Close up lens (diopters)

A question: If I have a lens wich minimum focus distance is 7.9" (20 cm) and don´t need a diopter/close up lens to focus behind a dome, what are the benefits on adding a +4 close up lens? It focus better? bigger? or only closer? The image become more sharp?

A dome port acts with the air and water to produce a virtual image close by in front of the dome. What is the radius of your dome port and can your lens focus close enough to get the virtual image in focus?

Small domes have a short radius and the virtual image is extremely close so a strong (+4) CU lens is needed. I am sure someone here will be only too pleased to give you the mathematical formula for working it out.

I use big dome ports with wide-angle lenses because I do not need a close-up lens. However, with the 12-24 mm Nikkor I found I needed a plus 2 dioptre lens even with a large dome. If you can get away without compromising your lens quality with a cheap supplementary lens - don't!

Using a medium dome port of 6" the virtual image I think is 12", so the lens focus on minimum 7.9" and I already try it several times, and it focus really close and really fast, but I wonder what benefit I can get on adding an B+W +4 close up lens on my lens...

Alex, this one is for you.

Adding a +4 is not needed. But Alex will argue that adding a lower power, +1 or +2 will improve sharpness.

Why a lower power add sharpness and not a stronger one like a +4 or +5?

Retaking this issue... I´still don´t know what are the benefits of adding a +2 or stronger diopter, as for say to a sigma 17-70mm behind a dome... focus better? sharper?... (macro works better?)