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I’d say there is more to it than whether it’s a doublet or a triplet. I have a 100 F/9 doublet that has better colour correction than some 127mm F/7 triplets out there. Having 1” less aperture helps with colour correction, being F/9 as opposed to F/7, being a well crafted fluorite doublet helps compared to a cheaper FPL-51 triplet.
By a formal definition an APO can have visible CA which is what blurs the line between APO and Semi-APO.
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