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Franco,
The exposure of extended objects is essentially determined by the f-ratio of the scope, as you have recognised. However that ignores the question of their (absolute) angular size in the sky vs the actual field of view in the camera.
In this respect you are right - a cheap f/4 camera lens will (potentially) image nebulae such as M42 as fast as an F/4 20cm newtonian. But what is totally different is the focal length - image scale, and the resulting magnification.
What is totally beyond small lenses - and small scopes - is serious lunar & planetary stuff.
This is where you need scopes with resolution better than 0.5 arc second (aperture > 25 cm) and focal length > 4 metres. And the mount to track precisely during an exposure; this is far from trivial.
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