re trying to say with the image. The right autofocus for you is simply the one that works.

Let's start by breaking down the underlying technology and then we can share our personal approaches.

The Complicated World of Autofocus Technology

First and foremost, autofocus is not a single, universal system. The way different cameras achieve focus varies wildly, which is a major reason why there is no one-size-fits-all solution for everyone.

• Contrast Detection: Compact cameras typically use a system called contrast detection autofocus. This method works by analyzing the contrast in the image. The camera will move the lens back and forth until it finds the point of highest contrast, which it determines is the sharpest focus. This can be very accurate, but it's also quite slow because the camera doesn't know which direction to move the lens to get into focus.

• Phase Detection: Most SLR cameras and the top-end mirrorless cameras use phase detection autofocus. This system is much faster because it uses a separate sensor or sensors on the main imaging sensor to measure the light from two different points, which allows the camera to instantly calculate the direction and distance the lens needs to move to achieve perfect focus.

It's also important to remember that all these systems are electronic and rely on the brainpower and energy of the camera's processor. This is why two cameras that seem to have similar autofocus systems might perform very differently—one could have a much more powerful processor behind it, allowing it to run a lot faster.

To complicate things further, every manufacturer—Olympus, Sony, Nikon, and Canon, to name a few—has its own unique language and options for autofocus. This means a setting that works brilliantly on one camera might not be the best option on another camera, even from the same brand.

Watch us discuss the technical differences between autofocus systems here.

AFC vs. AFS: A Continuous Debate

One of the first decisions you'll face is whether to use AFC or AFS.

• AFC (Continuous Servo Autofocus): This mode means the camera continues to focus all the time you are half-pressing the shutter button.

• AFS (Single Servo Autofocus): With this mode, the camera will focus once and then hold that focus until you take the picture.

For underwater photography, we would say that AFC is the way to go. As underwater photographers, we're rarely perfectly still. You are constantly moving, and so is your subject, especially if it's a fish or other marine life in the water column. For this reason, continuous focusing is almost always the right choice.

However, some older mirrorless cameras and compact cameras have very poor continuous autofocus performance, but very good single-servo performance. In those cases, that advice may not apply to you, and you might find more success with AFS.

See our explanation of AFC and AFS here.

The Back Button Autofocus Method

Another common question is about back button autofocus. Many underwater photographers like to use this technique, which involves setting a separate button on the camera housing to activate autofocus. When you press the shutter button to take a picture, it doesn't activate any autofocus. This decouples the focusing from the shooting.

This method is great for a lot of people. It allows you to focus very carefully and then take a load of pictures without the camera refocusing. Many truly excellent photographers shoot this way.

Alex's Take: I personally don't see the reason why I need to press two buttons when I could just press one. I generally have my autofocus set on the shutter release for most of the pictures I take. The only time I use back button focus is when I specifically don't want it on the shutter button. For example, if I'm shooting a split-level picture and want to focus on the underwater part and not focus again, or if I'm in a dark wreck or cave where the camera might struggle to find focus. With my Sony A1, it focuses so well on super macro that I've found I can just use the shutter release now, and it works perfectly.

Matthew's Take: I, on the other hand, am one of those people who uses back button focus exclusively. It's one of the first things I set on any camera I use. At this point, it's largely muscle memory for me more than a necessity. It’s just how I've gotten used to shooting, so I don't plan on changing it anytime soon.

Watch us discuss our preferences on back button focus.

Our Personal Approach to Autofocus

We both gravitate towards simple, effective modes and use them almost all the time.

Matthew's Simple Setup

I currently shoot with a Sony A1, and I always have AFC enabled. I literally have only two autofocus modes that I use.

• For Wide-Angle: I primarily use Zone and Auto. The Zone mode on the Sony covers about the center two-thirds of the screen. The A1 is reasonably good at finding the main subject in that box and keeping it in focus.

• For Macro: If the camera gets confused, especially in darker environments or with macro subjects, I'll switch to a smaller, single-point autofocus box. I can move this tiny box around the screen to pick a precise spot to focus on. It still uses continuous focus, so as long as I hold down the back button, it will stay with the subject as I move slightly.

Alex's Simple Setup

My approach is similar in that I also have a few favorite modes on my Sony.

• For Wide-Angle: I use Wide Area autofocus almost all the time. This mode turns on the whole autofocus sensor, and it's very good at picking out the obvious subject and focusing on its front. If a shark or diver moves across the frame, the camera has coverage on the edges and will stay focused on it. If the camera is struggling, I'll switch to Zone, just like Matthew.

• For Macro: I prefer to use a single point of autofocus but in Tracking mode. This is exactly the same as Nikon's 3D tracking. I start with the single point on the subject’s eye, and then I can recompose the picture slightly, and the camera will follow the eye across the frame. I usually use a small or medium-sized point for this. For really static subjects, like a nudibranch, I'll sometimes turn off tracking and just use a static single point.

The Future of Autofocus: Subject Recognition

Many of the newest mirrorless cameras come with subject recognition. While no camera has been programmed yet with specific underwater subjects, you can use their general animal, bird, or insect modes. The generic animal mode is usually the best bet for underwater subjects.

When the subject is small in the frame, the camera will pick up the whole subject. But when the subject is large, the camera is really looking for eyes. This feature works with newer cameras, and we have both been very impressed. The Sony A9 III and the Canon R5, for example, have a noticeable difference in how quickly they can figure out exactly where the eye is. I, Matthew, vividly remember the first time I took the A9 III on a dive, and it stuck to a seahorse's eye and never let it go.

This technology is a great example of autofocus being there to help you so you can focus on composition and lighting. However, it can sometimes be a bit too smart. We've seen it get confused by a moray eel's nostrils or a butterfly fish's false eye.

This is why, if you're going to use subject recognition, we recommend making sure it's easy to turn on and off. I, Alex, have one of the levers on my camera programmed to be the subject detection on/off button. This way, if the subject detection starts to get confused, I can turn it off and go back to a simple, manual system.

Here we discuss how subject recognition works (and fails) underwater.

Final Thoughts: Keep It Simple, Get the Shot

The most important takeaway is this: autofocus is there to help you. It's not about using every possible mode of your camera. The more thought you are putting into autofocus, the less thought you are putting into other things that matter, like light and composition.

The simpler you make it, the more likely you are to think about those important elements and end up with a better shot. The reason camera companies give us all these options is because subjects are different, the way people shoot is different, and photographers have different preferences. The choices that work for you are the correct ones.

So, don't feel like there's a right or wrong way to do it. Find the modes that work for you, keep it simple, and get the shots.

If you want to see us discuss this with all the gory details, check out our episode below.