How do the four main types of coffee roasts ( light, medium, medium-dark & dark ) compare in caffeine content?

If we use 35g of roasted beans to brew 500ml of coffee, here's how much caffeine, by weight, is stored in those 35g samples (noting that not all this caffeine will actually make it into your brewed cup of coffee):
* Light Roast: .49g (490mg) of caffeine
* Medium Roast: .52g (520mg) of caffeine
* French Dark Roast: .6g (600mg) of caffeine

In reality, 50g of dark roast coffee and 50g of light roast coffee have roughly the same amount of caffeine. Measuring by weight (mass) will give you about the same caffeine every time, regardless of roast level.
However, this caffeine myth does have an understandable origin.

Light roast beans are more dense than dark roast beans, so each individual bean has slightly more caffeine. Dark roasts are less dense, so each bean has slightly less caffeine.

Measuring by scoops (volume) doesn't take the density of the beans into account like measuring by weight (mass) does, which is where this myth is born. So one scoop of light beans may result in 70 mg of caffeine, while one scoop of dark beans may only produce 65 mg.

As you can see, 5 scoops of a light roast contains slightly caffeine than 5 scoops of a dark roast. It doesn't have to do anything with the roast level - it's all in the measurements.