FIGURE 1.10 TC-14E II, 1.4X (left); TC-17E II, 1.7X (center); TC-20E III, 2X (right). Autofocus works best at apertures of 5.6 or wider. Adding a teleconverter to your lens reduces the maximum aperture, possibly slowing down the focus speed or disabling it all together.
A TC-20E III increases the magnification of my telephoto lenses by two times, with a two-stop loss of light (Figure 1.11). With the 2X on, AF is slowed down significantly, and I will usually resort to manual focus (example: 600mm f/4 with 2X= 1200mm f/8.0).
FIGURE 1.11 Adding the 2X teleconverter to my 600mm lens and lowering the tripod to the ground gave me the best angle of view when this hooded warbler caught a crane fly.
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C O M P O S I T I O N : F R O M S N A P S H O T S T O G R E AT S H O T S
ISO 400 1/2000 sec. f/8 600mm f/4 lens with TC-20E III