Hi Cody,
General wisdom suggests:
Hi Power:
No more than aperture in mm x2 and no less than 0.5mm exit pupil
In your scope: 120mmx2=240 max useful power. Shortest eyepiece that may be of any use: 840mm/240=3.5mm. I suggest 5mm as a good place to start as 3.5 gives the marginal 0.5mm exit pupil and a power of 240 that might be good on paper but too high for all but the best nights depending on where you are. 840mm/5mm=168. 168x is still a reasonable power to see detail on planets If that gives you a really crisp image more often than not from your location, you can always get a 3.5mm EP later and have a meaningful increase over the 5mm. Alternatively, start with a 4mm and see if it works well. If not, resell and move up or down. For some, a zoom EP covering most of the above is excellent, but a 3-6mm Nagler zoom (for example) isn't exactly cheap.
Mid Power:
Aim for a 2-3mm exit pupil. Gives good contrast, works well on extended objects and provides comfortable viewing. Many say that this eyepiece might well become your most-used. I agree.
In your scope: 7*2mm=14mm. 7*3mm=21mm. So your mid power in that bracket perhaps, maybe more towards 14 as you already have a 28mm.
Low Power:
If you have a really really dark sky, then you can go as low-power as you like. Because your scope is a refractor, you don't need to worry about what is being said about exit pupil getting too large at very low powers if your sky is nice and dark. Who cares about wasted light when the view is epic (if the exit pupil is greater than your own pupil you are "wasting light"
). Very low power at that aperture under pristine skies is a sight to behold. For most of us though, the sky is less than perfectly dark, and a very large exit pupil will show an unpleasantly bright background sky, which reduces contrast and just doesn't look very pleasing. If you are anywhere near a city, I suggest a low power exit pupil no greater than 5mm.
In your scope: 7*5mm=35mm. You might not find an affordable one at that exact focal length, but there are some very nice 2" eyepieces available in that general bracket. However you may well find that your 28mm is perfectly good for low power work. So try as you go and get this one last perhaps.
You mention wide field. There are 2 reasons that this may be desirable. One is technical, the other superficial. At high powers, a large AFOV helps you track your target if your mount is not an EQ. At mid-low powers, it just looks pretty and provides nice context views. I like both.
As always though, YMMV! Too many subjective factors happening at the interface between equipment and living organism...
Have fun & enjoy the journey not just the destination.
