Barlow questions

1> If a Barlow lens is rated at 2x what is its focal length?

2> Are there different fl for 2x Barlows?

2> What is the relationship between fl and power?

The formula for Barlow magnification is (in one form)

M = 1 - D/F

where F is the focal length of the Barlow (always a negative number) and D is the distance from the optical center of the Barlow lenses to the projected focal plane (which would be at the field stop of an eyepiece, film of a camera at prime focus, etc.)

Obviously, for a 2x Barlow, F = -D. Yes there would be different focal length 2x Barlows. Pick any F you want and then make the Barlow long enough (or short enough) in physical length that D = -F. I only know the focal length of the 1.25" Tele Vue 2x Barlow because Al Nagler once quoted its focal length on the old Ask Al forum. For that one, F=-74.4mm.
--
Mike