Although very little (single digit percentages) light is lost to a teleconverter, the ratio of entrance pupil to focal length grows proportional to the magnification of the telecon. For example, if a 100mm f/2 lens has a 2x teleconverter attached to it, the lens will effectively become a 200mm f/4. This is because the lens will always have a 50mm entrance pupil (remember, the name f/number literally means the entrance pupil diameter is focal length/number)

Because the amount of light gathered by the aperture series is based on doubling (f/2.8 gathers half as much as f/2) the series progresses not linearly but rather along a log-base-2 line. The equation for the increase in f-number as a result of teleconverter is, approximately:

E = 2 x log2(M)

Where

• E is the (negative) shift in Ev
• M is the magnification of the teleconverter
• log2 is the operation logarithm base 2

So if you combine 2 each 2x teleconverter and 1 each 1.4x teleconverters (4.8x total) you would "lose" about 4.5 Ev or stops of light. If you attached that teleconverter to an f/2 lens, it would become a f/9.5 lens.

This equation could be combined with the expression of aperture in Ev to predict new effective aperture based on old effective aperture.