I cover the Raynoxi and other close-up adapters a bit in

https://www.pentaxforums.com/forums/pentax-lens-articles/152336-cheap-macro-b...lose-work.html
I have a few sets of cheap +1+2+4 dioptre adapters, a few +10dpt's (but not an Opteka) and a Raynox DCR-250. I know the Raynox is optically corrected and the others aren't, but they don't bother me much. Anyway, here are how the numbers work.

A lens' dioptre measurement is the inverse of its focal length, in meters. A +1dpt glass has a focal length of 1m or 1000mm; +2dpt= 500mm; +4dpt= 250mm; +8dpt (like the Raynox DCR-250)= 125mm. When stacking glass, just add the dioptres. So a +2dpt stacked on a +8dpt gives +10dpt. We use this dioptre-addition to calculate the effective focal length of any optics combination. A +10dpt glass on a 100mm lens is thus 10+10= 20dpt = 1000/20mm= 50mm.

And yes, we can treat stacked adapters like a lens. I can (and do) put a +2dpt onto my Raynox DCR-250 and mount them on an M42 bellows for an effective 100/2.5 optic. A bit soft and romantic, too! I'll let y'all figure out how to compute effective apertures, eh?

To calculate the magnification of any lens plus adapter, use:

M = F*D/1000

where M is magnification, F is host lens focal length, and D is dioptre of the add-on lens. Rewriting that to find D or F, we get

D = 1000*M/F

F = 1000*M/D

A +8dpt Raynox (or whatever) on a 100mm lens gives M = 100*8/1000 = 0.8x. On a 200mm lens, M = 200*8/1000 = 1.6x. Et cetera.

So magnification depends on the adapter's dioptres and the lens' focal length. Working distance is a different matter -- that depends entirely on the adapter's dioptres. Here is a table of working distances:

+1 >> 20-38" (500-950mm)

+2 >> 13-20" (330-500mm)

+3 >> 10-13" (250-330mm)

+4 >> 8--10" (205-250mm)

+5 >> 6.5-8" (165-205mm)

+6 >> 6-6.5" (153-165mm)

+8 >> 5" --- (127mm)

+10 > 4" --- (102mm)

And here's what all this boils down to:

* A close-up adapter can dramatically thin a shot's DOF.

* Close-up adapters don't do diddly-squat on short lenses.

* Stronger dioptres and longer lenses give more magnification.

* The stronger the dioptres, the closer the working distance.

Anyway, I have lots of fun with such cheap adapters, and with the Raynox too.