I find it is always much more difficult to get sharper images with my longer telephoto lenses, particularly at slower shutter speeds.
With this 2 times teleconverter, the lens being used looses two stops of useful light. Sometimes I find that I am trying to capture images at f/4 which gives an effective aperture with the teleconverter of f/8, hence the shutter speed becomes the equivalent of two stops slower. I am finding that even at higher ISO settings my shutter speed can get down to around 1/80 second which is very difficult with a lens and teleconverter combination of equivalent focal length to 600mm.
In order to help achieve stability of the lens and camera combination at slower shutter speeds I have made a minor modification to the lens mounting with the help of a Wimberley P40 lens plate and a few generally available items of hardware as shown in the following image.
The lens effectively rests on both the mount and also the arrangement of the nut and bolt (1/4″-20) and the bolt can be adjusted to the correct height to support the lens. A small rubber disk is used against the lens to prevent the bolt marking the surface of the lens.
The above arrangement seems to help significantly in achieving sharper images at slower shutter speeds. The two images below have been captured using this arrangement with the TC-20E III teleconverter.
The above is a beautiful water dragon posing in the sun light. The above image has been captured at 1/80 second at 600mm.
Both these images of the water dragon and the rainbow lorikeet have been captured with the Nikon D700 camera and 300mm f/4 AF-S lens together with the TC-20E III teleconverter.
The beautifully coloured Rainbow Lorikeet feeding on nectar. This image was captured at 1/250 second at 600mm.
I also use a really solid Gitzo monopod (GM5561T), it is a six section monopod, hence it folds up quite small, but it is extremely stable for all my long lens photography, particularly with the teleconverter.
The equipment I currently use for my photography is as follows: