Cantonese tones 2/5 and 4/6 confusion

[chrix]

sure, AFAIK Cantonese is the only Sinitic language that has vowel length distinction. Nevertheless, in language change often features that were phonemically irrelevant can become so.

That's how people believe tones developed. And that's why Cantonese has a 陰陽 set of tones and Mandarin doesn't, it has to do with voiced and voiceless consonants, influencing the fundamental frequency of the following vowel. In Cantonese it became phonemic, in Mandarin it didn't. While Sinitic languages (except for Shanghainese) now no longer have any voiced obstruents, Cantonese and others still retain this opposition since it had already become phonemically relevant. Also, toneless languages such as English are known for the same effect which is usually not picked up upon by speakers because it is not phonemically relevant in those languages.

So the question is not did Middle Chinese have vowel length distincion, but is there some kind of systematic correspndences between vowels in MC and the ones in Cantonese, accounting for the different vowel lengths. Your comment suggests so.