Diopter help

Hi,

So apparently I need a +4 diopter to use my EFS 17-85 behind Ikelite's custom zoom port for it. I've never bought/used a diopter before, so can anyone advise me on a decent brand and whether I really need +4?

Cheers

I had a 17-85 with my aquatica and never needed a diopter, must be an Ike thing.

B+W is a good and not so expensive brand, but I don't know if you will find it in Australia.