Nothing wrong with having trouble understanding this concept
When imaging the focal ratio is important because it means that you have more photons being concentrated into a smaller area (because of the shorter focal length). It isn't the whole story though.
If I use a KAF-8300 sensor with its 5.4 micron sized pixels on a 10"F/5 Newtonian it will perform virtually as "fast" as a 10" F/8 RC with a KAF-16803 with its 9 micron pixels.
Both telescopes are capturing the same number of photons as they are both 10" in diameter. As both cameras have different pixel sizes, even know the F/8 is technically a slower telescope it has been paired with a camera that has larger pixels.
So, in imaging, the "speed" of a system is to do with the sky coverage of each pixel (expressed in arcseconds/pixel) against the aperture.
To calculate the pixel coverage:
(Pixel Size*206.265)/Focal Length
(5.4*206.265)/1270= 0.877"/pix
(9*206.265)/2032= 0.914"/pix
The same principle works with visual astronomy. If I put a 5mm eye piece in the 10" F/5 I'll get a magnification of 254x. If I put an 8mm eye piece in the 10" RC I'll get 254x magnification. As both are the same diameter there is the same amount of photons at the same magnification.
Hope this helps John