300mm f4 lens with 1.4x converter produces f5.6.
Excerpt from LensRentals.com
The teleconverter reduces the maximum aperture of the lens by one stop (1.4x converter), 1.5 stops (1.7x converter) or 2 stops (2x converter). An f/4 lens becomes an f/5.6 lens with a 1.4x mounted. An f/5.6 lens becomes f/8.
- Teleconverters work very well with f/4 or f/2.8 telephoto prime lenses (300, 400, 500, 600), especially mounted on a tripod.
- Teleconverters work acceptably on high quality zoom lenses with maximum apertures of f/2.8 or f/4.
- Teleconverters don’t work well with consumer (f/5.6 zooms), medium range zooms, or wide angle lenses. Most manufacturer’s teleconverters won’t even mount to such lenses.
- Third party (Sigma, Kenko) teleconverters will mount to other lenses that the manufacturer’s teleconverters won’t mount to.
- Third party teleconverters don’t ‘report’ electronically to the camera body and may ‘fool’ the body into trying to autofocus when it shouldn’t. Results are pretty variable (often poor).