This image was taken with my 28-70mm lens set at 70mm. The branches in the foreground were about 6 feet away. A quick look at a Depth of Field chart would show that it is impossible to get both the foreground branches and the trees in the background clearly in focus.
However, even if the background were in focus, the fog would make the trees appear blurry. In this case, the best proceedure is to do as I did here, stop the lens down, I used f22, focus just behind the branches, I used about 10 feet, and let the background be out of focus. The branhes appear very sharp and the background appears naturally blurred.