Snapper School - The secret of 'Focus Lock'

The moray eel is blurred while the corals are in focus Coral in the foreground is sharp with the diver artistically blurred

MY CAMERA'S AUTOFOCUS SEEMS TO HAVE A MIND OF ITS OWN - HOW DO I BEND IT TO MY WILL?
One of the greatest frustrations with underwater photography is when your autofocus camera is deciding where it is going to focus and you don’t happen to agree! Most modern cameras have intelligent focusing ‘brains’ that are designed to try to get it right in a wide variety of situations, but none of them have been specifically designed for underwater use. They are designed to work in air, but in water there are countless semi-visible particles that can fox the autofocus system, causing it to choose the wrong point of focus.

SO WHAT CAN I DO ABOUT THIS?
As I mentioned, most digi-compacts have a system of intelligent settings, and the best time to play with them is when your camera is fresh out of the box. However, they can be tweaked whenever you like. You need to access the camera’s menu screens and navigate to the focus control adjustments. This will usually have a choice of three modes, labelled something like ‘intelligent Ai/Af’, then you get the ‘face detect’, and on most cameras there is a setting called ‘center’ or ‘central’. This last setting is what you should choose to have more control over the camera’s precision in focusing.

RIGHT - I'VE DONE THAT. WHAT'S THE NEXT STEP?
Remember last month’s technique of exposure lock? In short, you need to practise – with the camera in its housing – how to half-depress the shutter release, until a green light or (more commonly) a green square illuminates in the very centre of the LCD screen. You need to practise the light touch necessary to maintain and hold this so that the green square stays lit as you move around. This is quite a delicate technique, so I suggest that you practise at home as we are trying to achieve a slightly different result to last month.

MY LITTLE GREEN SQUARE IS LIT, SO HOW IS THAT NOW USEFUL?
You have told the camera the very point of focus you want it to concentrate on. Let’s imagine you have pointed the camera at a fish in a cleaning station; or, more precisely, you are trying to focus on the fish’s eye – as this is the part of the picture you definitely need to be sharp – but you would rather have the fish at the bottom of the frame as it makes a more pleasing composition. If you have practised your focus-lock technique, you should be able to choose wherever the point of sharp focus in the frame should be.

Pointing the camera at your desired object of focus and holding down the shutter release halfway will give you the control over this.

DOES THIS MATTER ALL THE TIME?
If you are using a wide-angle or fisheye lens, such precise focusing is less important, because such lenses have an ability to focus over a large depth of field. The latter phrase is bandied around a lot by photographers, and refers to the area in front of and behind the point of focus that remains sharp. With some wide-angle lenses, this area can extend from a few centimetres away up to the limits of visibility.

A wide-angle lens can be a handy tool, but if you are shooting fish portraits or using your zoom to isolate something from its background, focus locking can be a very useful technique. In the picture taken below left, the photographer has used this technique to focus on the corals in the foreground, while leaving the diver artistically blurred in the background.

HANG ON - ISN'T THIS JUST THE SAME TRICK AT LAST MONTH'S LESSON ON EXPOSURE LOCK?
The technique of holding down the shutter release is the same, and to tell the truth the focus and exposure locks work well in tandem. There will be times when they are not mutually compatible, but this is the exception – in these circumstances, I suggest you use the point of focus in the picture as your priority. Next, fine-tune the exposure using the ‘exposure compensation’ technique featured in Snapper School in the April 2008 issue.

MEMORY CARD

• Use the camera’s menu controls to access the focusing settings
• Switch from ‘intelligent’ or ‘face detect’ to ‘central focusing’
• Practise half-pressing the shutter release with camera in housing
• Try using your zoom a little to exaggerate the depth of field effect, and point the camera at different areas in the scene
• Try different points of focus and check the results on your LCD


• Paul Duxfield (www.paulduxfield.co.uk)