jackpi wrote:
On a zoom lens, a 1.4x TC will reduce sharpness by 15% to 25% and you lose one stop on your widest aperture (f/5.6 --> f/8). And your telephoto lens AF may be limited to a single focus point, if it works at all. I doubt you will need anything longer than 200mm in Cambodia and Thailand. In Alaska, your two zoom lenses should be fine, even for bald eagles. If you shoot anything that needs a longer focal length, you will need a tripod and a stable platform to shoot from.
Not sure what lenses and TCs you are referring to, but with the best TCs from Canon and Nikon, matched with appropriate high quality lenses, the loss in sharpness and contrast ranges from negligible to relatively minor. The absolutely stellar Canon 70-200 f/2.8 L IS USM II, for $1950, partnered with the Canon 1.4x Extender III for $429 makes for a fantastic and super sharp f/4 combo when a little more reach is needed.