why a diopter +4 on a zoom lens

Hello,

I used to take pictures with a olympus c5060.

Today a new world opend for my, i have now a Nikon D80 in a ikelite housing. First lens to use, the 18-70 in a dome port.

The shop give me also a diopter with it, +4.

I used it , today , and have some nice results , but also questions.

Why need a diopter in a dome port, and I don t need in a flat port.......

Do I always use it in a dome ???? or only with a zoom lens ?

Thanks for helping me out.

Best greetings/regards,

Fotoduiker.

hoi fotoduiker,

the dome becomes a lens under water. The lens itself will focus on the " virtual image" projected by the dome port, rather then the actual image.

Since this virtual image is very close (something in the range of 10-20 cm with a small dome), the lens has to be able to focus this close (and most zooms don't). The diopter will fix that.

do some searches with diopter, zoom lens, or virtual image. There are some posts that have explained it much better

Gerard

PS: i think it is also mentioned in the ikelite port manuals