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Thanks for that Syzygy. You've made me realise that my knowledge of what eye relief is was sorely lacking. Consequently I've done a little more research and tried to apply this to a barlow lense. What I've come up with is the attached ray trace diagram.
The top diagram shows how eye relief works with a standard objective->eyepiece scenario. The bottom one I've inserted a barlow and shown how the light rays are affected by this. Assuming that I've got it roughly right you can see that the eye relief is affected (lengthened) but how much would be dependent of focal length of the lenses of course.
How does this sound? As I say I have very little knowledge of optics (a bit more now than when I started though) so I'd be happy to be corrected. Anything that brings us closer to the truth is good.
Trav
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