Originally posted by wallyb Care to put any reasoning behind your insult? Because you are in fact wrong. The closer the virtual image of a lens is to the lens' hyperfocal distance, the more interference a filter may possibly have on the lens. The longer lens is, the smaller of an aperture it takes to get close to hyperfocal distance; as in, an aperture that no one would use, and most telephoto lenses wouldn't even be able to stop down to.
Ok,
for sake of argument, lets say the imperfect filter is a one diopter closeup lens.
Put it before a 1000mm lens, and it will half the focal distance.
Put it on a 100mm lens, and it will shorten it about 10%.
Imperfect filters usually have some wavy and/or wedge shape. But the same imperfection will have more effect when it is farther away from the sensor.