[dr_zoidberg]

And the amount of light that enters the sensor through the lens is insanely different. A couple of years ago I moved from a Canon G16 with a f/1.8-2.8 6.1-30.5mm (28-140mm FF eq) lens to a Olympus OM-D EM-10 with its pancake f/3.5-5.6 14-42mm (28-84mm FF eq) lens.

Long story short, the pancake gets almost 3 times as much light as the canon integrated lens, despite the higher f number, because of the areas and sizes involved on both. And if I had gone for an APS-C for FF camera, we'd be talking in the order of 5 to 10x more light (hence, better low light images, or the ability to use shorter exposures for the same results).

Phones f/1.8 cameras are great -- comparing to other phones. If you compare them to a camera (almost any camera), they're in trouble.

[Auracle]

Even different lenses with the same aperture on the same camera can have decently different t-stops.

[dr_zoidberg]

Yes, because the f number is a ratio between the effective aperture vs the length of the lens. A long lens with larger f number can focus more light into the sensor than a shorter one with a smaller number, say f/4.0 400mm against a f/2.0 50mm.