You remember the old adage "Oils ain't oils"? Well, eyepieces ain't eyepieces when it comes to scopes.
Telescopes we think of as focusing light at just one single point. Not so. In fact, a scope's focal point is actually a 'plane'. AND a curved plane a that! Depending on the design of the scope, this plane is either convex (refractors, SCTs, Maks) or concave (Newtonians). And depending on the focal ratio of the scope, the shape of this curve is either very deep (fast scopes) or shallow (slow scopes).
Eyepieces then in turn need to be designed to deal with one shape or the other. The way the EP is designed WILL influence the way it deals with the manner the focal plane of the scope falls onto it. This is why some eyepieces are brilliant in one scope, say a fast refractor, and poor in a fast Newtonian. Nothing wrong with the eyepiece. The two are just not an Optical Match. And unfortunately, many people write off this or that eyepiece because they see a whole bunch of aberrations, and those aberrations would not be there if they had put the EP into the right scope design!!!
Some eyepieces can work well with both convex and concave focal planes. Oddly enough, these also tend to be more expensive EPs too...
. BUT, they do not throw up the exact same image quality!!! There will be some compromise somewhere in the optical light train. Either a little field curvature, or a little softer in focus, or a little astigmatism right at the edge, or a shift in eye relief, or a little more eye strain, or eye placement becomes more critical... Something will change, however small. <
WARNING: another cliche> -
No Free Lunch here... But the nett result is still a very damn good image.
Barlows... double edged sword these things,
Curiously most people think these are somehow 'neutral' focal extenders, like maintain the native eye relief of the EP (which they don't, though close in most instances), or simply 'double' or 'triple' magnification. Yet they have other consequences on the way the light is dealt with by the eyepiece. Use the wrong eyepiece at the end of the wrong scope-barlow combination, and the image thrown up can be atrocious, even if the EP is a high end one or not.
But, use the right eyepiece in the right scope-barlow combo, and you've hit the jackpot!
I tend not to use barlows. Apart from the odd chance of an optical mismatch, for me it is more the case of another bit of kit that gets in the way in the middle of the night. It also introduces more glass between me and those precious photons I'm trying to hog for my eyes. And if the barlow is a cheapie, even worse in terms of poor coatings, lens quality, etc.
I do have a barlow, and I keep it only for one particular telescope I have, an 8" f/4 Newtonian, as with the shortest focal length eyepiece I have, it is only with a barlow that I can push the scope to the maximum the 8" aperture can give when atmospheric conditions are good. This combination gave me my best view of Saturn one night at the time - and in all honesty, I haven't used the barlow since! A barlow is a nice tool to have up your sleeve. But being able to use one very much depends on your scope/scopes and EPs too. Most people probably have one, but I'd say few actually use it much - they would rather have an EP that does the heavy lifting on its own.
~x.X.x~
There is a lot of bias when it comes to eyepieces. This then clouds objective reviews and recommendations. And I am not without fault. You NEED to look through a crap load of eyepieces. I may suggest this or that eyepiece, but best if you try out a mate's before you buy. You WILL change eyepieces several times over the years. Only this way will you come to your own unique eyepiece kit, with your own set of individual preferences. The EP's in my kit I've cherry picked for me, to suit my own set of preferences. Your own collection may well end up being 'near enough is good enough', and by me that is as valid as 'only top shelf performance',
Alex.