If you have some experience with video on land you will know that many professional videographers do not use autofocus but rely on follow focus devices. Basically those are accessories that control the focus ring of the camera and avoid the shake that you would create if you were turning the focus ring with your hand.
The bad news is that there are no devices to perform follow focus underwater and if you use a focus knob you will indeed create camera shake. This is the primary reason why I do not use focus knobs on any of my lenses with the exception of the Olympus 60mm macro and in those rare occasions I uses it I do not actually use to obtain focus but to ensure I am at the closest working distance.
So how do you achieve good focus if you can’t use a focus ring and continuous autofocus cannot be trusted? There are essentially three methods that I will discuss here and provide some examples:
- Set and forget
- Set and adjust
- Optimised Continuous Autofocus
You have noticed that there is still an option for continuous autofocus in the list. Before we drill down in the method I want to give some background on autofocus technology.
If after reading this post you are still confused I recommend you get some tuition either joining my Red Sea trip or 1 to 1 (offered in Milton Keynes area in UK).
Contrast Detect vs Phase Detect and Hybrid Autofocus
The internet is full of autofocus videos showing how well or bad certain camera perform and how one system is superior to another. The reality is that professional cameramen will use follow focus in majority of cases and this is because the camera does not know who the subject is.
Though it is true that one focus system may perform better than other you need to consider that Red cameras use contrast detection autofocus same as your cheap compact camera so clearly autofocus must not be that important.
The second fact is that any camera focus system needs contrast including phase detect. Due to scattering of blue light in water there are many situations where the contrast is low in the scene resulting in focus hunt of the camera autofocus system.
So my first recommendation is to ignore the whole discussion about which focus system is superior because the reality is that there will be situation where the focus will be difficult to achieve and the technology will not come to help. You need to devise strategies to make things work and this is what this post is about.
Let’s go now in the techniques.
Method 1: Set and Forget
As the name implies with this method we set focus at the beginning of the shot and never change this again. This means disabling the camera continuous focus in video mode. This is essential so that this technique works.
This works in three situations:
- Using a lens at the hyperfocal distance behind a flat port
- Using wet wide angle lenses
- Using fisheye lenses
Method 1.a Hyperfocal Distance Method
I am not going to write a dissertation on this there is good content on wikipedia worth a read: https://en.wikipedia.org/wiki/Hyperfocal_distance
The key concept is that depth of field at a given aperture and subject distance will reach infinity. The wider the lens closer this subject distance. For example a 14mm lens on a micro four third body at f/5.6 is 1.65 meters so if you focus on an object at this distance anything between 0.8 meters and infinity will be in focus. As you close the aperture the hyperfocal distance diminishes. This technique is good for medium or reefscape shots where you don’t mind that the whole frame is sharp in focus. It is not suitable for macro or close shots as the aperture required would be too small and diffraction would kick in.
Looking at the past CWK clips if continuous autofocus was disabled and he had focussed just at the start of the scene at 1.85 meters no focus was required until the manta was at 0.9 meters. Note that distances have to be adjusted to account for magnification of water effect.
Once you have your lens and aperture setting you can quickly work out some distances in your scene and fine tune your expertise.
Obviously shooting those shots with a flat port is not exactly the most common method however understanding this technique is paramount to the other two.
Method 1.bc Wet Lenses and Fisheyes
Fisheye lenses tend to have an incredible amount of depth of field even wide open and therefore the set and forget applies in full here without even bothering about hyperfocal distance. Usually focussing on your feet is all is required.
The real revelation to this technique are afocal wet lenses. Afocal means that the focal length of the wet lens is infinity and the light coming through does not diverge or converge. Together with the magnification factor typically 0.3-0.4x means you get to a fisheye situation without the same amount of distortion.
This is the primary reason to buy a lens like the Nauticam WWL-1 or even an Inon wet lens with afocal design.
My Tiger and Hammerhead videos are shot with the camera locked in manual focus after focussing on my feet.
Even when the shark hits the camera the image is in focus
I do not have technical information on newer Nauticam WACP-1 or WACP-2 so am not in a position to confirm if those lenses are afocal or not and therefore I cannot help you. I would think consideration on depth of field still apply. If Nauticam or a shop or user lends me a set up for pool testing I can provide optimise settings for WACP.
Set and forget is the number one method for wide angle and reefscapes underwater and it is easy.
Method 2: Set and Adjust
As the name implies this method sets the focus at the beginning of the shot and then adjusts when required this is necessary especially in macro situations.
The set and adjust method varies depending on how the camera managed push on focus. If the camera manages a refocus using a half press shutter no other settings are required other than disabling continuous auto focus.
For cameras that do not have a refocus half shutter setting you need to operate in manual focus and the set a custom button to perform a single auto focus.
In both cases you need peaking to be active during the shot.
- Set the focus as required using half shutter or AF On button
- Observe the peaking to ensure the subject is in focus if required moving the camera.
- In case of loss of focus refocus using the shutter or the AF On button
This method works well with macro where typically you set focus and then move the camera back and forth to keep focus, in those cases where you want to switch focus on another part of the frame you refocus. This would have helped Brian in the two crab situation.
As the refocus does bring a moment of blur in the clip you need to ensure that when you trigger the refocus the camera will succeed this is best achieved when using a single area of focus.
Method 3: Optimised Continuous Autofocus
Although autofocus has some risks there are situation when this is required those include:
- Shooting aperture that do not have sufficient depth of field to warrant a set and forget
- Using dome ports and rectilinear lenses from what I have experienced those lenses do not work well with hyperfocal distances due to physics of dome ports
Obviously the best option remains using a wet lens and set and forget however there are instances where we absolutely want straight lines for example shooting divers or models. In those cases we will use a dome port and as we can’t use a focus gear because the camera would shake we need autofocus.
Focus Area Settings
Cameras have a selection of modes to set the area that will be used by autofocus:
- Face / Animal recognition -> locks on recognised shapes
- Multi area -> selects the highest contrast area in a number of smaller area of the frame cameras have up to 225 or more areas and you can customise the shape of it
- Single area -> an area of selectable size and position in the frame
- Tracking -> tracks the contour of an object in the frame
Face recognition and animal recognition are not useful in our case.
Tracking requires the object to keep the shape within the frame this is useful for nudibranches for example or anything that does not change shape in the frame, a fish turning for example will be lost by this method so this is seldom used. To be honest this fails also on land most times.
So we really are left with multi area and single area.
My advice is to avoid multi area because particles in the water for example can generate sufficient contrast to fool the camera and make it lock on it.
So the best option is to use single area, I typically set this to a size smaller than the central third of a nine block grid. With this configuration is also possible to focus on a subject off the centre by moving the area within the frame. This setting works well when the subject is tracked by our movement and the subject is in the centre which is the majority of situations.
This video is shot on a 12-60 mid range zoom using single area AF for all scenes including macro.
The single more significant risk for single area is that if the centre of the frame goes to blue water the camera will go hunting so if you are shooting in caves or on a wall make sure the AF area is on one side of the frame to avoid hunting or lock occasionally focus to prevent the camera seek focus that won’t be found.
Achieving focus in underwater video requires different techniques from land use and a good understanding of ports and optics.
If you think you are not skilled enough and need help from autofocus my advice is to get an afocal wet wide angle lens. This will transform your shooting experience and guarantee all your wide angle to be in focus. If you work in a macro situation you need to master the single AF setting of your camera and make sure you are super stable.
The most difficult scenario is using dome ports and this is one of the reasons I do not recommend those for video. If you are adamant on rectilinear lenses than the specific settings.
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