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So I have been wondering if a barlow (say 2x) will produce the same result of a half eyepiece.... So would a 10mm with a 2X Barlow produce the same result as a 5mm? (Assuming the FOV is the same on the 10mm and the 5mm)....

Since I dont have any combination that would be "equivalent" I cant test the result

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Using a Barlow effectively increases the focal length of the telescope, leaving other factors the same. So, the magnification will be the same as if you were using a 5mm eyepiece.

However, a cheap 10mm eyepiece usually has better eye relief and apparent field of view than a cheap 5mm. So, you are actually better off with the Barlow than getting a 5mm.

The only downsides are you increase the number of surfaces (and thus loose light to reflections), and there might be some issues with focus if you have short focuser travel.

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    $\begingroup$ I keep thinking that there's got to be a catch somewhere, somehow. Ignoring reflections and chromatic and other aberrations due to imperfect optics, we can always get a boost in eye relief by using a Barlow with a lower magnification eyepiece without sacrificing anything else? That seems like an actually really good reason to use a Barlow lens! $\endgroup$
    – uhoh
    Jul 30, 2022 at 23:11
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    $\begingroup$ @uhoh the main catch is that the Barlow costs money. If it were free, they'd integrate it into every eyepiece. $\endgroup$
    – Greg Miller
    Jul 30, 2022 at 23:59
  • $\begingroup$ @GregMiller IIRC some high end eye pieces do include a built in barlow; used with variable powers so they can gain economies of scale by reusing the same fancy optics for multiple eye pieces within a single series. $\endgroup$
    – Dan Is Fiddling By Firelight
    Jul 31, 2022 at 23:45

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