Avoiding lens flares is the responsibility of three people: the camera operator, the first camera assistant, and the key grip (or grip department in general). It’s the first assistant’s job to keep an eye on the lens as the lights go up, and if any bright spots appear in the glass they ask the grips to set up a “lenser” (a flag that blocks the offending light). The grip department generally keeps an eye on the lens as well, and often they’ll set up a lenser without being asked. Last but not least, the operator is expected to see the flare if it strikes the lens and affects the image, especially in the film days where they had the best view in the house.
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