As I understand it, a larger sensor size will require longer focal lengths to match an "equivalent" view angle as a smaller sensor. As focal length increases, perceived depth of field becomes more shallow. Which is to say, a lens at f/4 on a 4x5 camera will will be much more shallow than an equivalent angle lens on a smaller film/sensor size.

the F number is calculated by f/D where f is the focal length and D is the diameter of entrance pupil.

For a F/1.0, 50mm lens, you need a 50mm front element.

In Opal's terms, this means their f/D ratio is 1.8. However, it doesn't mean it can capture equal number of photons with a full frame system with a f/1.8 lens, since their sensors' surface area is not equivalent.

Another difference will be in DoF. Since the sensor is smaller, there'll be a big crop factor, it'll multiply everything. Hence the resulting DoF will not be anything like a full frame system. In other words, almost everything will be in focus.

To add a cherry on top, they're not telling the T number of their lens, which defines how much light actually passes through it. F number and T number can be vastly different, intentionally or unintentionally, due to design parameters and features of a lens.