The physics behind a 35mm f/0.9 lens for $350

For the wide aperture lovers, the way to get an  f/0.9-35mm lens for $350 has been here for a while (see video at the end of the post). However, did you know why using a 50mm, f/1.2 lens with a speed booster actually gives you an  f/0.9, 35mm lens? There is quite a bit of math involved but Jimmi Kai manages to squeeze a pretty understandable version of it into a nine minutes video. (Along with some other great tips and physics nuggets)

You’d want to watch the video to fully understand the physics behind it, but in a nutshell, Jimmy uses a cropped-sensor camera, along with a speed booster and a full-frame lens. This combo gains an extra stop of light while also re-adjusting the light beam to use not spill over the sensor edge.

To validate the theory, Jimmy tested three combinations of camera/lenses:

A cropped sensor A6300 with a 35mm lens @f/1.4

A full frame A7R2 with a 50mm lens @f/1.2

And a cropped sensor A6300 (again), this time with a LensTurbo II and a 50mm F/2.0 lens

As you see, they all kinda perform the same

So what about that f/0.9, 35mm lens for $350?

Well, once you are convinced with the science Jimmy showed in the previous video, It has to do with the video below which show a combo of a canon FD 50mm f/1.2 for about $290 along with a Roxsen speed booster for about $80

[35mm f/0.9 for $350 | Tested and Explained: Physics of Speed Booster and Crop Factoring | Jimmi Kai via reddit]