[sudosysgen]

Yes, somewhere around 24mm. You'd need to step down a bit more at 90mm.

However, at 90mm the object would also be farther away, so you wouldn't need to stop down a lot more than you'd expect.

[bosie]

at 90mm (at 1:1 or higher magnification) stepping down will not solve the issue, you still need to heavily focus stack.

[sudosysgen]

If you want to take the images in the article of a ~6mm object even at 90mm stopping down would be possible. You'd be putting the object at around 18cm instead of 2cm though.