PART 5 – FOCUS
Most cameras will have an auto focus mode with the option of switching to manual if desired. Unfortunately not all underwater housings will have the controls to let you access this. Some give you a choice of being able to manually change either focus or exposure. If this is the case then I would highly recommend leaving focus on auto and having full manual control over exposure.
Focus is critical to get right.
A picture can be a little too dark or a little too light, or even a little oddly framed, and still be acceptable to most audiences. But a picture that is even slightly out of focus is very hard to accept.
Professional cameras don’t have this option but most others do. Auto focus is a wonderful piece of technology, but no matter how wonderful, it still doesn’t know what you, as a camera person, actually want to focus on. It will take its ‘best guess’ as to what’s right and that generally is what is biggest and most central to your picture frame.
Let’s look at an example of when auto focus can really mess up your videoing.
You are in open water with a dolphin or two. There are no well defined images in the water except the dolphins themselves. Unless the dolphin is large in the frame the auto focus will have a terrible time trying to find an image to resolve itself on. Without the dolphin as a reference in your monitor you may notice that the auto focus is ‘hunting’ in and out trying to find a subject. Often it will settle on focusing on the small bits of dirt or bubbles on your lens port. (Note – always keep your port clean and check it constantly.)
Then a dolphin comes into frame. Will it be sharp? Who knows, it’s pot luck. If you then stay framed on the dolphin long enough, maybe just a few seconds, the auto focus will recognize the image and be happy. Sadly in those first few seconds of video, which may be the most important, the dolphin may not be sharp. Then as soon as the dolphin leaves frame the auto focus will become worried again and may start looking for a new focal point.
This may seem only a minor problem at the time, but believe me, when you get back into the edit room and look at the video, the first few seconds of out-of focus images will drive you crazy. Hence the advantage here of manual focus which you can set yourself.
For this type of shot with the dolphins it usually works out that wide angle framing is best and thus will have a good depth of field. Preset the focus to around 1 to 2 metres. This will generally give sharp images from the lens port to infinity. A point of reference for the pre-focal point could be one of your fins. Simply point the camera down to you feet and focus on the tip of your fin. A brightly coloured one would be most effective. Perhaps use your dive buddy’s bright tank. This pre focus trick can also work quite well for the auto focus especially if you do it moments before the dolphin enters frame.
With manual focus, although you do have greater control over what is sharp, the difficulty arises when you want to zoom in and alter the frame size. Now the 1-2 meter focal point won’t have enough depth of field and so you will have to adjust your focus to compensate. On zooming in the focus becomes much more critical and will have to reflect the true distance of the subject as the depth of field will now be quite small. We look at depth of field later.
With auto focus, zooming in, as long as you remain on the subject, can be very good, but if you are not already framed up on the subject the auto focus will have a devil of a time trying to resolve the image.
Ideally your image should be crisp and sharp. Even if the visibility or light values are bad.
A ‘hunting’ auto focus may give you a ‘soft’ unusable image while all the action is going on regardless.
Here, the auto focus has almost resolved the image for you but not quite in time.
(Next time we look at Lenses, Depth of Field and Angle of Coverage)