A dark band along one edge of a flash photograph can occur when the exposure exceeds the setup's normal flash-sync conditions. Check the camera, flash and trigger before trying to brighten the strip. The area may have missed the flash illumination, so an exposure adjustment cannot guarantee recovery of the detail that was never adequately recorded.
Verify the timing hypothesis
Nikon explains how a closing shutter curtain can block flash illumination at speeds beyond normal synchronization. It also says that most modern cameras with dedicated hot-shoe flashes make this out-of-sync behavior very difficult to obtain. Keep both points: a dark edge is a possible symptom, not proof that every modern flash photo has a sync error.
Record the body, shutter mode, flash, trigger and exposure settings. Check which normal sync speed applies to that combination. Do not borrow a number from a different camera model. Compare the saved image with the scene, and distinguish an abrupt edge band from ordinary uneven lighting or an object physically blocking a lamp.
Make a supported still-life comparison
This is a reproducible test plan, not a tested flash repair. Use an ordinary still subject and stable lighting. Keep the camera position, lens, aperture and flash placement recorded, then make a comparison at a documented supported normal sync setting. Change one timing-related condition at a time rather than increasing every exposure control together.
Inspect the affected edge and central subject in the saved files. Record whether the band changes, disappears or remains. A brighter center with the same dark edge is not a successful correction. If the proposed timing change makes no relevant difference, investigate the other causes instead of insisting on the initial explanation.
Enable high-speed sync only when supported
Nikon's article gives a compatible-camera route through Custom Settings > E1 and an option labeled Auto FP. It directs users to confirm the body and Speedlight combination, and notes that non-Nikon flashes may not fully work with that feature. This is not a universal menu instruction for every camera or trigger.
High-speed synchronization has different light-output consequences, so compare the resulting subject exposure rather than assume that a faster shutter is a free upgrade. Use the actual supported system and its instructions. The built-in flash and external flash roles should also be checked instead of treating every light source as interchangeable.
For a hypothetical catalog shoot, a creator chooses between a supported normal-sync setup and a compatible high-speed setup, then evaluates both for the product's detail. This example has not been shot. It does not promise that changing a menu setting will solve an unidentified dark band.
Accept detail across the full frame
Require an adequately lit subject across the intended image area, with no unexplained edge band and no lost highlights introduced by compensation. Keep the accepted settings and source frame. Reacquisition is often the useful option when the missing illumination prevents a faithful rendition of product detail.
Imagild can prepare a creative treatment from a suitable image; the camera's shutter and flash timing remain external capture conditions. Generating plausible detail in the band should be treated as an alteration, not proof that the original scene's texture or lettering was recovered accurately.