Lenses used behind a dome port may require the addition of a +dioptre. The dome creates a curved image that sits 25 to 30cm out front and if the lens can't focus this close then a dioptre is needed.
My 18-55mm does work behind the 6" dome without a dioptre but performance improves with a +4. At the 18mm end there is less barrel distortion and the corners are sharper.
I never knew whether it also allowed you to get closer to the subject so some pool testing was required.
Adding the +4 brought the closest focus from 21cm to 13cm, and sharp corners only required f/16 rather than f/22. Of course you can use wider apertures if the corners are not critical.
