Originally posted by Canada_Rockies I believe that any lens that uses drop in filters in the rear is designed to work best with a filter installed.
No. For instance, as has been mentioned the Tamron 60B was designed with the filter in place, whereas its predecessor as designed without a filter place, despite having the slot. Also, we have a very extensive thread in the 300mm lens club, with many example photos. And interestingly enough comparissons with and without filter show different results. Some lenses work much(!) better with the filter in place and other visibly better without the filter.
Ben