[StephenBatey]

In theory, the marked focal length should be accurate to about 1%. Yours clearly isn't within this limit.

There are a number of factors that could cause a change in focal length when no change should occur, and as pointed out, focusing is one of them.

I haven't bothered to check, but from memory infinity for a camera lens is usually taken to mean about 1000 times the focal length; in the case of a 70mm lens if I've used the Windows calculator correctly that would be around 83 yards.

For what it's worth, I'm not a regular here and only just saw your post.