Sony F-717 digital
Sunpak MZ4400AF flash off-camera
47.5mm w/ OM 50mm f1.4 reversed on front, f8
1/500 second, ISO 100

Pushing the limits
acorn barnacle Sessilia feeding

There are just some images that I consider a worthwhile challenge to capture. This was one of them, though I'd still like it to be far better. What you're looking at here is an acorn barnacle (order Sessilia,) those little pale crusty things that decorate pilings, ships, and even whales the world over, and this is how they feed.

The delicate-looking "nets" (actually their feet, believe it or not), are swept through the water rapidly, much like the porcelain crab does, gathering microorganisms. The motion is very rapid, 1 to 3 sweeps a second, and only a diaphanous blur to casual observation.

Since my photo subject here is only the size of a small pea, getting any kind of detail at all required using a macro trick called "lens stacking" – putting a 50mm lens reversed onto the front of a telephoto. The built-in lens of the Sony F-717 was at maximum tele, even though the focal length seems to deny this, but since the digital sensor is much smaller than a 35mm film frame, it is comparable to 190mm in film terms.

What this does, however, is drop depth-of-field, the range of sharp focus, down so small that only by catching the feeding appendage at a precise point would it even be the least bit sharp. Off by a millimeter, and the image would be unusable. Notice the speed of the sweeping motion above? Uh huh, now you're getting the idea. Basically, this kind of photography is strictly hit-or-miss, but mostly miss. Over and over again.

cilla of feeding acorn barnacle SessiliaAfter an uncountable (but truly disgusting) number of failures, I managed to get a frame that showed the fine feathery "toes" of the feet (cirri) in decent detail. Something I would only attempt with a digital camera ;-). And while it could be better, I know the hurdles I had to go through to get it, and I'm proud of it. Web display doesn't do it justice either, since much of the detail gets lost when you size the frame to get an overall view. I've included a full-resolution detail inset at right. There is no way for me to determine an accurate measurement of the little combs that do the food capture, but I'm estimating between 0.2 and 0.5mm. Bear in mind that this is taken through the sides of a basic consumer fishtank, not produced for optical qualities, and several centimeters of unfiltered water. I'm pleased with it.

Barnacles were long thought to be molluscs, shellfish like mussels or oysters, but they're actually crustaceans – directly related to crabs. Their anatomy is pretty bizarre, and Charles Darwin devoted quite a few years of study to them while working on Origin of Species. For my part I have mixed feelings, because while I think they're pretty cool little critters, they're also very sharp and tend to coat every rock where I go snorkeling – this results in no small number of cuts and scrapes on each dive.