[astro744]

A 40mm at f10 would give 4mm exit pupil and when combined with a 2x Barlow would give twice the magnification and yes, half the exit pupil, ie. 2mm.

You could build your eyepiece collection around exit pupil alone and if your eyepieces all had ther same apparent field of view (AFOV) the magnification and true field of view (TFOV) would be in step.

For an f10 instrument with 1.25" focuser your maximum exit pupil will be 4mm, (40mm Plossl). An f10 instrument with 2" focuser can accept say the Tele Vue 55mm Plossl and this will give you an exit pupil of 5.5mm.

A nice range of exit pupils would be say 5-7, 3.5-4.5, 1.5-2.5, 1-1.5, 0.5-1. The actual number isn't that critical and you may want to cover more in the 0.5-1 range for planetary observing to cover a range of seeing. The 5-7 range is for wide field roaming under dark skies or simply as a low power finder eyepiece under any sky conditions. 4mm is usually a good wide field roaming power if the sky background isn't dark (suburban/city lights) so as to give you more contrast. 2mm is great for bringing out faint galaxies (ie good contrast) and gives you a nice dark background and there is still plenty of light in the object.

0.5mm is about the smallest you want since the image really gets quite dim beyond this point but not impossible. In fact under exceptional seeing you could add a Barlow and work in the 0.25-0.5 range and an object like the Moon could handle it.