Originally posted by RBellavance I'm pretty sure that it effectively "eats" 1.4 stops of light. However, it is usually rounded to 1 stop for ease of calculation. The correct answer would be 4.5, IMHO.

I'm pretty sure that this is not correct

One stop more or less means half or double the light. The max aperture is a function of the focal lenght f, thats why it's called f/2.8. So, a 28mm f/2.8 lens has an effective max. inner diameter of 28/2.8=10mm or 28mm/10mm = 2.8. When stopping down, say, 2 stops to f/5.6, this results in 28/5.6=5mm. The area that 'lets in' the light grows with the square of the diameter, so it's half the diameter, quarter of light (two stops).

Therefore one stop more is 1.4142 (square root of 2) times the diameter, resulting in 1.4141 ^ 2 = 2 times as much light.

If you increase the focal lenght f by 1.4(142) using a TC, while of course the inner lens diameter remains the same, you lose one stop = aperture value times 1.4.

With the 28mm lens from above:

28mm x 1.4 (from TC) / 10mm = f/4