View Full Version here: : Barlow Questions

07-04-2007, 05:52 PM
Simple one: what is the effect of using a 2x Barlow on the exit pupil size and field of view of an eyepiece?

At a guess, the exit pupil size and FoV are both halved?

So one more, does a Barlow normally have any effect on the eye relief of an eyepiece? Not?

07-04-2007, 10:04 PM
Hmmm.... a more thorough Google search shows that my guess to the first question was correct.

The second is less obvious, but I imagine it's fixed by the optics of the eyepiece?

08-04-2007, 01:27 AM
As I understand it - not at all. That is set by the eyepiece geometry More knowledgeable ones will know more...
For the earlier point.
Pupil exit size X Focal ratio = eyepice size or
Pupil exit size = eyepiece / Focal ratio
so for a .5mm exit pupil with an F4 scope barlowed (X2) to F8 gives an eyepiece of 4mm minimum size.

True FOV is the smaller of that given by the Aparent field of view or that limited by the field stop.
AFOV = eypiece field of view (approx 50 degrees for plossls) / magnification where magnification = focal length / eyepiece size.
also Focal length = Aperture X Focal ratio
TFOV = (field stop diameter/Focal length) x 57 degrees (approx)
so a 1.25" eyepiece with a maximum practical field stop of 28mm in a 4.5" (114mm) F4 scope barlowed (x2) has a TFOV of
[28/(114X4X2)]x57 = 1.75 degrees (maximum), and a 4 mm plossl gives a FOV of 50/[(114x4x2)/4] = 0.22 degrees or 13 minutes. a 40mm eyepiece (if there were such a 1.25") gives a FOV of 2.2 degrees capped at 1.75 degrees as noted above.

Maybe this helps:screwy:

10-04-2007, 11:25 AM
Yes, exit pupil and AFOV halved for 2x barlow.
Eye relief is extended, more so for long F.L. EPs. Annoyingly so (increased blackout / kidney bean) for many 25+mm EPs, negligibly for 10mm and under.