Having tried my lovely new Nikon 105mm macro lens out on a recent trip to Scotland, I had borrowed a Nikon diopter and also used a teleconverter. Now I had discovered that I was able to get some pleasing images, I was interested in making some measurements.
By using different combinations of these three items (lens, teleconverter and dioptre) different magnifications can be obtained, but as usual there are trade offs to be made. On one hand, a teleconverter increases magnification without needing to get closer (ie same working distance), but the optics absorb some light, which means either brighter illumination or a larger aperture, which in turn reduces the (already very narrow) depth of field. On the other, a dioptre lens increases magnification without absorbing so much light, but reduces the working distance.
So which is the best combination to choose? The first step, I felt was to try all the combinations and measure the magnification, working distance and depth of field. This would provide me with objective data to make decisions.
It seems to boil down to working distance- the dioptre is likely to give better results if the subject is suitable- shorter working distance (so hopefully less backscatter) coupled with higher magnification. However, the teleconverter will help with more skittish subjects because it gives high magnification at a longer working distance, but relies on not having too much suspended matter in the water. Using both is a possibility, will require some care to ensure that the appropriate part of the image is in focus, due to the wafer thin depth of field.