This is a technique that is pretty much second nature to anyone shooting a wide angle lens underwater. Unlike above water photography most shots are taken into the sun to either produce the clearest blue or to use the sun ball as a feature in the image. Getting the exposure right takes practice, however, a general rule of thumb is to expose for the background water column (dialing in some underexposure to manage the sun), with a little and then use fill flash to brighten up the foreground subject. The following is a good example. This was taken very early in the morning shooting up the reef at around 15m below the surface. We went into the water in near darkness, so the sun is very low on the horizon, if the sun is high in the sky it is much harder to balance the exposure.
The real challenge is positioning the two flash guns to illuminate the reef fully, in this case I have strong light fall off to the right of the frame, not disastrous, but this would have been much better if I could have got the balance better. Difficult to do in current whilst floating free off the edge of a reef. The next shot is another example this time shooting along the side of the reef with better strobe balance but not so interesting a subject.
Another place that this technique is used is in outdoor portraiture and in particular for weddings where there are frequently very strong light level variances in a shot. In the photograph below, the bride would have been a dark silhouette if I had not fired in some fill flash. In the second shot, she stands under a shade to make a speech again without the flash the background would have either blown out or she would again be a shadow.
So whilst fill flash is used in many situations, some of which I have a little experience with, I had not really thought of using a flash gun for landscape photography, but am always willing to try something new. I have chosen dawn rather than dusk as I can make time in the morning more easily than the evening, and this project is "optional", so forgive me for a little artistic license.
The challenge with any use of fill flash is always going to be balancing the flash and ambient light. In particular when shooting at dusk or dawn there are 3 elements to any exposure, the sky (very bright), the background (very dark), and the foreground (dark, but within flash range). There is no solution that will work for all of these as the next two shots indicate. In the first I have exposed for the scene and asked the flash for TTL without any adjustment. The result - crap, the shot is badly exposed and rather boring.
Trying again I have now exposed for the sky, accepting silhouetting of the background and dialed in -1eV on the flash gun. This is a little better, however, the bright foreground still looks fake to me and I am not sure how to make it look good. Anything set against a rising or falling sun should be dark and the brain knows it. However, it creates an interesting look.
The other potential problem with this technique is the limitation of camera mounted flash, anything in the middle ground will be dark and again there will be a visual contrast between the fore and middle ground that is not very convincing. To get around this I could use multiple flash units slaved to my camera or battery based studio lights. But, hang on, this is meant to be landscape photography, I don't want to compete with Gregory Crewsden.
Now, having been very skeptical about this technique I did find a place where I felt it worked and I was quite pleased with the result. I shot the following directly into the rising sun, using -2eV fill flash to splash the tree trunk and just bring out some detail. I think this still looks a little false, but in an interesting way.
Well, it was fun to play with this concept, but I cannot see it becoming part of my normal practice, especially as I want to work on some much larger objects in the future. Apart from the challenge of generating enough light in a subtle way this technique has another technical drawback and that is the sync speed limitation of the camera. For the 5D2 this is 1/200s, not normally a challenge, but when shooting into a rising sun it is. the last shot needed f/11 to avoid problems with the shutter speed, fine if that's what you want, however, I suspect this is a technique that would work well with shallow Dof. There are always ND filters.