Originally posted by Marc Sabatella I guess that depends on what aspect you find difficult. The Raynox gives you very little control over working distance period. Maximum focus distance is fixed at something around 6 inches or so (I forget the specifics; longer for the 150 than the 250). Minimum focus distance is determined by the lens' own focusing characteristics, but in general, its not that much closer than the maximum......
So, prime + Raynox = fixed working distance, fixed magnification. Zoom + Raynox = fixed working distance, variable magnification. Whereas a true macro prime gives you a range of working distances, with magnification directly (well, inversely) related to working distance. Only a macro zoom would allow you control working distance and magnification independently.
I want to expand quantitatively on Marc's important comments. The following applies to thin-lens optics.
In the situation where a close-up lens is attached to a camera lens focused at infinity:
Working_distance_mm = 1000/diopter_strength
The focal length of the camera's lens doesn't matter.
For example, a Raynox DCR 150 (4.8 diopter) close-up lens gives a working distance of 1000/4.8 = 208mm = 8.2". So if 200mm (8") is a good working distance for your purpose, choose a roughly 5 diopter close-up lens. If you need 400mm (16") you'll want a 2.5 diopter close-up lens, etc.
The focal length of the camera's lens in combination with the close-up lens determines the magnification.
Magnification=Focal_length/Working_distance
So, continuing the Raynox 150 example, a 200mm lens yields about a 1:1 magnification, while a 50mm lens yields a 1:4 magnification. A nice magnification range I think, with a reasonable working distance. A 100mm macro lens at 1:1 would have a 200mm working distance for comparison.
Compare this with a Raynox DCR 250 (8 diopter) close-up attachment which has a working distance of 125mm (5.12"); closer, however a shorter camera lens is required for magnification.
It looks to me like a Canon 250D or Raynox DCR 150 (or any other good achromat in the 4-5 diopter range) would work well in combination with a long zoom lens for hand-held macro work.
Dave
Disclaimer; the above is approximate & neglects things like the actual location of a complex lens' principal plane, lens hoods, lens recess, etc. Still, I believe it is reasonable guidance for choosing close-up supplemental lenses for macro work.
In real life, the working distances estimated above are maximums, and the magnifications are minimums. For maximum magnification estimates, the lens' effective extension while focusing must be taken into account (but the effect isn't large for normal lenses.)