Feathering means adjusting a light’s aim so the subject is illuminated by a chosen part of its distribution, commonly with the centre aimed past them. It is a way to control where light lands and how its level changes across the scene.
Why it changes the image
A fixture or softbox may have different intensity and apparent emitting area when seen from different directions. Turning it can change the subject’s exposure, reflections and the amount of light reaching the background. Feathering is therefore not simply another name for dimming.
Practical example
When lighting two people at different distances, try aiming the brighter portion toward the farther person and letting the nearer person use a weaker part of the field. Compare readings at both marks. The useful angle depends on the actual modifier and positions, so a fixed rotation does not guarantee equal exposure.
Feathering a soft source
A softbox should be evaluated as an illuminated surface, not just as a centre point. Turning it changes the width visible from the subject and can affect softness as well as output. A grid can make the change more pronounced. After adjusting the aim, check the lens for flare and the entire blocking area for uneven levels before locking the setup.
Related terms
Falloff · Spill · Soft Light · Grid · Incident Light Meter
References
Elinchrom: even illumination and feathering · Profoto: feathering your light