The main consequence of slow shutter speeds in macro photography is that they allow us to burn in blue backgrounds, which dramatically changes the appearance of our images. For clean blue backgrounds it is best to shoot into open water. A slightly upwards camera angle is good because it can help reduce the exposure time. It is not always possible to isolate a subject against open water, yet we can still get blue backgrounds from reef or sand so long as we are deeper than about 8 metres and follow a few simple rules.
First, we must aim our strobes so they light only the subject - we won't get a blue background if we light it with flash. Second we should chose a shooting angle that puts the background as far back from the main subject as possible: this helps with the lighting but more importantly ensures we get good subject separation and the background is out of focus, less distracting and, of course, blue.
A particularly important
consideration, if we are using sand or the reef as a background, is that it must be brighter than the main subject. If the main subject is brighter than the background it will be overexposed when lit with both ambient and flash light. Dark coloured subjects are ideal, white subjects will hardly ever work. If your subject is a similar brightness to the background choose a camera angle that casts it in shadow or silhouette.
Aesthetically, we should also consider the colours. If we are in tropical seas then our backgrounds will be cyans and blues, so we should select subjects with complimentary colours. I find red, orange and yellow subjects most attractive with cyan or blue backgrounds.
Judging the exposure for the image is most easily done with spot metering - spot meter on a nice mid-brightness area of the background and
Choosing a yellow, orange or red subject will provide a good colour contrast with a cyan or blue background.
Nikon D100 + 28-70mm lens. Subal housing. 1/180th sec @ f4.8. 2 Subtronic Alphas on 1/64th power.
set the exposure. If the camera doesn't have spot metering then you are best to take an average (matrix) exposure for the whole scene and then underexpose this by 1/3 to 2/3 of a stop. This underexposure accounts for the main subject which should be silhouetted or slightly darker that the background - remember the rules
Wide aperture macro also works on shots that are only lit with flash. Try to select backgrounds of strong, solid colour as details will be out of focus. Nikon D100 + 105mm lens. Subal housing. 1/180th sec @ f5. 2 Subtronic Alphas on l/32nd power.
above! TTL will do a good job with the main subject if it is large in the frame. If not it is best to bracket the TTL (if your camera lets you) to underexpose slightly by about 1/3 to 1 stop. With manual flash I find I often reduce the flash lighting by about 1 stop from a flash only exposure.
As I said above, we use slow shutter speeds in macro photography to get balanced light images (i.e. blue background and flash filled foreground) while maintaining the large depth of field achieved with a small aperture. The obvious problem with slow shutter speeds is keeping the camera still - so that we do not create ghosting of the main subject from movement during the exposure.
The best advice I have here is to select subjects where you can rest the camera on the sand to take the shots. I found this technique particularly suitable on muck sites in Sulawesi. Choose subjects that do not have loads of wavey tentacles, polyps, fins etc, as these will undoubtedly move around during a long exposure. But
Select a shooting angle that accentuates the subjects separation from the background in the shallow depth of field, to give the image a three dimensional look.
Nikon D100 + 105mm lens. Subal housing. 1/180th sec @ f5. 2 Subtronic Alphas on l/32nd power.
there is no miracle cure, and I recommend taking several exposures to ensure you have a sharp one!
The alternative approach with a moving subject is to pan with the subject, allowing the background to streak and accentuate the movement.
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