Summary: When you spend £800 on a lens you need to know that it's protected from everything including dust and curious children with sticky fingers! What you do not want is the filter to affect (other than it's design parameters)the outcome of the shot. This looks and feels every penny worth of it's cost.
Summary: Bought to protect the front of the 100-400mm Canon lens and does what it says. Haven't used it outside a lot and have been shooting birds from inside recently, so have removed it to get cleaner shots. I haven't noticed any drop off in quality when using the lens in good light, but for low light conditions, the autofocus does hunt around a bit.Happy with my purchase, and well worth the additional expense to protect a high value lens.