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Originally Posted by yagon
Does image quality deteriorate with use of a Barlow?  |
As you are adding more glass to the light path yes it will.
The question is by how much and a good quality barlow with good multicoatings will give you a decent view, whilst a cheapy will be less sharp and suffer from reflections on bright objects and excess light scatter (think driving with a dirty windscreen at night).
To answer the other Q, its a matter of comfort while viewing. The eye relief on a 10mm plossl can be a bit tight for some tastes whilst the 20mm will be more comfortable due to the greater eye relief.