Additional Information

Coverage Explanations

The Doubler adds a 2x crop.
The Blackmagic B4 mount adds a 1.1x crop.

With the lens doubler engaged & the corrective element, only APSC is covered (w/ a bit of vignette).
The crop factor is around 3.8x (So an 8mm B4 lens is equivalent to 30mm FF lens).

Light Loss

Note that the adapter and the doubler introduce a loss of light.

Calculations:

• $Apertur{e}_{final}=Apertur{e}_{initial}\times Adapter\times Doubler$
• $Apertur{e}_{final}=1.8\times 1.1\times 2=3.96$

For example, my f1.8 B4 lens lets light through as if it were an f4 lens.

Crop Factor Calculation

	Full Frame	B4	Crop factor
Width	36	8.8	4.09
Height	24	6.6	3.64

So the crop factor is between 3.6x and 4.1x
I found the experimental crop factor to be around 3.8x (w/ slight extra cropping in post).
It really depend of what you consider usable (how much you crop to remove the vignette and if you want to crop at all).