>>2662308>The lens in question was a Canon 24-105mm f/4 IS that returned from rental looking quite normal with the renter taking equally normal photos.>But a quick look through the glass reveals a fairly significant problem a fly deep inside the lens.http://www.lensrentals.com/blog/2015/06/i-dont-know-why-it-swallowed-a-fly-weather-sealed-lens-with-a-fly-insidehere's their general "why dust doesn't much matter" post:
http://www.lensrentals.com/blog/2011/08/the-apocalypse-of-lens-dust