Similar to other 24-70mm lenses, the length of the lens extends when the focal length is changed. At the wide end @ 24mm, the lens is at its shortest length. As you zoom in, the length of the lens increases, reaching its longest length at 70mm. The Nikon 24-70mm behaves completely differently – its shortest length is at 50mm, while zooming out to 24mm extends the lens quite a bit. Take a look at both lenses at 70mm:
Read more: http://photographylife.com/reviews/tamron-24-70mm-f2-8/2#ixzz2yCT5RznJ