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I will try and explain the idea as I think about it.
Firstly, imaging uses a camera that accumulates an image. the longer you collect light the brighter the image. Your eye doesn't work that way. It simply responds to the light coming in right now. So imaging systems and visual systems work quite differently.
With an imaging scope, the focal ratio determines the size of the image on the sensor. A long focal length scope, for example an 8" at f10 (typical SCT) will have a focal length of 2000mm and will produce a quite large image compared to a much faster 8" at f5 with a focal length of 1000mm. The amount of light being collected is exactly the same as the aperture is 8" in both cases BUT the amount of light being collected by each pixel on the sensor is more with the faster scope. So it can collect the light in a shorter time (hence it is faster!) This has benefits as guiding is easier, the sensor will stay cooler, so less noise and you can get an image quicker or get a deeper image in the same time.
As your eye cannot accumulate light like a camera, the f ratio no longer matters (as far as the overall brightness of the image is concerned) the only factor is how much light is collected, which is a function of aperture. Having said that, the eyepiece that you use will vary the "magnification" of the object you are looking at. If you increase the magnification with a shorter focal length eyepiece, it spreads out the light over a greater area, in effect reducing the surface brightness of the object, but not the overall brightness. However (just to complicate matters) higher powers also have the effect of darkening the sky background which can make dim objects "pop" into view as the contrast improves.So often times for visual observers, particular objects require an ideal amount of power to be seen best, and you sometimes have to try a few eyepieces to find the one that works the best on that object in your scope.
I have tried not to get to technical with the above, hope it helps!!
Malcolm
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