There are many gaffers who advanced from that to directors of photography, the latest I can think of is Claudio Miranda (http://www.imdb.com/name/nm0592073/) or even Robby Baumgartner (http://www.imdb.com/name/nm0062373/?ref_=ttfc_fc_cr266)
I do think that even though it is important to have a knowledge of lighting and framing, I consider that as a cinematographer you have to really understand how light works in many different setups and a very good way to know that is by working with it.
At the end of the day there are so many good camera operators out there that you can hire for them to help you framing but there are not too many good gaffers who have an understanding of light AND arts, which I consider essential if you have to describe a mood to your gaffer and you are unsure as to how to light it.
If you become a cinematographer through the camera ranks your path will follow something like:
- camera intern
- 2nd AC
- 1st AC
- Camera operator (if you are lucky) / DIT
Through all that way you will only decide on something when you are a camera operator and if you are a camera operator you will need to know how to block a scene, etc.
Nowadays it is a little bit easier to understand scenes in the camera department because there are monitors everywhere so if you are a camera intern, 2nd AC or 1st AC you will have the fantastic opportunity to see how the blocking is done, how it works for the scene, almost the finished product lit and etc.
If you go through the electrician way:
- Best Boy
(assuming you do rigging and etc when you are an electrician and a best boy)
So, now you want to become a cinematographer and the cinematographer is telling you what light he wants to create an effect, you place the light, you turn it on, you direct it and then you can go to the monitor to see how that light is actually affecting the scene, ain't that amazing?
And you get to discuss with the cinematographer why he / she decided to choose that light for that particular scene.
Again, what Stuart said is true, the path that you have to choose is the one that works for you.
Have a good day.