Focussing Techniques for Video – Part I Problem Diagnostic

Thanks to Brian Lim and WK’S gone diving for providing some examples.

When I started thinking about writing this post I thought of presenting a whole piece on the theory of focus and how a camera achieves it however I later decided it made more sense to start from example and then drill down on the theory based on specific cases.

So we will look at three common issues, understand why they happened and then discuss possible mitigations.

Issue 1: Wide angle Manta Focus Hunt

This clips has been provided by WK’s and has been taken during a trip to Socorro

The water is quite dark and murky and there is a substantial amount of suspended particles in water otherwise we would not have mantas. The water is also fairly milky and therefore the image lacks contrast which is not ideal for the camera to focus as all cameras, including those working on phase detection AF need contrast.

WK’s had a flat port and was shooting quite narrow aperture at f/7.1 which should ensure plenty depth of field on his 14mm lens.

In this clip you can literally see the autofocus pulsating trying to find focus the hunting carries on until the manta is very close at around 15 seconds in the clip. At that point the clips is stable however the overall approach has been ruined.

Diagnostics

The key observations are that the subject was not in focus at the very beginning of the shot and then you can distinctively see how some fairly bright particles come into the scene at 0.04 for example and disturb the camera process as they create a strong contrast against the black manta and the camera can’t decide who is the subject so it starts hunting. When the manta is close and well defined in the frame the camera knows she is the subject and therefore focus issues stop. The white particles in the water when the manta is far are large and bright enough to be picked up by the matrix point of the camera AF this is true regardless of the manta being in the frame and the same would have applied if another fish was doing a photobomb.

Solution

The problem in this clip is not new to video shooters similar things happen when you have the bride walking to the altar and someone the priest or the husband steps into the frame and they are far apart. On land you would keep control using manual focus or if you were really daring you would use tracking. In our case WK’s does not have focus gear and it is not possible for him to manually change the focus.

WK’s could have used tracking  if available on the camera. With tracking you need to ensure that the camera can lock onto the manta and then if it does that the manta does not turn or change shape and nothing bigger comes in front. At this point everything would work. This is a high risk technique only worth trying in clear water and when there are no particle in the water so in this scenario not advised.

The last option and the solution to this issue was for WKs to switch to manual focus and engage peaking. Use a single AF on to focus on his feet or an intermediate target and then check the manta was in focus. If focus was lost WK’s could have triggered AF again at least being able to control how many times the camera was refocussing.

Issue 2: Macro Subject Switching

This other clip has been provided by Brian Lim and it is a macro situation.

We can see that there are particles flying in the water and some other small critters at close range. The main subjects are the large crab and the two small crabs in the foreground.

Brian is not happy about the focus on this shot as not everything is sharp.

Diagnostics

Despite the murky water Brian has correctly locked focus on the crabs in the foreground and due to the high level of magnification the camera does not have sufficient depth of field to make the small and large crab crisp in the frame. It is possible that Brian could not detect on this screen that the crab behind was not sharp which could be avoided with peaking. In any case it is likely that there is no possibility to have this shot sharp end to end. Brian is super stable in the shot so he was set to make it work.

Solution

Brian does not have a focus gear on this camera this would have been required to pull focus in the same shot on the small crab and then go onto the larger crab.

However even in this situation in manual focus Brian could have shot two clips focussing on the two different focal planes and then managed this in post. It is critical to be able to review focus on screen when we shoot or to review right after before we leave the scene.

Issue 3: Too many fish and too much water

The last clip is mine and is taken during a recent trip to Sataya reef.

I have deliberately left this clip uncut because it lets you see that you can use autofocus in water behind a dome port and for most part it works but there are some pitfalls so the most photogenic dolphins at 00:50 are initially blurred.

Diagnostics

I was not expecting the sheer amount of dolphin on the day and certainly I was not expecting them this close so I had a standard zoom lens at 24mm FF equivalent behind a dome port. In most cases I managed to have some fish in the AF area of the camera but at 00:45 and 00:58 the camera does not have anything in the middle of the frame and goes on a hunt.

Solution

Working with a dome port and a lens of that nature does not warrant you will have enough depth of field to leave the camera locked even at f/8 so some refocussing activity was indeed required. In this case I was using a single AF area in the centre and in those moments the camera has just the blue and nothing to focus on and goes on a hunt, as soon as the subject is back in the AF area the camera locks back in. Note that the AF change speed is not fast enough to follow when the dolphin come too close therefore here the only real solution was to have a wider lens, however I could have avoided the hunt if I had set the camera to AF lock and intercepted the moment the AF area was empty preventing the camera to re-engage.

Summary

In all examples of this post the issues have been generated by a lack of intervention. All the situations I have analysed could have been dealt with at time of the shot for most part and did not require extra gear. I believe that when we are in water there is already lots to think about and therefore, we make mistakes or not apply the decisive corrective action that would have saved the shots.

In the next post I will drill down in focus settings and how they can help your underwater shots and also discuss how those apply to macro, wide and mid shots. I am also happy to look at specific examples or issues please get in touch. Specific coaching or troubleshooting is provided in exchange of a drink or two.

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2 thoughts on “Focussing Techniques for Video – Part I Problem Diagnostic”

  1. So for issue 1, you’re saying to use manual focus, and focus on something far since the aperture is relatively small?
    What if the object is coming towards you? Won’t it lose focus once it’s close enough? Also with manual focus, the ring is on the barrel of the housing and changing it will cause camera shaking. How do you account for these issues? Thanks!

    1. The clip was shot at 14mm 7.1 on GH5 if it had focussed at 2 meters anything between 1 meter and infinity would have been in focus. In essence he only had to focus once and disable continuous autofocus

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