It is important to understand that an "F stop" is ascertained by calculating the ratio between the diameter of the aperture at the back of the lens, to the length of the tube to the front element. By adding an extender ( 1.4x or 2.0x ) you lengthen the tube and the ratio changes to a greater number.
The 1.4x will increase that radio by one unit F stop. The 2.0x will increase it by two F stops.
Any lens which goes beyond f5.6 will function, but lose the auto focus functionality. You will also start to lose image quality with a 2.0x extender....and yet the 1.4x maintains image quality....This is a huge subject and impossible to cram into a few short paragraphs......Read up about it all over the Internet. Very interesting and it will improve your results...
Generally a rule-of-thumb is NOT to use extenders on zoom lenses, and just stick to using them on prime lenses.
Hope this helps
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