Nikon Speedlight SB-800 review and tips - manual modes
Distance-priority manual flash (GN)
Guide Number (GN) of a speedlight at a certain position of its zoom-head measures the speedlight's flash output - the higher the number the greater the intensity of the flash. GN is defined as f-stop multiplied by flash-to-subject distance and determined experimentally when the best exposure for the subject at a given ISO (usually ISO100) is achieved.
Distance-priority manual flash is a new and very convenient manual mode which does not require doing any calculations for flash output. You simply need to set ISO (that is done in custom settings), focal length of the lens, your preferred aperture (same as on the lens) and distance to the subject on the speedlight and flash output is then controlled automatically based on these settings as well as the speedlight's guide number, which is a known constant at a given zoom-head position. It is also possible to directly dial in flash output compensation in 1/3 stop increments, which is very precise and convenient as otherwise one would have to tweak aperture settings.
If you use this mode on a modern Nikon (D)SLR with a CPU lens, focal length of the lens, ISO and aperture are communicated to the flash automatically so you only need to set the flash-to-subject distance. However, this mode is most useful when the speedlight is used with manual cameras. For instance, it can be used with a fully mechanical Hasselblad 503CW (or any other mechanical camera for that matter) to create nice fill-flash in the following way: use a standard PC-PC cord to connect the flash and the lens; determine exposure for background/ambient light and set respective shutter speed and aperture on the lens; set film ISO, focal length of the lens (note: in terms of 35mm format), aperture (same as on the lens) and flash-to-subject distance on the speedlight; enter flash output compensation if necessary and you are ready to shoot.
Manual flash (M)
If we re-write the equation above and factor in ISO sensitivity, we get the following:
f-stop=GN * ISO sensitivity factor/flash-to-subject distance.
This is the most basic manual mode where you have to use the speedlights' guide number table (which lets you know the flash's GN at a certain zoom head position and flash output level), ISO sensitivity factors table and the equation above to calculate the aperture.
On the flash you need to set focal length of the lens, flash output level (from M1/1 for a full power burst to M1/128), ISO and aperture. The good thing is that the speedlight shows appropriate shooting distance after you input these parameters so that at least you can check if the distance is correct. Alternatively, you can tweak aperture and flash output settings until the right distance is shown but then again the GN mode above is more straightforward.
As in all the other modes, if you use the speedlight with modern electronic cameras and CPU lenses then focal length of the lens, ISO and aperture settings are fed to the flash automatically from the camera body. You then simply need to input flash output (and check the distance if necessary).
Whereas I seriously doubt that many photographers would use this fully manual mode too often Nikon, of course, could not have omitted this very basic feature in their top-of-the-line speedlight.
Repeating flash (RPT)
This funky mode has been present in several generations of Nikon's top-of-the-line speedlights and in theory allows creating stroboscopic effect for moving objects. The flash, basically, fires repeatedly during a single exposure so that the moving object is captured on the recording media (film or digital) at several points of its trajectory.
This mode is much more manual than regular manual modes get. First you have to figure out flash frequency (i.e. the number of times the flash fires per second) and the number of repeating flashes per frame that would produce desired stroboscopic effect for your subject. Once determined, you set the numbers along with the flash output level (in the range between 1/8 and 1/128 of full output) on the speedlight; on a modern camera with a CPU lens focal length of the lens, ISO and aperture settings are communicated to the speedlight automatically (note that the aperture will have to be quite large as otherwise flash range is going to be very limited). When all these settings come together the speedlight shows the appropriate flash-to-subject distance (at this point you are likely to have to readjust your subject's movement, frequency and the number of flashes). Furthermore, you have to also determine a suitable shutter speed, which will have to be rather slow as otherwise synchronizing opening and closing of the shutter with the subject's movement would be very challenging. In addition, a relatively large aperture and a slow shutter speed are bound to impose restrictions on ambient light, i.e. you will have to shoot in rather dim surroundings.
In reality, all the parameters mentioned above are highly interrelated and you will have to tweak them back and forth until the overall scheme becomes feasible (during this process you will also realize that the number of subjects suitable for this kind of shooting is very limited). Indeed, making this work constitutes a pretty good operations research problem solving which is not going to be a piece of cake and will chip away at the creative intentions of the photographer (good luck if you would like to use this mode with a manual camera and film). For one thing, I am yet to see a single creative photograph made with the use of the mode (there is one picture in the end of this brochure but its aesthetic value is rather questionable).