victor:
Warren
Can you please give as clear a description of diopters, I still cannot get my head around them.
A diopter is really just a magnifying glass that you can use to adjust the refractive power of a lens. Practically, what this means for photography is that it allows your lens to focus closer (if you use a + diopter), which is useful in particular in macro photography as it will allow you achieve a higher magnification of your subject than if you were to use your macro lens alone. I guess technically you can also have negative diopters which would make your lens focus further away, but there's no real practical purpose for using one as far as I know.
For example, for macro, I have two lenses I use. One a 60mm and one a 105mm. Both can reproduce a subject at 1:1 magnification (i.e. at closest focus for both lenses, shooting the same subject will be the same level of magnification). The difference really is how far I am from my subject. The 60mm does so while being closer to the subject than the 105mm, which is further away.
The implication of being further away varies depending on conditions. Further away might mean more water to contend with (and if water quality if bad, this might affect your lighting as well as an increased chance of things like particles in the water getting in the way). On the flip side, though, if you're further away, you may have better success getting certain shots (particularly of skittish animals) since you do not have to be as close to get the same level of magnification.
So why add a diopter? Simply because you want to achieve a greater magnification than your native macro lens will allow. Which lens would you add it to? Well, technically you can add it to either lens, but it would likely be better to add it to a longer focal length macro since you have greater working distance to begin with. The diopter will shorten your working distance on both lenses, but in the case of the 60mm, it will be much closer than with the 105mm with a diopter on in either case. The problems associated with being too close are simply you might scare away whatever it is you're shooting or even accidentally bump into it with your port. As well, when you're that close, lighting might get a bit tricky. With the 105mm, you're going to have more working distance and these problems, while they may still exist to some degree, will be easier to work with.
You can add either a wet diopter or dry diopter. You may have heard about people having "Woodys" - they are hopefully referring to an add-on diopter that you attach to the front of your port. It is a wet lens that allows you to focus closer. A dry diopter is a lens (looks like a regular filter) you screw onto the front of your lens, so it is inside the housing. You have to make sure that this diopter does not interfere with the port as it extends the length of the lens, so when focusing, you have to check to see that the port is long enough to accomodate it's addition.
As well, you can get varying degrees of power on diopters. I don't know if the wet diopters are available in varying degrees of power, but I have a set of 3 dry diopters that I use with my 105mm (+1, +2. and +4). The greater the power, the closer the focus is.
Does this help?