SUMMARY
Teleconverters offer increased focal length but come with trade-offs like reduced light, sharpness, and autofocus speed. Photographer Jan Wegener's extensive bird photography suggests their utility varies greatly. They perform best on fast prime lenses and when subjects are already at a manageable distance, preserving detail without excessive atmospheric distortion. For longer reach needs, especially with slower lenses or frequent use, a native longer lens often proves superior.
TAKEAWAYS
Teleconverters degrade image quality and autofocus speed, especially on slower lenses.
They are most effective on fast prime lenses (f/4 or faster) and when subjects are closer.
Frequent teleconverter use (over 60% of the time) suggests a longer native lens is a better investment.
Build your kit around your most-used focal length without an extender; use teleconverters for exceptions.