This article is an adaptation from an article that I had on my old site. I still get requests from people about the technique and it is actually hard to find info on online. So, I thought I would update it and put a quick tutorial back up.
One of the challenges in underwater macro photography is focusing on your subject. Much of the time, auto focus works fine, but for situations where it doesn't, or if you prefer to focus manually (many macro photographers do), there is a technique called "Lock and Rock" that works very well once you get used to it. I personally feel it works best with cameras that have focus peaking, but some users do it straight through the viewfinder. For the purposes of this tutorial; we will assume you are using focus peaking.
Before we get to the focusing part, there are a couple quick things to do once you locate your subject and before you get lockin' and rockin':
1. Make sure your camera and strobe are on and strobes are positioned.
2. Switch your shooting mode to “Manual”, or select your macro preset.
3. Prepare or "lock in" your manual focus distance.
How I lock in my manual focus distance: I put my finger in front of my camera lens at the closest point it will focus and give the shutter a half-press to activate the auto-focus. With your camera in Single Shot AF-S focus mode (or equivalent setting - don't use a constant focus mode), it will automatically focus on your finger and set the focus to the shortest (or desired distance). Once you get it focused at the closest distance, activate your manual focus mode and your camera will now be set and locked to your closest focal distance. With a little practice, you can do this in as little as 2 seconds. I have my approximate focal distance memorized by feel, and I can lock in my focus very quickly.
Now, if everything is set up correctly, if you put your finger in the front of the lens, you should see your finger surrounded by little white dots which adjust as you move your finger in and out of focus. This is the Focus Peaking effect that we mentioned earlier, and is very useful for macro photography. Now all is left to do is to take the photo.
Line up your shot with the camera lens very close to your subject. Holding your camera as stably as possibe, slowly move back and forth until you see your subject start to peak in and out of focus. This technique is called “rocking”, and is very useful in macro photography. You will know when to shoot when the white dots appear over the area that you want to be in focus. I like to pull my peaked area just a hair to the front of the subject, rather than hitting it spot-on.
Once your subject has the white dots where you want them, press the shutter release, and you should have an in-focus macro shot. This technique takes some time to master, but is really a great way of taking consistent super macro shots. It may sound super hard, and be very difficult at first, but I promise that if you stick with it, you will come to depend on it as a technique in many situations. With my Sony Rx100 rig, I find it much faster and more accurate than auto-focus.
MIf the photo is not to your liking, you can easily make adjustments to your aperture value (on smaller sensor cameras, be careful going to 8.0 and above, since you will start losing sharpness due to diffraction). You can also quickly adjust the shutter speed up or down to give you more or less ambient light by turning the rear dial.
Lastly, when in manual focus mode, many cameras have a magnified MF assist mode, which will give you a multiplied view of the subject. It can be a very helpful tool in precisely locking in your focus.
That's really all there is to it. Get out there and get lockin' and rockin'!