Canon Extension Tube EF 25 II

This product is for use with Canon EF mount lenses and not for Sony a7 series cameras.

It is compatible , just be aware that this is to help you focus close (micro) not a telephoto use .

If the lens has a Canon EF or EF-S mount, it will work - if the lens's focal length is adaptable to the extension. If the lens has autofocus, it will drive its motor and will always work in manual mode. If the lens has a Canon FD or R mount, it will not work.

Basically you will use this so your Canon EF 70-300mm f/4-5.6L IS USM Lens will be able to focus closer than the lens's native close focusing distance of 3.94' / 1.2 m. We were not able to find information as to how much closer but we would guess that bout 2' would not be unreasonable. People with telephoto lenses by this to do exactly this, get a closer focusing distance. This can be an excellent too.

A 2X converter will double your focal length, an extension tube will reduce the minimum focusing distance to your subject.

Yes it will work with an EF-S lens on a Full Frame body. It's recommended for lenses with a focal length of 50mm and higher.

I need to qualify my last response. If you place the Extension tube between the EF 100mm and the RF adaptor (that you already have?) it might work. You say "as the lens is already adapted to the camera." Does that mean you already have an RF adaptor? My hesitation is that rear focus is so much closer on a mirrorless camera that adding an extension tube potentially creates a problem. The R series and the EOS with a mirror series are very different.

Yes, you can stack extension tubes for greater macro capability.