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Note how the focus range increases as the aperture is stopped down.

The range of the depth of field varies with the focal length of lens being used - the wider the angle of the lens, the more the depth of field so focusing is much less critical.

The effect of using different film

In order to achieve correct exposure you have to take the speed of the film into account. The faster the film, the quicker it reacts to light and vice versa. Common film speeds are: 50 asa 100 asa 200 asa 400 asa

Just as with aperture and shutter speed, there is a relationship between the film speeds in that 100asa is twice a fast as 50asa so 100asa is ONE STOP faster than 50asa and vice versa. For example, if the correct exposure for 100asa is 1/ 125th @ F8, it must be altered to 1/125th @F5.6 for 50asa (or any combination which achieves the same amount of light).

If you want to freeze the movement of a fish you will need to use a fast shutter speed but will have to "OPEN UP" the aperture to compensate. This in turn will limit the depth of field so you will have to be extra careful with the focus.

Exposure meters and light readings

The precise level of available light is best measured by using a light meter. This can be either a hand held meter or one built into the camera.

Fortunately, light levels underwater are fairly consistent in that the light is diffused but care should be taken when taking a light reading to make sure the correct exposure is achieved.

There are three main types of light metering systems:

Centre weighted

This is the most common and is similar to that in the Nikonos 1Va and V cameras. As the name implies, the meter takes more into account from the centre of the frame than from elsewhere and this does suit the majority of subjects underwater. The usual weighting is 75% from the centre circle of the frame and 25% from the rest of the area.

With this sort of system, care should be taken when the sun or any brighter area is in the frame as it may fool the system into thinking there is more light available than there actually is and this will result in under exposure. In this case tilt the camera slightly down or away from the bright area but still include some of it and this should achieve a correct exposure. With an automatic camera this is not practical as you will be pointing incorrectly so you will have to take a reading with the camera pointed down, note the reading and set the camera controls manually.

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