One difficulty I’ve seen in learning languages is matching orthography to pronunciation - especially vowels.
English has several distinct sounds that a native speaker will describe as the vowel ‘e’, for example, while a language like French would use a wider variety of symbols to represent the same set of sounds (é, è, ê, e). Moreover, mispronouncing these sounds in French seems to affect the meaning of what you are saying to a greater degree. And I may be incorrect, but I get a sense that fewer sounds map to each orthographic symbol in French vs. English (that there are more ways to pronounce ‘e’ in English vs the same symbol in French).
Is there a formal concept of how much phonology-orthography correspondence varies across and within different languages? Further, are there any theories about how phonology-orthography complexity affect word comprehension, and difficulty in acquiring a language as an L2?