Absolutely - you can't buy an f/7 Newtonian in any aperture, even if you want to - which staggers me.
Manufacturers are driven by lowest-cost - and the shorter the tube the cheaper it is - oblivious of the consequences - knowing that most don't know why they SHOULDN"T buy short focal ratio Newtonians.
Which leaves making one as the only viable alternative.
BTW here's the maths
http://www.telescope-optics.net/newt...berrations.htm
and note: "This means that one could see it (coma) at the edge of a 40° AFOV eyepiece at ƒ/4, regardless of its focal length"
This suggests widefield eyepieces (65 degrees and more) in an f/4 newtonian are rather pointless. You can struggle with Parracors and the like, but it would be a lot better to start with an f/7 mirror.