STEP 4 FLASH-TO-SUBJECT DISTANCE

I use through-the-lens (TTL) control of flash output, like a good many other underwater photographers, and it's a fair question to ask why we bother about flash-to-subject distance at all, when there is this magical facility on modern flash units which does all the work for you. Well, it can't do quite all of it, and that is the problem. If the flash is too far away, as in picture A, then enough light will not be delivered to illuminate the soft coral, as has happened here, when the flash was probably three feet away.

Deep red soft corals, in particular, tend to soak up light more than we might expect.

The converse can happen if the flash is too close to the coral, when it can overpower the TTL process and lead to burn-out, as in picture B. Here the flash was only 6in away. The soft coral is just about acceptable, but the white hard corals that surround it are over-exposed. I chose this subject deliberately to have white areas in the picture, as you can see the difficulty this can lead to. Again, I try as a general rule to choose subjects that will not provide a conflict of interest, as has occurred in this shot. The optimum flash-to-subject distance for the finished picture was between about 12 and 15 inches. How do I know this? Well, using Velvia 50 ASA, my chosen f-stop of f16, and my SB-25 (set routinely on -0.7 exposure values), experience has taught me that this distance will permit the TTL to work to its top performance and give me the exposure that I want.

By running a test film using each aperture in turn, and varying the flash-to-subject distance in a repetitive order, from very close to far away, I have found that with Velvia the optimum distances are very roughly about 6-9in at f22; 12-15in at f16; 18in-2ft at f11; 2ft 6in-3ft at f8 and 3-4ft at f5.6. These distances are only approximate, and again, I try to bracket the flash-to-subject distance from too close to too far away when taking the picture. A dark subject will need more light, while a paler one will need less, as we have seen. As a final note, I always hand-hold my flash as it gives me a better feel for where it is when I'm busy looking through the viewfinder. Also, I use an aiming light, as seeing it through the viewfinder helps me direct the flash with more accuracy. The aiming light is also useful to determine the colour of a subject when making the initial choice of what to shoot.

STEP 5 WHERE TO FOCUS, AND DEPTH OF FIELD

For me, there is really only one place to focus in this picture, and that is right at the front, on that red coral. I have thrown away more shots than I would care to mention because they were softly out of focus where the eye notices it most - at the front. With my SLR, I use auto focus to do the job for me, while Nik V users will have to focus manually. With wide-angle lenses and small apertures, depth of field is huge, and most of the rest of the picture will also be sharp as a result. This is yet another reason why shallow water shooting is so convenient, as the high f-stops allow you to have so much of the picture in focus.