If interested, I added a very simple demonstration of TTL BL direct flash, showing how it is protected by D lens distance in a good case (meaning, which is sometimes accurate, and sadly sometimes not, sometimes it is worse. Primes are better than zooms).
Flash Indoors
And also showing how FV Lock can affect the metered value, apparently when TTL BL direct flash no longer pays attention to the D lens distance protection.
It will be trivial to reproduce this to see it for yourself, which is the purpose. It is certainly something we ought to know, but one point is, if your TTL BL direct flash is coming out underexposed, FV Lock may override it. FV Lock is not a feature on entry cameras without a commander however.
It is not a beginners subject, more for those that have explored their flash system, and who have questions. And not sure how organized this is... it may relay on information at other links, but the links are provided.
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