You can extend the magnification of many Barlow lenses by increasing the distance, as explained e.g. in this PDF:
The image quality decreases slowly, so there is no "hard break" beyond which a Barlow can not be used - it also depends on the complete optical system.
Also - a barlowlens may work well in a range of 2x to 3x (with the Zeiss being even somewhat better) - but still it is not the same quality as what you can do with a high quality zoom, were several lens groups are moving up and down to optimize optical performance.
Direct T-2 Adaption possible - extends magnification up to 3x - without quality loss
The Carl Zeiss Abbe Barlow lens consists of two parts. The front barlow section ends with a photographic T-2 thread suitable to directly connect onto standard DSLR T-rings. , at the separation level a T-2 photo thread - mounted with our Baader ClickLock eyepiece clamp with the known rotary clamping system. A direct T-2 adaptation is possible.
An enlargement of the imaging scale up to three times is possible without loss of quality. On the first T-2 thread you can connect normal T-rings, so you have a 2x Barlow lens - with 1¼ " barrel - in front of every camera, webcam or DSLR camera housing. If you add T-2 extensions the magnification can be extended by up to 3x. We recommend using our T-2 Varilock clamps for stepless adjusting of the optical length: Varilock 20-29mm (# 2956929) or Varilock 29-46mm (# 2956946)
Spectral transmission curve of the Zeiss Abbe Barlow
This (german) document attests to the exceptional performance of the multi-layer anti-reflection coating applied onto the new 2016 Carl Zeiss Abbe-Barlow-Lens made in Jena / Germany. Carl Zeiss has chosen to develop and apply the very best coating ever made available to the most discriminating visual observer. The coatings feature a 0.2% loss over the exact spectral region where the night-adapted human eye is most sensitive. This extremely high efficiency also translates to near-zero coating scatter for the highest contrast ever seen. No lens-system has ever been coated to more stringent requirements for visual observation and for VIS-photography as well.
|Outer Connection (lens sided)||Barrel, 1¼"|
|Inner Connection (eyepiece/-camera-sided)||Clamp, 1¼"|
|Outer Connection (eyepiece/-camera-sided)||Thread, T-2 (M42 x 0,75)|
|Magnification (x)||2x - 3x|
|Barrel Size (in)||1¼"|