Bill_de wrote:
If I'm not mistaken the T/C brings the actual focal length to 420mm. Having it on a DX camera multiples the equivalent focal by 1.5x.
In other words, if you were shooting with a full frame camera like the D750, you would need a 630mm lenses, or lens +T/C, to get the same image. From a kayak that is pretty impressive.
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Sorry Bill but you are mistaken. No sensor is going to make the focal length of your lens longer. It will be a 420mm no matter whether the lens is on a full frame or crop sensor camera. The only thing that changes is the field of view not the reach.