Read the room before you open a case
Walk in and identify three things: where the ambient light comes from, what color it is, and where the deep holes are. A typical shop has overhead fluorescents or LED strips at 4000 to 5000K, maybe a roll-up door throwing daylight down one aisle, and a dead zone under every bench and behind every machine. You are not replacing the ambient. You are filling the holes and adding direction so surfaces have shape. Set your base exposure for the existing light first — usually something like 1/60 at f/5.6, ISO 800 — and only then decide what the flashes are doing. If you start by firing the lights you will end up with a frame that looks like a crime scene photograph.
Two lights, three jobs
Light one is the worker. Put it on a stand at about eight feet, roughly forty-five degrees to one side of your subject and slightly in front, shooting through a small umbrella or bounced off a nearby white wall. That single light gives a person at a bench a modeled face and separates them from the machinery behind. Light two is the hole. Place it low and behind the subject aimed into the dark area — under the bench, into the corner, along the wall — at maybe a quarter of the power of the first. It reads as though a shop lamp is back there, which is plausible in a shop, and it stops the background from going to black. If I have a third light I use it as a kicker down a row of machines to give the aisle depth, but two is genuinely enough.
Distance does more than power does
The thing that trips people up is that moving a light does not change brightness a little, it changes it a lot. Light falls off with the square of the distance, so doubling the distance from a subject costs you two full stops. The practical consequence is that a speedlight at four feet is a soft, controllable, low-power source, and the same speedlight at twelve feet is a hard, expensive one running near full power with a two-second recycle. Get the lights close and feather them past the subject rather than pointing them straight in. It also means a light placed three feet from a background will make a bright hotspot; move it to eight feet and the falloff across the wall becomes gentle enough to look natural.
Color, flicker and dust
Gel the flashes to match the ambient or the mismatch will be obvious on every skin tone and every white surface. Against 4000K shop LEDs a quarter to half CTO on each head gets you close, and then you set white balance to the ambient and everything agrees. Watch shutter speed for flicker: cheap LED strips pulse, and at 1/250 you can catch a dark band across the frame. Staying at 1/125 or slower usually fixes it. Keep the flash heads capped when not in use, because sawdust and grinding dust will find their way into a zoom head and a fresnel that will not retract is a dead light. I wipe everything down in the truck before it goes back in the bag, and I never put a light stand where a forklift turns.
Frames worth having in a shop set
Establishing shot down the main aisle, one frame per major machine with an operator working it, two or three tight frames of hands and material because those are what people look at, a clean shot of the finished product, and one wide from a corner at about six feet showing the whole operation. That is ten to fifteen frames and it takes about three hours with two lights. The hardest one is always the wide establishing shot, where the windows or roll-up door will be several stops brighter than the interior — the same balancing act as any interior with bright glass in the frame, solved the same way with a flash-ambient blend or a return visit at dusk. For the underlying physics of why light placement matters more than light power, the inverse-square law explains the whole thing in one equation.
Frequently asked questions
Do I need radio triggers or will optical work? In a bright shop with obstructions, radio. Optical slaves miss constantly once a machine is between the heads.
What power setting should I start at? Quarter power at eight feet, ISO 400, f/5.6 is a reasonable first guess indoors. Adjust from the histogram, not from the rear screen.
Is it safe to fire flash near operating machinery? Never at someone running a saw or a press. Photograph the operation staged and stopped, or shoot ambient-only with a faster lens.