Curmudgeon wrote:
Has anyone tried a teleconverter on a macro lens?
You mention having a 90mm macro lens in a followup. I suspect it's one of the Tamron lenses (there have been a long series of versions).
You might find this interesting. I happen to have one of the very early, manual focus versions of that lens: a 90mm f/2.5. It's one of the interchangeable mount versions (they made two of those). This lens actually was offered with an optional, matched 2X teleconverter, which I also have. To be totally honest, I've never tried them together. The 2X teleconverter on it would make for a 180mm f/5 combination and I already have a 180mm macro, so don't really have any need for this combo, which is unlikely to have anywhere near as good image quality as an actual 180mm lens.
I've adapted this vintage lens for use on both DSLRs (Canon EF mount) and mirrorless (Canon EF-M mount). I've also used it and various copies of the other manual focus version on a variety of vintage film cameras (In addition to the more modern ones, I have the interchangeable "Adaptall" mounts for Konica K/AR, Pentax bayonet, Pentax M42, Olympus OM, Nikon F, Canon FD and Minolta MD mounts.)
The early Tamron 90mm were only capable of 1:2 magnification on their own. Tamron offered a dedicated extension that could be used to increase the lens' magnification to 1:1 (which I don't have... instead I just use standard macro extension tubes in whatever mount I've got the lens set for).
For what it's worth, the minimum focus distance (MFD) of a 180mm macro lens is roughly 18 or 19". In comparison, the MFD of a Tamron 90mm is 11.81" (latest F017 version, others are similar). Presumably the MFD of a 90mm + 2X teleconverter would be very s1milar to that of a 180mm lens. So at full 1:1 you might expect to gain right around 7.5" to 8" distance with a 180mm lens. However MFD is measured from the plane of the image sensor to the subject. A significant portion of that distance is occupied by part of the camera (approx. 1.75" with a DSLR), while even more is occupied by the lens and any attachments to the front of it. What's left is the actual "working distance" between the front of the lens and the subject.
For example, the Canon EF 180mm I use is about 7.5" long... add the 1.75" of the back focus distance within the camera and deduct the resulting 9.25" from that lens' 18.9" MFD. This leaves a little over 9.5" of "working distance" between the lens and the subject (assuming the lens' 3" deep hood isn't fitted and there is no filter or macro flash mounted to the front of the lens).
The Tamron 90mm (F017 version) is 4.61" long and adding the 1.75" back focus distance ends up a little under 6.5" total. Deducting that from the lens' 11.81" MFD will leaves a little under 5.5" of "working distance" (also assuming the lens' 2.5" deep hood and filters/flash aren't fitted).
So when you compare "working distance" instead of MFD, you only see a gain of about 4" going from 90mm to 180mm focal length.
Both the above lenses are internal focusing, meaning they don't change size when they are focused closer. This is not the case with all macro lenses, including some of the other versions of Tamron 90mm. If you have one of those, it's probably shorter than the F017 version when set to infinity, but actually might be a little longer when focused to full 1:1 magnification.
Finally... something else to consider is that with longer effective focal length you get significantly shallower depth of field at any given aperture. That compounds the fact that the longer focal length is also a lot more difficult to hold steady and avoid camera shake blur. I usually use a tripod with my 180mm and switch to a 100mm or 60mm for any handheld macro work.
P.S. No, for what the original poster (Curmudgeon) is asking about, a macro extension tube or bellows (which is actually just an adjustable, extra long extension tube) would NOT help. Those increase a lens' magnification ability, but will not allow the OP to stand farther from the subject. For that OP needs more focal length... hence the idea of using a teleconverter. A different lens with a longer focal length would give similar increase in distance from the subject.