I debated for a long time between the 1.4x and 2x versions for my EF 100-400mm IS L II. I wanted extra reach for wildlife, but I also wanted to stay on a tight budget. I found a used 1.4x III last year and have been fairly happy with it.

Focus is a little more hit-or-miss at maximum range, especially in backlit situations, but that is expected. The 1.4x extender gives a useful reach boost without the bigger 2-stop penalty of the 2x version.

Paired with the EF 100-400mm f/4.5-5.6L IS II, it becomes roughly a 140-560mm f/6.3-8 lens. On modern RF bodies with the EF-RF adapter, it can still be quite usable. On older bodies like the Canon Elan 7 or 5D Mark II, though, autofocus is the limitation. Those cameras generally require f/5.6 or faster for autofocus, so with this lens/extender combo, I would treat them as manual-focus-only.

The Good:

• Adds meaningful reach with only a 1-stop light penalty.

• Small and easy to pack compared with a longer lens.

• Good match for compatible Canon L telephotos.

• Maintains autofocus and EXIF communication on supported setups.

• Professional build with weather resistance.

• More practical for the 100-400mm than the 2x version for most of my use.

The Bad:

• Only works with select compatible lenses and cameras.

• Image quality depends heavily on the base lens and shooting conditions.

• Autofocus can be slower or less reliable, especially at the long end.

• On older f/5.6-limited DSLR/film bodies, the 100-400mm combo becomes manual focus only.

• Get it if you regularly need more reach from compatible telephoto lenses.

• Get it if you use newer Canon bodies that handle smaller maximum apertures better.

• Pass if you often shoot in low light and cannot afford the 1-stop loss.

• Pass if you rely on autofocus with older bodies like the Elan 7 or 5D Mark II when using the 100-400mm.