Teleconverter Calculator — Effective Focal Length & Aperture

A 1.4x teleconverter loses 1 stop of light (aperture multiplied by 1.4). A 2x teleconverter loses 2 stops (aperture multiplied by 2). A 200mm f/2.8 becomes 280mm f/4 with a 1.4x TC, or 400mm f/5.6 with a 2x TC. This affects both exposure and autofocus performance.

Most cameras require f/5.6 or faster for all AF points to work, and f/8 for the centre point. A 2x TC on an f/4 lens gives f/8 — only centre point AF on most cameras. Modern mirrorless cameras often autofocus at f/11 or even f/22, making TCs more practical.

Yes, teleconverters add optical elements that can reduce sharpness, especially at the edges and at wide apertures. Quality varies greatly — first-party TCs designed for specific lenses perform best. Stopping down 1-2 stops from maximum helps. A 1.4x TC typically loses less sharpness than a 2x TC.

A TC increases reach with maintained resolution but loses aperture and some sharpness. Cropping maintains aperture but loses resolution. For high-megapixel cameras (45MP+), heavy cropping can match TC results. For lower resolution cameras, a quality TC often produces better results than cropping.