Yes. A teleconverter increases focal length, but it also reduces the amount of light reaching the sensor, so the maximum aperture becomes smaller.

A simple rule is:

• 1.4x teleconverter: lose 1 stop
• 1.5x teleconverter: lose about 1.2 stops
• 2x teleconverter: lose 2 stops

You can calculate the new f-number by multiplying the original f-number by the teleconverter factor:

new aperture = original aperture × teleconverter magnification

Examples:

• 200mm f/2.8 with a 1.5x teleconverter → 300mm f/4.2
• 200mm f/2.8 with a 2x teleconverter → 400mm f/5.6
• 70-200mm f/4 with a 2x teleconverter → 140-400mm f/8

So the focal length and the f-number are both multiplied by the teleconverter factor. In practice, this means less light, potentially slower autofocus, and possibly reduced image quality depending on the lens and converter.