This is largely down to the compexity of zoom lenses. On the big bucks pro jobs this is less of, or not a problem, when compared with lenses at the budget end. Some zooms can have as many as 20 lens elements. Zooms also come in a variety of zoom length flavours. A 3x zoom like 100-300 will produce better images than a 10x 28-300, as distortions within the design are easier to correct.
Now add a converter with between 5 and 7 elements and not only do you have a lot of glass, but the converter will magnify any defects from the zoom before adding its little bit. There is a thread running on the subject in the Lens Forum - TC Musings. It goes to show its not an easy question to answer.
Don
|