Iin the 1980s, I used mostly Tamron lenses and can clearly recall my irritation and very visible dust right through the lens. Later, Nikon lenses also showed prolific amounts of dust, and only one Canon lens had it to memory. Personally I will not, and never have, now buy a lens with any trace of dust inside it. All equipment is bought in excellent to mint, and condition verified with seller/dealer with detail photographs and then again in person. A large amount of dust close to the rear element will have an effect on imaging quality (softening, flare).