ならいざ知らず・はともかく: the difference
ならいざ知らずN1
Meaning
It would be one thing if …, but; X may be different, but; Setting aside X
Past papers
Example
１万円は社会人ならいざ知らず、小学生にとっては想像を絶する金額だ。
I don't know about working adults, but for children, 10 000 yen is an unimaginable amount of money.
はともかくN2
Meaning
Setting aside, Apart from
Past papers
Tested once in past papers
Example
冗談はともかく、そろそろビジネスの話をしよう。
All jokes aside, let's get down to business.
How to tell them apart
「〜ならいざ知らず」: the first part is excusable, the second is not; sarcastic and critical.
「〜はともかく / 〜はさておき」: neutrally sets the first part aside and moves on to the second; no anger or blame.
Typical pairs: 「子供ならいざ知らず、大人……」, 「昔ならいざ知らず、今……」, 「初心者ならいざ知らず、プロ……」.