difference between barlow and telenegative?
Posted 06 September 2010 - 09:00 AM
Posted 06 September 2010 - 11:27 AM
The real question here is what exactly did you purchase?
Posted 06 September 2010 - 01:30 PM
One on left.
Posted 06 September 2010 - 02:05 PM
Posted 06 September 2010 - 02:12 PM
Tele - distance (or length)
Negative - to ruduce - minus
A barlow plays with the focal length of an eyepiece by reducing it (negative distance)
A 25mm EP becomes a 12.5 with a 2X barlow
Magnifiction of a scope is the scope FL divided by the EP FL
So, a 2X barlow reduces an eyepieces FL by half, a 3X reduces the EP FL to 1 third.
Posted 07 September 2010 - 06:57 AM
I purchased a 2x telenegative and am wondering if it has any different limits/applications than a barlow?
Well, technically a barlow is one form of telenegative. In other words, not all telenegatives are barlows, but a barlow is one kind of telenegative (with just one cemented doublet).
Of course, it doesn't help that some "barlows" are really telextenders/telenegatives but not really barlows.
My Zeiss barlow is a barlow, but my Baader VIP barlow isn't, since it has an air-spaced doublet.
My former TMB barlow was a barlow, but my former Orion Shorty plus "barlow" (three lenses) and former Powermate¹ (two doublets) weren't.
In practice, I used all four without worrying too much about the number of lenses and lens groups .
¹(TeleVue is a bit more precise in its use of words and won't refer to a PowerMate as a barlow)