The teleconverter, the wildlife photographers best friend but not for the reason most think. The teleconverter is a tool permitting the photographer to narrow the angle of view and isolate the subject better with narrower DOF. These main attributes of the teleconverter seem to get lost with the main obvious attribute, a bigger subject in the viewfinder. One of the drawbacks of the teleconverter is their sucking up of light. You getting nothing back for their hunger, you simply have less light to work with. In the case of the 1.4x, you loose one stop of light and the DOF is only 1/1.4x or 70% of that of the prime lens. The 1.4x was once supplied with the 600f4 but now you have to buy it separately. It was once my main teleconverter but for the last two years, I hardly use it. I have all three, use each when the subject requires it.