How to Get a 35mm f/0.9 Lens for Just $350: Speed Boosters Explained

You would think the physics of lenses and light are pretty set in stone—and yet, somehow, people still get really twisted around when it comes to things like crop factor, depth of field, and speed boosters. Hoping to end (or at least quiet) this debate, photographer Jimmi Kai created this very informative, easy-to-understand video.

The video specifically focuses on so-called Speed Boosters, and how these focal reducers can, in fact, turn a 50mm f/1.2 lens into a 35mm f/0.9 lens.

It all comes down to a basic understanding of f-stop, how it’s calculated, and how it’s different than t-stops. Jimmi covers it all, explaining in plain English how crop factor impacts the light being projected onto your sensor, illustrating how speed boosters compensate for crop factor, and sharing side-by-side images comparing an APS-C camera with a speed booster against a full-frame camera without.

Jimmi is not infallible, and he’s already had to issue two minor corrections for his video:

  1. The lens opening is determined by the opening of the iris inside of the lens, not by the front element.
  2. In the tests, I should have compensated for the concentration in light with ISO. This explains why the speed booster setup gave a brighter image than other setups in these settings.

But for the most part his math, physics, and logic all hold up. If you own a 50mm f/1.2 lens you can turn it into a 35mm f/0.9 lens for the price of a good speed booster. Pretty neat.

So if this deep dive into the physics that we shared last month was a bit too technical for you, and you need a more straight-forward explanation of the mechanisms and math at play here, check out the video up top.