First off Ruu, there need not be a "highly technical explanation" for your allegedly deficient intelligence. Could be a much simpler reason--no experience in working w/ close-up equipment or accessories. Easy to fix, but not necessarily cheap. Another reason though may be how the industry tosses terms around in an often confusing fashion.
Like macro. Used to be, shortly after mastering walking upright, we defined macro very narrowly. It was a specific lens, ground for flat field, allowing one to typically work with reproduction ratios of better than 1:10. (Just make a fraction out of that ratio number and you've figured out repro ratios--it is a comparison to life-size. A 1:2 macro lens delivers a 1/2 life size image on the film...opps, of course I mean
sensor.) Macro since has become a catch-all term, ill-defined but to most folks it means getting closer to your subject. I would argue though that isn't why people use them (to get closer to their subject). They really just want small things to appear bigger within the frame, so they don't have to crop a lot and lose sharpness and/or introduce noise (or what us troglodytes used to call grain). Getting closer can be one way to accomplish this but certainly not the only way.
Rather than lurch off into excessive details about close-up work I'll try to just answer your questions about lenses like the M67 vs the +1,+2 etc. low cost diopters. First. Think of either like a a pair of reading glasses (which use the same diopter ratings). The higher the number, the closer you can hold something (like fine print) to your eyes. Works the same way on camera diopters. Higher numbers allow for getting physically closer, hence create a larger image. They do NOT lose light however, anymore than stronger reading glasses make things look darker. Possibly Ben W. was thinking of extension tubes (which do lose light) or an actual macro type lens but diopters don't lose light.
With DSLRs you could buy a macro lens which is designed to focus closer w/out attachments, typically at reproduction ratios of 1:2 or better. True macro lenses are pricey and yes, are not as "bright" (ielose light), since their maximum aperture is typically smaller than an equivalent standard lens but they really aren't part of your question, though of course they could be a great solution for close-up work.
The eBay solution is limited for several reasons, notably that you want to use them underwater and they weren't designed for that. So what happens is they seem to magnify a lot on the surface but uw, not so much. More critically though they are a single piece of curved glass, usually w/ out a lens coating to reduce flare and are likely to cause some image quality loss due to their generic approach. Fantasea's lens is multi coated to more effectively transmit light & color to the sensor and they use multiple elements in multiple groups for better optical correction and to give a stronger correction uw. (Some other things too but believe it or not, I am trying to be brief.)
Now I gotta say I was a bit thrown by the notion of a diopter getting you a "little further" from the subject (it does just the opposite) and somehow getting more light from the strobe. Uh uh. Basic physics. "Light falls off inversely proportional to the square of the distance". If you move your strobe further from the subject you will not get as much light on the subject, period. Angles aside, if what you want is to make small stuff bigger in the frame you either increase the focal length of the lens (zoom it to a higher focal length) or you move in closer. Either way you will magnify the image size in the frame but you also increase problems with depth of field and increase subject or camera movement, which reduces image sharpness. (That is a different kettle of nudibranchs though so let's not open it more than that.)
Well, so much for brevity. I use ReefNet's SubSee diopters (a true uw +5 & +10) and love 'em. Haven't heard folks complain about Fantasea's diopter either. Kind of a "get what you pay for" thing. The SubSeas are not only impeccably sharp but their swing-away adapter system is so much easier than unscrewing or even bayonetting in a diopter lens that can be dropped or needs to be tucked away when not in use. Obviously your budget and current camera are factors here, as is your personal desire to educate yourself enough to enjoy this sort of work/play. Your willingness to ask questions proves this is not about being an idiot--far from it. // ww
Bubble anemone detail/ Raja Ampat
Taken w/ +10 SubSee
Coleman shrimp on Fire Urchin/Tulamben, Bali
Taken w/ +5 SubSee
// ww
