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.
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.
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"|