Does Barlow reduce the quality of the image and FOV?

[blusky]

Get a very different picture when using 40mm Plossl+2x and a 20mm.

[great_bear]

A good quality Barlow will not reduce image quality to the extent that it is visible compared to an EP of the same focal length, and in some cases may actually improve upon it.

However, longer focal length eyepieces (over 20mm) tend not to work well with a Barlow, as the amount of eye-relief becomes too much, and also the edge-of-field can become vignetted thus looking fuzzy.

The apparent field of view should remain the same.

Also, the amount of magnification that a Barlow produces is dependant on the distance between the Barlow lens and the focal plane of the eyepiece. A 1.25" 40mm Plossl will likely have a focal plane close to the bottom of the chrome barrel, thus producing less magnification than you might expect.