If we consider the following as the "normal" set up of the camera
- T4 1/2, 24Fps, ISO 640
And then you want to go to:
- T2.8, 60Fps, ISO 800
You have to take into consideration what you get and what you lose.
First of all, the T STOP:
From T4 1/2 (which is the equivalent to F4.5 without starting talking about F and T differences) to T2.8 you gain 1 1/2 stops of light because of the way T stops work.
T2 >> T2 1/3 > T2 1/2 > T2 2/3 >> T2.8 > T2.8 1/3 > T2.8 1/2 > T2.8 2/3 >> T4 > T4 1/3 > T4 1/2 > T4 2/3 >> T5.6, etc
So from T4 1/2 to T2.8 you have to open up the T Stop 1 1/2 times.
Hence you have, already, more light coming into the camera.
Second thing, FPS:
From 24Fps to 60Fps you lose around 1 1/2 stops of light, because from 24fps to 48fps there is 1 stop of difference and then from 48Fps to 60Fps is about 1/3 or 1/2 stop (more or less)
Then you are almost even, the amount of light that you got by opening the T Stop in the lens has been sucked up by the change on the speed, so the only thing that you can change in camera if you want to get a bit more of light is the ASA.
From 640ASA to 800ASA there is 1/3 of difference (the difference between these ASA numbers is 1 stop 100 > 200 > 400 > 800 > 1600)
Take a look at the following link to see the ASA Speeds and the differences between all the ASA speeds.
Hence, you end up with a setup which will give you around 1/3 or 1/2 stop more of light when you go from - T4 1/2, 24Fps, ISO 640 to - T2.8, 60Fps, ISO 800.
Is there an easy way to make this calculation? I suppose that it all comes with practice and knowing how T stops, FPS and ASA work together.
(Now I hope I haven't messed up with my calculations haha)
Have a good day.