If you use the same film type in 2 different Canon cameras would you see much of a difference.
problem with your analogy is, the 5D MK3 and 650D don´t have the same sensor.
"i see bigger differences when i change from one film type to another on my analog camera."
In this statement, it seems that you are comparing film types. Ie different ISOs, film styles and processing quality. And in your original post, you are now comparing cameras
it makes absolute sense to me what the OP says.
film in analog cameras = sensor in digital cameras.
if you buy a cheap film, the quality of the images will be worse then with expensive film.
the grain is bad, colors are worse, resolution is worse.
with all the talk about how great FF is, you could expect a big difference in image
quality when going from APS-C to a FF sensor.
but that´s not the case. all you get is better quality when using high ISO.
and even this difference is only visible to pixelpeeper.
the normal customer, who prints 13x18cm, will not notice it.
Some posters have suggested an extender to work with your 70-200 f/4. I seem to have read somewhere where that the 5D3 doesn't AF @ f/8. This supposed to change with an update coming out this year.
a 1.4x teleconverter will work fine (f4 + 1.4 extender = f5.6).
but yes for f8 you need the coming firmware upgrade expected for april.