〜ならいざ知らず／〜はいざ知らず

Meaning

States "if it were the case of ~ that would be another matter, but it is not," provisionally granting a special case while contrasting it with a different reality. Often followed by surprise or criticism.

Examples

新人ならいざ知らず、ベテランがこんなミスをするとは情けない。 If it were a newcomer it might be understandable, but for a veteran to make such a mistake is pathetic.
昔はいざ知らず、今どきそんな考え方は通用しない。 That way of thinking may have worked in the past, but it does not apply nowadays.
専門家ならいざ知らず、素人にこの修理は無理だ。 An expert might manage it, but this repair is impossible for an amateur.