I am referring to filters to protect your lens. I just purchased a Canon 100-400 and ordered a UV for the end. In talking to a relative about my purchase (he's an avid bird photographer)was ask why I would want to use a filter as they tend to block a certain amount of light entering the lens. When doing some reading I realized that depending on the filter you buy you could be losing up to 7% of the available light. I tend to use a filter on all of my lenses for the protection that they offer. How bout everyone else ? Do you have a filter on your lenses ? If so which do you use for this purpose ?