Quote:
Originally Posted by PeterHA
Based on this I would expect, at least for visual use, more FLP-53 doublets to perform on par with the few CaF2 based doublets around (old Vixen and old and new Tak) but that seems not happen to often. I wonder why.
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The reason might be that most FPL53 doublets are not made to the same standard as CaF2 ones. AFAIK no "premium" APOs (AP, TEC, LOMO/LZOS etc) are doublets these days, which leaves 'cheap' (but still quite good) ones made in Taiwan/China (Orion, Skywatcher etc) to compete with Takahashi. Not a fair comparison I'd say.
But we have many premium
triplets on the market and I'm yet to hear about Takahashi CaF2 consistently outperforming FPL/OK4 based AP, LOMO, LZOS or TEC. Unit to unit variations might make one scope better than the other one
sometimes but not always. Otherwise Takahashi would be crowned world's best APO, and many people would vigorously contest that!
PS - Takahashi is predictably vague in their explanation on why is CaF2 better than ED glass :
"While ED and fluoro-crown lenses can achieve Abbe-coefficients approaching fluorite, they tend to absorb more light in the visible spectrum. This means that fluorite yields a brighter, higher contrast image."
Transmission of FPL53 :
340nm to 380nm (UV region) is > 90%
380nm to 420nm (violet region) is 99.6%
420nm to 700nm (extended visible region) is 99.7% (average)
700nm to 2000nm (IR region) is 99.6%
in the visual peak at 550 to 600nm is 99.9%
we have to ask
who will actually detect that 0.1% ?
Again, quality of manufacture (*) will far outweigh the choice of glass in a modern APO. To me at least Tak is using CaF2 as a marketing tool; which is fine, as long as we do understand the whole story
(*) not to say Tak is lacking in any way; they are in fact always top notch. But so are the others on the "premuim" list