Hi Leinad and many thanks for starting this thread.
It is just curiosity on my part but it appears to be an accepted fact that a barlow, apart from increasing the focal length of an optical train, will also increase the effective eye relief of any given eyepiece. I have two problems with this:
1. With the eyepieces I use, the eye relief remains unaltered when using them with a barlow.
2. On the internet I can find examples of those who attest the truth of an increase in eye relief and those who state that it is false but I can't find a web page which explains this phenomenon in optical terms though there has to be a web site out there that can shed light on this subject.
I understand the principle of placing a negative lens in an optical train to increase focal length complete with the arithmetic involved (I used to make my own barlows from old camera telenegative lenses in the bad old days) but I think i'm missing something here as I don't get the increase in eye relief with my plossls. The type and focal length of an eyepiece may be a large factor in whether a barlow increases the eye relief. And, of course, I may be doing something wrong when checking for eye relief.
Anyway,this should be an interesting thread, thanks once again Leinad and lets hope someone in the know can shed light on the subject.
Cheers
Malc