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The IQ's fine with the 500N closeup aux lens on the 135L. Maybe not as good as a true macro, nor nearly as flexible (it has a restricted 'in focus' range like extension tubes), but it's much easier to carry than an extra lens.
One big advantage for a closeup aux lens like the 500N is that it doesn't reduce the amount of light getting to the sensor, as an extension tube does, big time. This isn't a problem if you're shooting a static subject using a tripod, but the close up aux lens is a great solution for walkabouts and for when you're traveling light. If you're interested in this type of gizmo, I highly recommend the Canon 500N, which has two optical elements that provides relatively good performance right into the corners on FF . I've also used a single-element B+W closeup aux lens, and it shows some CA at the edges and in the corners (it's less well-corrected).
The light monopod I mentioned is a Manfrotto 676B Digi monopod. Unfortunately, it seems to be out of production. Here's the vital stats on the three monopods that I use. The Feisol is most all-purpose, but the Digi easily beats it for travel with light gear, up to a 1D-series with 70-200/2.8L IS or 28-300L IS.
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