i have been wanting to buy a 70-300mm lens for some time, but the money has been spent elsewhere, so as a cheaper alternative, i was going to put a 25mm ex tube on my 40-150 lens, but will i be able to af?
Have a look at the EX-25 product page
where it states that "Manual focusing use is recommended.
". Also note that all the extension tube will do is improve your macro capability. Using it you will not
be able to focus on distant objects. For that you would need something like the EC-14 Tele Converter