Flash F 280 guide number

The Guide Number (GN) will not change with shutter speeds. The flash device is independent of your camera and usually does not know the shutter speed on the camera, except the most modern Nikon CLS system, and possibly few newer.

Guide number changes with ISO speed only. Guide number is the measure of Maximum output possibly from a fully charged flash. You can use a fraction of it in Auto or manual modes, but still the GN does not change.

If you use shutter speeds between any slow and up to 1/1000 your camera shutter will catch all the light produced by the flash. At 1/2000 shutter that may be faster than the duration of the flash light, so you will catch less light, but again the GN does not change.

If you do not have user's manual, you can compute your flash GN for any ISO setting from the flash calculator, by multiplying the aperture by the distance from the calculator dial on your flash, if there is one. You can also test your flash and compute GN from whatever setting you discovered was perfect.

http://www.mir.com.my/rb/photography/hardwares/classics/olympusom1n2/shared/flash/f280.htm

In FP sync (stroboscopic multiple small blasts) mode, the total power (energy) of flash that determines flash GN is divided by the number of flash blasts needed to cover desired duration of shutter slot travel time across the film. The flash GN in table found for FP mode above is reduced from 28 (m, 100 ISO) to 2.6, more than 10 times.

In X-sync flash mode the guide number is always the full number, that is the 28 (m, 100 ISO), regardless of the shutter speed.

http://www.star.ucl.ac.uk/~rwesson/esif/om-sif/flashphotogroup/f280.htm

Right down the bottom of the page is a link called "manual" - click it and a pdf file of the manual will load up.