Macroless macro

I have a Sigma 70-300mm APO macro Super lens with an AML macro screw-on lens.
This lens served me very well on my F90, and does a good job as a telezoom on my D90, but it will not focus in Macro mode (even manually) with the AML lens attached.
My camera manual isn't much help, so can anyone enlighten me as to why this is so??

PS This message has taken over 20 minutes to type, as I'm on a ship off the west coast of Shetland and it's tossing about like a cork!!!

Jim

so,, kinda seems that if you move the camera to manual focus mode, decoupling the camera, that since the camera has a FMOUNT, it would work the same as your old film cameras, could see that the built in green light won't detect enough contrast to say it is in focus, but I would think that if you looked thru the viewfinder, you can focus with your eyes, or on live view, zoom in,, and focus.

if not, you might want to google on the web for more details.

Just a question, if you are using the Sigma 70-300mm APO macro Super lens and it is a macro already why are you using a screw in macro filter? Is the 1 to 2 ratio not close enough for you? Also will the focus work correctly without the closeup filter attached?

What happens when the screw on is removed..

Hi Don,
Thanks for your answer.
The lens is really a zoom with a Marco facility which can be switched on when the lens is zoomed out. It is only true macro when the AML is screwed on.
Jim