Q) I can only afford one long telephoto for bird photography. Is there a particular focal length that’s more versatile than others?
A) In my experience, there are two answers to the question. A quality 300mm ƒ/2.8 telephoto with matched 1.4x and 2x tele-extenders is very versatile while being reasonably priced and compact in size compared to an expensive and weighty 400mm ƒ/2.8 or 600mm ƒ/4 telephoto. The 300mm ƒ/2.8 focuses closer, maintains excellent sharpness even with the extenders attached, can be carried in a photo backpack and enables you to work with a fast ƒ/2.8 300mm, a lightweight 420mm at ƒ/4 (with a 1.4x extender attached), and a light yet reasonably fast 600mm ƒ/5.6 (with a 2x extender attached). If your D-SLR has a less-than-full-frame sensor, you’ll have the additional advantage of a 1.5x or 1.6x magnification factor, yielding as much as 960mm.
The other good option is a quality 500mm ƒ/4. This lens has a size and weight that allows handholding if necessary, focuses reasonably close for smaller birds and will maintain excellent sharpness with either the 1.4x or 2x matched tele-extenders available from the lens manufacturers. The Canon 500mm ƒ/4 also has image stabilization, which adds to its versatility and value.
This snow goose was photographed in southwestern Louisiana with a Canon EOS-1N and Canon 300mm ƒ/2.8L lens with an EF 2x tele-extender attached.