Nikon SB-28 - Manual (Page 55)

Exposure compensation in Repeating Flash Mode
The flash shooting distance calculated in step number 4 is the correct
exposure for the first flash in the sequence. Therefore, repeating flash at
this flash output level will result in overexposure of the overlapped images.
To prevent this, use a smaller aperture.
NOTE
Use fresh or fully charged batteries when performing repeating flash.
Allow enough time for the flash to recycle between each repeating flash.
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