Ok so let's play out my actual use case.
I have the 105Z so not worth it at 100mm.
I have the Plena probably not worth it.
So at a lets say 300mm, is there any chance I can stay as distant as the 105 and get the same or larger magnification.
We have a lot of nasty things in AZ including tarantula hawks (big wasps that sting tarantulas) whose sting is one of the worst in the world. I want to keep a distance.
- The 105 macro lens is a stock 1:1 macro lens. Add say a 20mm tube and it becomes roughly a 1:0.83 macro lens(something 0.83x the sensor size will fill the frame so for a FF sensor something roughly 30mm x 20mm will completely fill the frame) but of course you have to get very close to see that magnification. Pretty cool for static macro subjects but not as useful for live subjects that fly or scurry away if you get too close.
- Say that 300mm was a 300mm f/4 PF F mount lens with its native 0.24 magnification(roughly 1:4). Add the same 20mm tube and the magnification only increases to roughly (1:3.3) That's a small improvement in magnification at the new MFD of the combo but not a huge change though a lot more working distance than the 105mm macro lens.
But you won't be able to work nearly as close with the 300mm PF plus tube combo as you could with the native 105mm macro. For live pseudo-macro subjects like amphibians, butterflies, dragonflies, some small snakes and the like that larger working distance is a very good thing but for pure macro magnification on static or very cooperative subjects you'd need a roughly 230 mm long bellows to get the 300mm lens to focus as close as the 105mm macro with its native 1:1 macro ratio and that's more of a studio solution and not very practical in the field. You'd also likely run into image quality problems if you tried that much extension as the lens was never designed to be focused that close.
Realistically I use tubes on telephoto lenses but then it's not a question of macro ratios but just reducing relatively long MFD to something better for the situation. I've often used a 36mm macro tube on my 600mm f/4 E-FL when working small birds from blinds or otherwise at very close distances that would otherwise be too close for that lens. I've also used tubes on the 300mm for pseudo-macro subjects like those described above, often with a 1.4x TC which doesn't shorten the MFD but does increase the effective focal length without changing MFD which is another way to increase magnification.
FWIW, the 100-400mm Z lens focuses very close to yield a 0.38x magnification ratio (roughly 1:2.6 so something 2.6 times the width and height of your sensor would completely fill the frame) which is pretty good for a lot of live subjects though not nearly a 1:1 macro lens which is great for the smaller subjects.... if you can get close to them.
But getting back to the thread title, an extension tube could definitely be used with the Z 100-400mm lens and help improve its already good close focusing distance. I don't know at which focal length the 0.38x macro ratio is defined but for the sake of discussion if you can achieve that at 400mm then adding say a 36mm tube would increase its macro ratio to roughly 0.47. If the lens is spec'd for its 0.38x at 300mm then add a 36mm tube and you'd be roughly 0.5x or 1:2 macro ratio.
The caveat posted above about focus breathing and whether the 100-400mm lens is true to focal length at MFD still applies so those numbers above are just estimates.
