A gobo, short for go-between optic, is a cutout placed in front of a light to cast a patterned shadow into a scene. The pattern can be representational (window blinds, a leaf-dappled forest floor, the slats of a venetian door) or abstract (broken shapes, geometric grids), and the gobo can be a precision-machined metal disc, a sheet of cardboard with holes cut into it, an actual branch held up between the source and the subject, or anything else opaque enough to break the beam.
The term originated in theatrical lighting, where gobos slot into the iris position of an ellipsoidal spotlight (a Leko in stage terminology, a Source Four in modern use) and the pattern is projected sharply onto the stage. The same hardware appears in photography as the optical snoot or projection attachment, which mounts on a strobe and accepts standard B-size or M-size gobos for hard-edged pattern work.
Even without dedicated hardware, gobos are simple to improvise. A piece of foam core with venetian-blind slats cut from it, held a foot or two in front of a bare flash, throws a recognizable window pattern across a portrait subject. A branch of leaves held in the path of a backlight gives the dappled look of a forest clearing. Crinkled aluminum foil with holes punched through it produces a starfield. The closer the gobo is to the source, the softer the shadow edge; the closer it is to the subject, the sharper.
Gobos are a subtractive tool, like a flag, but they shape light rather than removing it wholesale. Where a flag kills illumination across a region, a gobo carves the existing beam into a recognizable pattern. The two are often used together, with a flag at the edge to keep spill off a wall and a gobo at the center to texture the light that does reach the subject.
Hard sources work best. A bare bulb, a Fresnel, or a tightly focused spot produces a defined shadow; a large softbox blurs the gobo pattern into uselessness because the light wraps around it. For this reason, gobos are usually paired with bare reflectors, snoots, projection attachments, or daylight in a window. Some photographers use a hard key light through a window-blind gobo as the entire scene, treating the projected shadow as the only artificial element in a frame otherwise lit by ambient.
Pitfalls include making the pattern too recognizable, which calls attention to the lighting instead of the subject, and using gobos that introduce sharp distortion on a face. The eye reads strong shadow patterns as cinematic but only when the rest of the lighting supports the implied source. A venetian-blind shadow with no other directional information looks staged; the same shadow with consistent warm key and a hint of haze in the air reads as a real room with real windows.