Hey, Tony,
Here’s a nice experiment:
Fit a 200mm telephoto lens to your very old manual 35mm SLR camera.
Go outside and put it on a tripod and view a scene.
Open up the iris to max (F4 on mine).
- Result: the viewfinder is very bright.
Stop down the iris to min (F32 on mine).
- Result: the viewfinder is now very dim.
What we observe:
- The focal length has not changed and thus the field of view has not changed – we see the same image, only brighter or dimmer.
- Therefore, the focal ratio is what affects the “brightness” of an image.
Cheers
Dennis