DreDaze wrote in post #17000498
why? those gears and shafts would be operating the same why they are when you zoom it...
Think about a lever, where a small force on one side can make you move a very heavy stone on the other side.
When you zoom, the linear rotation gets translated into a number of different movents forward and/or backwards inside the lens. And these movements need not be constant with relation to the turn angle of the zoom ring. Not only that - in some situations, the movement forward/backward inside the lens may even switch direction.
This means that when you zoom using the zoom ring, there is a well controlled force used to move the different groups and barrels inside the lens. When you instead push/pull the outside of the lens, you can have dead-lock situations where push or pull is stuck at a local maxima where it isn't clear if the rotation should be clockwise or counter-clockwise.
If you have a lens with zoom creep, it's common that that creep only relates to a limited part of the zoom range. The lens weight is the same all the time, but it may be only a limited part of the zoom range where the force is enough to create a rotation. And in some parts of the zoom range the lens can get into a deadlock.
You have probably seen some lenses where you zoom and the lens becomes longer and longer and then suddenly changes direction and for a little while becomes shorter. These same switching of direction can also happen with a number of internal groups. And any such switching of direction represent a point where the forces on the internal mechanism with escalate towards infinity if you try to push/pull instead of rotate.
So while the 100-400 is a push-pull lens, not all lenses can support push-pull just because there are parts of the zoom range where a element group either slows down greatly or even switches direction.
i'm not sure about the tamron, but i know there are many sigma 50/150-500mm owners that use theirs as push pulls with non repercussions
It's all a question of how the different parts inside the lens moves during the zoom operation. What works for one lens gives zero proof what works with a different lens - whatever manufacturer, model or focal length.
Another thing is that for some lenses both the zoom and focus operation results in changes to the total length, which means that push-pull will try to affect both mechanisms - both trying to change them and inducing wear.
Below is some examples of the cam slots inside lenses, and the changes in angle of these slots decides the "lever" effect, i.e. difference in force needed when affecting them through rotation or through push-pull, or if there are even parts of the zoom range where the push-pull will result in a deadlock: