Coral Hermit Crabs are generally a few millimeters in size and live in holes, abandoned by worms in the Madrepora Coral. It is best to photograph them with a strong diopter, typically +10, in front of a 100mm or similar focal length. At minimum focusing distance, autofocusing with this setup is close to impossible. It is advisable to lock focus at this distance – for maximum magnification - and move the camera back and forth until the creature is in sharp focus. Swiveling on a well-placed pointer with the camera sitting on the left wrist helps greatly. Once in position is a matter of waiting for the crab to come out of the hole before clicking. Given the reduced space between the front element of the diopter and the subject, the main challenge is to bring enough light in. With both strobes placed on the side of the diopter and attached to it, slightly angled them to the hole. This insure enough light on the subject and also a ‘wrap around’ soft lighting, ideal for reaching far into the crab lair.
The settings I used with a full frame camera, 100mm macro lens and +10 diopter are f29, 1/125 sec, 250 iso, at minimum focusing distance. Bear in mind that diopters increase magnification but also amplify light. Consider this when setting your strobes light output and maybe take a test shot on another similar Madrepora Coral with no crab in it to verify your setting prior of shooting your designated subject.