Back focus with 2x extender in low light

I'd appreciate your views on a problem experienced yesterday.

Trying a Kenko 2x extender with a 70-200 2.8 lens around park today it was great in nice light but back focused in shaded areas.

Was using centre point only without recomposing so know exactly where it should have focused. For example - one shot I focused on back of a squirrels head with it facing away from me at an angle. The image shows focus on the tail a few inches back. Is this typical when there is little contrast?

Lens is Sigma 70-200 2.8 APO DG HSM with Kenko Teleplus Pro 300 2x extender.

Have you got the assist (expansion) points enabled? It may be that there was better contrast under one of these and the camera chose a point just outside the marked centre point region.

Bob

Expansion points currently disabled. Manual focus point selected.

I'm now guessing the lens with extender needs different ma adjustment than the lens by itself?

I would not be surprised. even without the tc, the lens may need different MA figures at different focal lengths. The TC may add another variable. And then different lighting conditions can also need different MA values

Also remember that the AF point in the viewfinder is only a guide. The actual AF point is generally a quite a bit bigger, and may not be centered fully on the indication. The AF system will generally lock onto the CLOSEST point with highest contrast, in the correct direction, that is captured by the AF point.

Alan

Thanks for the replies

Sounds like I just need to adjust technique to compensate in low light