Originally posted by demp10 Good lens design takes into account the extra piece of glass that is the rear filter, which among other things will affect focusing. Most lenses with this feature are shipped with a clear glass to be in place all the times when a filter is not used.
Thanks for the reply, demp.
So, pretty much all larger "super" telephoto lenses would have this feature?
I am not into optics/physics, but how would this affect focusing?
The only think I remember reading about this was, a while back, a member of this forum acuired a FA*400/2.8 ... he had a tough time at first with the focusing until he was pointed to this specific "piece of glass" (the rear filter) that had to be inserted, which he did. Of course, he had good results with the lens afterward.
I still don't undertsand the concept though.
JP