The EOS-1D X & f/8

[weixing]

Hi,

Edit - just to explain further the 2x extender means that now your lens is effectively only using half the the front area both horizonally and vertically, so it's gathering a quarter of the light or two stops. That's why you also halve the resolution of the lens which has a varying effect depending on the resolution of the sensor.

No... you still use the full aperture of the lenses... the light gathering capacity is still the same. The lost in 2 stops is due to the increase in magnification... basically longer focal length = higher magnification. When the magnification is increase and the aperture remain the same, the image brightness will be reduce.

Anyway, there is a simple formula for calculating focal ratio: focal ratio = focal length / aperture
Base on this formula 600mm F4 had an aperture of 150mm (600mm / 4 = 150mm), so when focal length is double, the focal ratio is reduce to F8 (1200mm /150mm = 8 ).

Have a nice day.