Aperture Priority

The photographer chooses the f number and the camera automatically works out the appropriate shutter speed. As the name 'aperture'suggests the emphasis is on the selection of the aperture by the photographer. The majority of underwater photographers favour Aperture priority as they can directly regulate depth of field. A general rule of 'thumb' is to consider the priority of the subject you have chosen. I will take this concept one stage further. Ask yourself this question:

'What is my priority with this photo opportunity?'

Is the priority having sufficient depth of field for the subject? Or, is it to have control of the shutter speed in order to capture a moving subject in sharp focus? This is the decision of the photographer with each individual subject chosen — there is no right or wrong way.

• Aperture priority is an advantage when shooting both close-up and wide angle. In close-up photography it is important to achieve a good depth of field and by selecting an aperture of f 16 a generous D of F is assured.

FIG. 3.7 In this example I have selected f8 for an adequate D of F with wide angle and the camera has chosen 1/125th sec as the correct shutter speed to expose the scene before me.

• We are fortunate that wide angle lenses provide the largest depth of field possible, so when shooting seascapes and sunken shipwrecks it is better to set Aperture priority in order to maximise depth of field so that the resulting image shows sharp detail from foreground to background.

• Another advantage of Aperture priority is that stepless shutter speeds are automatically selected by the camera. This means the shutter is not confined to set increments such as 1/60th sec, 1/80th sec, 1/125th sec. The on-board computer selects speeds of 1/96th sec or 1/243rd sec. The camera does not display these intermediary speeds, just the ones that we are all accustomed to, and by using stepless shutter speeds we get the optimum accuracy of exposure.

• An advantage, which is often overlooked in Aperture priority mode, is the amount of corresponding shutter speeds available.

Set to Aperture priority the on-board camera chip has 13 — 13 shutter speeds to choose from as opposed to just eight — eight apertures to choose from when Shutter priority is set.

Shutter Speed Range

4'' 2'' 1'' 1/2 1/41/8th 1/15th 1/30th 1/60th 1/125th 1/250th 1/500th 1/1000th sec

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