We shot all of these with a Canon 20D digital SLR camera and a Canon 17-40mm zoom lens with no lens filter. We've never found a filter, regardless of claims of multi-coating, that did not increase flare with this lens in strongly backlit pictures such as numbers 2, 3 and 6, above. The zoom capability is really handy, but increases risk of flare because of the increased number of lens elements needed in a zoom lens.

All photos except numbers 9 and 12 used flashbulbs fired by radio-slaves triggered by a hot-shoe mounted radio transmitter. This equipment allows hand-held, backlight-only shots such as 2, 3 and 11 without the need to block the visible output of a flash near the camera that would be used to trigger an optical/UVslave for the backlight. We had poor results with attempts to use optical/UV slaves (e.g. Wein and Firefly) for this sort of shot. A slave unit placed directly behind the human subject of the photo (needed to provide the rim-light/backlight) simply does not see enough UV light to be triggered. The radios are not perfect however; some small and twisting passages seem to absorb the radio frequencies, yielding worse results than the optical slaves, so we carry both types into the cave.

On photo 1 we used a snoot on the flash that lights Ken Walsh's face. The snoot restricts the ouutput from a Vivitar 283 strobe to a 6-degree beam. The backlight in photos 1 through 8, 10 and 11 is an M3B bulb in a Honeywell flash gun, fired by a radio slave. The front light in photos 4 through 10 is the venerable Vivitar 283 strobe in the "manual" setting (full output). The artificial lights in photos 12 and 13 are the big (dangerous) No. 2B screw-base flashbulbs. The main light in photo 12 and the back light in photo 13, both of the entrance to Sinks of Gandy Creek, are daylight. In order to get the aperture we wanted (f/8) at 100 ISO, the shutter speed was a slow 0.6 seconds. Doug Medville was able to hold perfectly still for six tenths of a second (sounds easier than it is) as is evident from the lack of any light trail around his headlamp during the tripod-mounted exposures.

Photos 9 through 11 are of the same scene with different lighting - a section of Randolph County's Nelson Cave. Photo 11, lit by a single electronic flash held at arm's length below the camera, does a decent job of showing passage configuration, but is rather flat and lifeless (an on-camera flash would have been even more lifeless). Photos 10 is the same setup with a backlight. Photo 11 is backlight only. Photos 10 and 11 demonstrate how the backlight-only effect draws your attention much more to the shape of the cave; photo 10 is a person in a cave, whereas photo 11 is a cave with a person in it.

Thanks to Ken Walsh and Hazel and Doug Medville for playing along.

For more information on digital photography with flashbulbs, see here.