Spent 4 months cleaning every lens element before I learned the dust was behind the front group
I was fixing a sticky focus ring on a customer's 50mm and figured while I had it open I'd do a full clean since he complained about specks in his shots. Spent a whole evening with the bulb blower and lens pen, got it looking spotless, put it back together. Two days later he brings it back with the same dust spots in the exact same places. I felt like an idiot. Turns out the dust was sitting on the inner surface of the front element group, which I never touched because I assumed the outer glass was the only spot that mattered. A guy at the shop on 5th Street watched me struggle for ten minutes then just said 'you pulling the front group off or what.' Now I always split the front group first, takes two extra minutes and saves me a return trip every time. Anybody else have a job where the obvious fix turned out to be the wrong one for way too long?