A little help with extenders.

[papatooth]

EF 2XII Extender, Extension Tube EF 25 II, EF 1.4X Extender, what is the difference between them? I just bought a 100-400 zoom and want to reach for the stars or at least bring 200 yrs closer.

 

[dxqcanada]

An extension tube physically increases the distance between the lens and film plane ... thus changing the focal point.
This is normally used for Macro photography as it decreases the minimum focus distance.

A teleconverter uses optics to magnify the image.

So you will want the extender.

 

[usayit]

You loose 1 stop on the 1.4x and 2 stops on the 2x. Both have optics to maintain infinity focus. Both will degrade the final IQ a little.

100-400L is not ideal to be used with either. Depending on your camera, you will loose AF ability. Ideally, they are used by telephoto prime shooters for when they need a bit more focal length without the extra weight of another optic.

 

[Overread]

To clarify:
Extender - this is what Canon have labled their teleconverters as, but everyone else (shops and other manufacturers) call them teleconverters and most canon shoots still call them teleconverters too.
These essentially magnify your image and thus give you a longer focal length for your lens - the amount you get is the number of the teleconverter (1.4 or 2) times the focal length of the lens you are attaching it to. So adding a 2*teleconverter (extendor) to a 100mm lens would give you a 200mm lens.
However they are not ideal on all lenses and the 100-400mm is not going to work well with the 1.4 and would be very hard to use with the 2*TC. As USAyit said you idealy want to use them on prime lenses or very high quality zooms (eg a 70-200mm f2.8) and on zooms only the 1.4 will give acceptable image quality in most situations.

Extension tube - a different item here, this is a hollow tube that allows your lens to focus closer and thus achive a greater level of magnification than previously possible. However in doing so you lose the infinty focus of the lens. Depending on the lens and the amount of tubes you add you can have anything from only a few feet down to a few millimeters of distance that you can focus over.
Typically these are used for close up or macro work - and a common setup is a "cheapmans macro" which is a 50mm f1.8 lens with around 50mm worth of tubes; the rough maths is that adding the same length in tubes as thefocal length of the lens will give you 1:1 (true) macro magnification; of course in the real world you don't want to go adding more than around 50mm of tubes because you do lose light (image quality) as well as having a more difficult setup to use.
Note that Canon tubes are not worth getting since they arevastly overpriced for a tube with nothing in it and most people will use "Kenko AF tubes for Canon". The AF tubes are important as there are even cheaper tubes on the market, but they lack the AF contacts in teh tube construction. This means that not only can you no longer use auto focus, but you can also not control the aperture blades on the lens - this meaning that you have ot shoot wide open (biggest aperture smallest f number) which is not ideal for closeup work.

 

[Stosh]

As usayit already said, the 100-400mm is not well suited to use with extenders. You will certainly lose autofocus with the 2x, and probably lose autofocus with the 1.4x on all but the brightest of scenes and even then it will be slow.

Worse yet, the image quality goes downhill fast. I have found that I can get almost identical image quality from cropping the original image as I can by adding the extender. Cropping is certainly better from the standpoint of light because you don't lose any like you would with the extenders.

Prime focus lenses have the resolution to be magnified and still have excellent image quality at the sensor.