don't fall into the common trap of thinking that a faster scope (f4, f3 etc) will give a wider field than a slower scope (f6, f7 etc).
The focal ratio is only part of the story.
The more critical point is the Focal Length
, not the ratio
You need a short focal length to achieve a wider field.
As you go up in apature, so does the focal length.
As an example:
a 12" f4 newt will have a focal length of 1500mm.
a 10" f4 newt will have a focal length of 1200mm.
The 10" will give a wider field
I hope that is what you want to know, and that I made sense
A Barlow will give you a smaller field of View (FOV). Object will appear larger