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Alex,
If you want an RFT you can use f/7 - theres no real need to go down to f/5 these days unless compact size is what you really want. The benefit of f/7 is that the ED f/7 doublets can also give perfect high powers too, far better than any f/5 achromat can.
By way of example:
Low power: if you assume your eye pupil could be 6mm, for a 4" scope that means the lowest useful magnification is 17X. This corresponds to a 42mm eyepiece, and I happen to have a 40mm (close enough) which means it will work fine as an RFT.
Note that this lowest power is determined by m = D/d
where D the ratio of the objective of the scope, and
d = the diameter of your eye pupil.
It is not governed by the f/ratio of the scope. If you choose an f/5, it will only need a 30mm eyepiece to give the same exit pupil and the same field I have in mine.
High power: at x1 per mm of aperture you should just be able to discern the maximum detail in images that a scope can provide. If you use more power (say 1.4X per mm) you should clearly see diffraction rings around bright stars, but not really any more detail.
For an f/7 scope, a 7mm eyepiece gives x1 per mm, and a 5mm gives x1.4 per mm. On my scope, 102X and 148X respectively. On an f/5, those eyepieces will be 5mm and 3mm. And I don't like using eyepiece that short.
Happy hunting !
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