Why is my flash photo half dark?

Because the shutter speed was above the camera’s sync limit, so the whole frame was never open at once. The flash lit the part the travelling slit had reached, and the rest stayed dark with a hard edge between them.

A hard-edged dark band across part of the frame is the signature of one specific cause, and it is not a faulty flash.

What is happening

A focal plane shutter is two curtains. Below the sync speed they part completely before the second closes, so the whole sensor is uncovered at one instant — which is when the flash fires. Above it, they travel as a moving slit, and a single burst illuminates only the strip the slit has reached.

The figure that governs it

Published per body, and belonging to the shutter. No flash raises it and no trigger raises it. One product in this catalogue prints a high-speed ceiling of 1/8000 s, which is the other route: that mode replaces one burst with a pulse train covering the whole travel, and it costs output because the same energy is spread out.

What to do instead

Keep the shutter at or below the published limit and control exposure elsewhere — a smaller aperture, or less ambient light. Two titles here print the abbreviation for the high-speed mode, so the capability is listed on the product rather than assumed.

What it is never

Not the guide number, not the energy in watt-seconds, and not the recycle time. Those describe output and readiness; this is a question of when the frame was open.

Where this page and the maker’s own manual disagree, the manual decides. Every focal length, aperture, guide number, stop of stabilisation, load rating and sealing claim on this page is reported from the manufacturer’s own documentation, with its unit written as published, and named as such — none of it is measured here. An engraved figure is reported as the maker engraves it, and is never restated in another convention, because the two are not the same number. And where two light sources are set beside each other: a guide number and an energy in watt-seconds describe different quantities and cannot be compared with one another. A guide number depends on the sensitivity and the reference coverage the maker chose before printing it; a watt-second figure describes the energy a capacitor holds before the tube fires. Neither is the light that reaches a subject, which depends on the modifier, the distance and the room.