When 300mm does not equal 300mm?

[ahab1372]

The focal lengths are always approximately. On top of that, some if not most lenses change the focal length when you focus on a different distance. Focusing on a far away subject makes some lenses shorter, some longer, depending on the design. Exceptions I think are most cinema lenses, which is one of the reasons why they are so expensive. So if you repeat your experiment focusing on a subject 50 or more feet away, the results might be different. I would love to hear if you actually do repeat it.

Arnt