Lighting a backdrop

[stumac]

I'm using a white backdrop and was wondering what is the best way to evenly light it with speedlites, is there a modifier I should buy??

 

[JonathanRyan]

It's going to depend on the size but in general you want something that spreads light evenly across the back but stops it from heading towards the camera.

Those small softboxes should do the job. Or you could try something like a stofen with a bit of black card taped to the back.