Close-up lenses with dome ports (Ikelite)...

Ever notice that Ikelite's port recommendations for their 6" and 8" ports with lenses that don't have a great short focus distance are always to use a +2 or +4 diopter?

Why never a +1 or +3?

Just curious....

Mainly you want to re-establish the negative dioptric strength of the dome used, a 6'' dome port has around -4 of dioptric power when immersed (some wiser mathematician would be able to give a more exact figure, but lets rough it for now ) and throws a virtual images at +/- 12 inches away from the center of the dome, that is where a faraway subject would show up, this close distance makes it hard on any lenses to focus on the virtual image and some won't even be able to do it, so adding a +4 more or less bring the focusing scale to a normal behaviour, the virtual images located at 12'' from the dome is easily focused on (if you looked at the focusing scale on the lens, it would show around infinity whit the +4 close up lens attached.

The 8'' dome normally should require around +3 (or 2.95 or something), but as anybody who ever looked for one can tell you, they are bloody rare, so a +2 is logically used and is enough to let the lens focus scale work in this situation.

Close up lens kit mostly come in +1, +2, +4, the thinking being that if you need +3, you would stack the +1 & +2 (yuck , like this will yield good results!!!)

There is very good reading available from www.scubageek.com its written mostly by no less than Les Wilk from Reefnet (and dad to fellow pixel member Keri Wilk)

Wow Jean, thanks. Far more answer than the (perhaps goofy) question deserved.

I use a +5 with my Nikkor 18-55mm f/3.5-5.6G with good results with a 6" dome port. Using the +5 instead of a +4 means that I get a bit better magnification. I may even try a +6 one day.