Extension tubes are to enable the lens to focus closer but often preclude distance focusing. Tele converters magnify the image so with a 2x converter your 300mm lens becomes a 600mm lens but at the cost of two stops of light and probably the ability to auto focus. Converters also degrade the quality of the main lens to a certain extent, the 2x more than the 1.4, so stacking converters is generally not recommended. If you are getting camera shake, up the shutter speed increasing the ISO if necessary. The longer the focal length of the lens the more critical careful focusing is. Hope this helps.
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