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Appendix B. Guidelines For SharkEye
Photography
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Light levels
should be high enough to keep exposure times below 1/20 second with a 6.5 mm wide angle lens. Half
the maximum exposure time for each doubling of the lens focal length. Longer exposure times result in blur because
the diver or ROV pilot can rarely hold the camera steady enough. As a guideline, a 150 watt light are sufficient for
close-up shots to 1 to 2 feet (depending on visibility). Doubling the range to 2 to 4 feet will require a four fold increase
in light power (600 watts). Due to the high red light absorption characteristics of water, artificial light will not be
effective for color photography beyond about five feet.
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Close-up work
requires the use of a stand-off mechanism to obtain proper focus. The distance range that is in focus
is very small for close-up work, so without a stand-off, the diver will not be able to keep in the focus range. Mount two
rods, one on each side of the camera. Determine the proper focus setting by immersing the camera in a bucket and
focusing on objects at the bottom of the bucket. For example, we found a 312 step setting to work best for rods
protruding 9” from the face of the view port. The left to right distance in the image with the 6.5 mm lens will be about
1.3 times the distance of the object. For example, if the object is 9” away, the left-to-right area viewed will be 12” long.
Top to bottom will be about 1.1 times the distance of the object.
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Increase the distance range in focus
(depth of field) by closing the lens aperture somewhat. For each doubling of
the f-stop number on the lens, the depth range will roughly double as well. Unfortunately, exposure time will double
too – so there are limits. For example, when focusing on objects a distance of 9” away, perhaps the focal range is
8.5” to 9.5” (1”) with an f-stop setting of 1.4 (lens wide open). The exposure time with a 150 watt light may be 1/200
second. You could close the lens to f-stop 5.6 (1.4 x 4), thereby increasing the focus range to 4” from 1”, and
requiring a lengthening of exposure time to a still acceptable 1/50 second. Closing the aperture down somewhat will
also improve image quality because the less precise edges of the lens play a lesser role.
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Find good camera settings and an imaging strategy
, then stick with them. The feedback process through the
umbilical is slow. The diver or ROV can’t hold still for even several seconds, so you can’t keep adjusting the image
hoping to improve it. Instead, find the best setting for the type of shot you are doing. For example, you may find that
end shots of oil rig anodes require a stand-off distance of 2 feet and an exposure time of 1/30 second. Dial these
numbers into the camera, then ask the diver to move into position and aim the camera. On the diver’s command
(when he obtained the most steady position and proper aim), acquire the image by left-clicking in the left display area.
Ask the diver tyo relax and wait for the image to download. If the decimated image looks good, left click on the right
display area and inspect the cropped image. If this looks good too, save the image. If not, ask the diver to repeat –
possibly after doing some adjustments for focus or exposure.
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Keep the camera neutral buoyant
. Attach proper flotation, so the diver will not have to strain holding the camera.
Straining will increase movement and reduce the diver’s ability to aim.
Obtaining good underwater images is no accident, but requires careful planning. The beam of the lihght source must
cover the field of view of the lens, and ‘hot spots’ must be minimized. Light levels must be sufficient to keep exposure
times short enough to avoid blur, or a more stable diver/ROV position must be sought. The light source must be placed
so as to minimize back scatter. Distance to target must be short enough to obtain good image detail in the applicable
water conditions. The F-stop of the lens should be selected to obtain a sufficient depth of field, and the camera must be
focused correctly.
Here is a gallery of good and bad images, with suggestions for improvement. Please make sure to become very familiar
with the operation, capability and limitations of your camera before embarking on an offshore operation.
