Using your thumb and forefinger on each hand, make a finger frame, and hold it out at arms length.
Now imagine that your entire field of view is limited by the bounds of this finger frame. Anything outside of the frame is not visible to you.
If you wanted to see a larger (wider) field of view, you have two options.
The first is to move the finger frame closer to your eye. You'll still see the bounds of the frame, but you'll be able to see a large field inside the frame.
This is the analog of moving from the 40 mm flange depth of DSLRs to the much shorter 16–20 mm flange depth of mirrorless cameras.
The throat size of a lens mount is the inner diameter of a mount, minus the tabs that are used to mount lenses.
Now increase the size of the frame by moving your hands apart. This is the analog of increasing the throat size.
Throat size also gives us a some information on the potential of the mount and it is vitally important for calculating the angle of incidence that we will explore later.
The inner diameter of a lens mount represents the size of the lens opening ignoring the tabs on the mount. Both short flange distance and large inner diameter back lenses increase the angle of the rays that hits the sensor.
The outer diameter of a lens mount is the full diameter of the bayonet mount, which in most cameras represents the end of the metal mount. The outer diameter of the mount plays an important role in determining the approximate outer diameter of the rear part of a lens.
Flange distance, also known as “flange focal distance”, “flange back distance” or simply “register”, is the distance between the mounting flange (which is the outer part of the lens mount when viewed from the side) and the film/sensor plane. Just like different mounts have differences in throat size, inner and outer diameters, flange distances also often vary greatly between different camera systems.
Angle of Incidence
Remember that light from a point source of light strikes every point on the front of a lens as a cone and is refracted through the lens to be focused back to a single point on the imaging plane. The throat diameter combined with flange distance determines the maximum possible angle of incidence of the marginal rays from the lens, which is important in designing lenses.
A very steep angle of incidence - light hits the "film" at a very steep angle (as far from telecentric as you can get). Modern sensors do not like angled rays.