Ok so I thought I would go out into the back garden and play around with the extension tubes for a bit. So I happily snapped away for a bit and then retreated within the house to have a look at the results.
Well once imported into lightroom it was apparent that they were quite blurry or rather soft. Why? Well in some cases the speed just was not fast enough, and in others the aperture was to wide. With the extension tubes on (only the 16mm in this case), the depth of field is very narrow so a small aperture is needed, and given that shallow depth of field every tiny hand movement can throw the focus off, therefore a fairly high speed is needed.
Therefore I headed back outside to take the same shots but with the camera set to manual with an f stop between 16-20, and speeds between 1/125 and 1/250, letting an automatic ISO of between 400-3200 compensate for the small aperture high speed combination.
The results – success – below is a shot of the empty seed head of some grass.