GH4 SpeedBooster +1.3 Stops...How do you calculate lens Aperture?

Y

Yavor

Member
OK, I'm going to show my ignorance. The Metabones Speedbooster XL for the GH4 has a light gain of +1.3 stops. I assumed this is F stops.
It must not be. Its not EV. On a F2.8 lens, it reduces it to F2.0.... not F1.5 . And this relationship is not linear with lenses of higher wide-open F ratings.
Nobody has addressed this. What IS this relationship? Bob:undecided
 
Hi Bob,

This is really most simple, it goes by the same factor of x0.64 as stated in the XL's specs.
F2.8*0.64 = F1.792 - which would be ≈ F1.8
So here you got the 1⅓ stops difference.

Works the same with the previous x0.71 boosters.
F2.8*0.71 = F1.988 ≈ F2.0
1 stop.

Best,
Raz.
 
Just to be clear, when im shooting at f2.8 with the older .71 speedbooster, it really means im at a 2.0? (Even though the display reads 2.8)
 
There are passive speedboosters (with no electronics) which cannot transmit aperture value to the camera body at all. However active ones (with electronic lens contacts) should make aperture adjustable through the camera and display the "corrected" value. A F2.8 lens turns into a F2.0 lens (in the case of 0.71 boosters) and should be displayed that way on the camera.
 
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