[mikeyjames (Mick)]

Hi all,
I'm just asking this out of curiosity.

I have been trying to understand why a 'fast' scope is fast. I've scoured the net and found a thousand pages that tell me the following:

F Number = Focal Length/Aperture
F5 = Fast
F10 = Slow
Fast = shorter exposure time.

I know all of this, but I'm finding that an explanation about 'why' this is so is a lot more difficult to find. The two explanations I have seen that seem to make sense are (as I understand them):

1) That at same aperture, a longer focal length = higher magnification. This in turn spreads the same amount of photons over a larger area on the sensor, which means less information captured at each pixel and therefore longer exposures are needed to fill the gaps. I suppose just like resizing a small pic to much larger and it looks like crap (unless you're on CSI Miami where they can resolve a suspect's fingerprint from a low quality video feed at 100m).

OR

2) Someone gave an example of standing in a 10m diameter pipe. When standing 40m back = F4, 60m back = F6, 100m back - F10, and so on. The person asked us to think what the target would look like at the F4 stop, and so on at each stop. As we go further back the field of view gets smaller and the image loses brightness, until maybe several hundred metres where you can barely make our anything.

Both of these make a bit of sense to me. I like the second one because I can actually visualise what is happening, but the fact that I like it doesn't mean it's right.

Are either/both/neither of these on the right track? If not, can someone explain why a 'fast' scope is actually fast.


Thanks
Mick