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Just wondering if anyone has experience using these. Is the loss in picture quality noticeable? I have canon 24-105mm F4 IS L series lens which is great for most applications but have been looking for some extra zoom for a few shots reccently and don't really want to part with 2 gorillas on a new lens.

Thanks in advance for any info/advice.

Extension tubes have no glass in them, and simply move the lens further away from the camera in order to focus closer. At large amounts of extension there is some light drop-off and some added magnification, but there's no other loss of quality.
On a zoom like the 24-105mm/4 using tubes will be confusing, as you'll find that (because of the optical design of the lens) the zoom and focus controls will dramatically affect each other. It doesn't seem like a very useful combination.

If you're looking for "extra zoom", you might instead be thinking of a teleconverter (Canon does confuse the issue by calling their TCs "Extenders"). TCs have lenses in them which magnify the image (typically by 1.4x, 2x, etc) and this has the drawbacks of limiting the quality and losing light (a 1.4x TC loses one stop, a 2x loses 2 stops).

However the Canon TCs won't fit the 24-105mm/4 - you might find a 3rd-party TC that will fit the lens without damaging the rear element of the lens, but it's not a configuration I would recommend.

Are you talking about extension tubes or teleconverters? Extension tubes are normally used for macro work and have no glass in them, so don't degrade through poor glass. TC are used to give more reach or focal length and degrade the image to some extent. It depends on the TC in question how much. I am not familiar with Canon TCs, but would assume they are better matched to your existing lens, though f4 is considered by many slow for a TC. I am sure someone who has one could be more informative.

Extension tubes just have air in them, so the 3rd party tubes with their cheap third party air... work just as well as Canon ones. But since you can get a set of 3 Kenko tubes for the cost of one Canon one... go third party. I did, and they work just fine.

That's if you want extension tubes. If you actually want more zoom, you need a teleconvertor.