Roy C
Occasional bird snapper
Steve, both combos lose 2 stops but stacked 1.4's do give 1.96 x mag, with one 1.4 tc you have a focal length of 420 (300*1.4) Now when you add another 1.4 tc to that set-up you have 420 * 1.4 = 588mm. (588/300 = 1.96).I don't see your point about the magnification Roy.
A 1.4x converter gives 40% increase in mag and a loss of one f-stop.
Two stacked 1.4x converters would surely give 80% increase in magnification, and 2 f-stops loss of light.
Surely it is 1.4 +1.4 (40% + 40%=80%) rather than 1.4 x 1.4? hence the difference in magnification.
Steve
I think I read somewhere that a 1.4tc is actually something like 1.416 so that it should come to 2x