What are the differences between the French and English [i] and how does it affect the perception?

I think there is a bit of confusion about the translatability of the IPA. The [i] in English is lengthened or not lengthened based on convention of transcription rather than because of a belief in some underlying difference in pronunciation. [i:] is the typical modern transcription and [i] is used as a compromise in some contexts (like -ity) where different speakers pronounce different vowels. As such I think the comparison between French and English should happen outside the traditional transcription and on particular uses. I don't know enough about French to offer an opinion but thought I'd caution against oversimplifying the English situation. See http://www.phon.ucl.ac.uk/home/wells/ipa-english-uni.htm