Exposure Control

Properly capturing an image requires the correct shutter speed and aperture combination to be determined and set, either automatically or manually by the photographer. A given light intensity can be properly exposed on film or a digital sensor with not one but a wide range of shutter speed and aperture combinations. Determining which combination is 'best' depends upon the intended result, ISO setting, ambient light, flash power, and lens focal length. ADJUST APERTURE TO MATCH FLASH-TO-SUBJECT DISTANCE @ FULL POWER

METER THE MIDWATER AREA

ADJUST SHUTTER SPEED FOR UNDEREXPOSURE OF -1 to -2

Diving in tropical water I normally keep the shutter speed at 1/125 and adjust aperture based solely upon flash-subject distance at full power, Manual exposure mode. In very shallow water I will raise shutter speed to the highest flash sync speed (1/180 second) to prevent burnout. In deep or dark conditions I may reduce the shutter speed to a minimum that matches the focal length and magnification factor of the lens in use. Ideally the shutter speed/aperture combination will allow a proper flash exposure at or near full power (assuming use of TTL flash) and ambient light underexposure between -1 to -2.

For example at ISO 50 at 30 feet, flash output at 3 foot subject distance of f/5.6 (fish or wide angle photos), the light meter reading taken from midwater on an average day in the tropics should give a shutter speed around 1/60. I set the shutter speed manually to 1/125 until going back into the shallows, where I'll dial it up a stop or to the highest sync speed.

APERTURE CONTROLS FLASH EXPOSURE

SHUTTER SPEED CONTROLS BACKGROUND BRIGHTNESS

SHUTTER SPEED & LENS FOCAL LENGTH

As a general rule the shutter speed should not be longer than the reciprocal of the lens' focal length to prevent blurring of an otherwise focused image. The image will be slightly unsharp overall. You must also account for the multiplication factor when the lens is used with a digital SLR (most are 1.5 or 1.6x) that has an image sensor smaller than a 35mm film frame.

Film or Full-frame DSLR 50mm = > 1/60 second 100mm = > 1/90 second

APS-C DSLR (1.5x crop) 50mm => 1/90 second 100mm = > 1/180 second

If you must use a slower shutter speed the camera and subject must either be completely motionless or add a burst of flash to create a sharp image. In the underwater environment, the flash option is the preferred method.

For macro the shutter speed should be left alone since the resulting aperture/shutter combination will result in a terribly long exposure thus producing camera shake/blur (1/8 @ f/22) and cyan shadows. This will happen automatically if you shoot on any of the automatic exposure modes (A,S, P, Green, Scene) with a small aperture. This technique can be used if you intend to create an artistic blur effect as long as the camera is held perfectly still during exposure. If your camera/flash has a Trailing Curtain Flash mode use it in this situation to create a blur that trails behind a moving fish as you pan to follow it.