leftj Loc: Texas

Great experiment. I too have a Tamron 18-400mm. I am going to conduct the same test comparing it to my Canon 24-70mm L.

sippyjug104 Loc: Missouri

imagemeister gave you what I believe to be the correct answer. The zoom and its focal distance are related to each other. This would have been more apparent if the comparison were against two prime lenses with one being a 400mm. I doubt that standing so close in the room that you would even be able to focus the 400mm on the keys.

leftj Loc: Texas

And yet, he did so why even make that statement?

Gene51 Loc: Yonkers, NY, now in LSD (LowerSlowerDelaware)

Almost. Focus breathing is endemic to nearly all internal focus lenses - those lenses that move internal elements, decreasing the focal length to minimize the minimum focus distance and whose front elements don't rotate or change physical length when you focus. Regular, non-internal focus lenses simply move the glass further away getting you greater magnification, usually at the expense of an FStop or two.

Older macro lenses use a simple design that moves the group of glass away from the sensor, and these are great for focus stacking and other applications where breathing would present a problem. On the other hand, many new lenses are internal focus, including primes, and they all suffer from some focus breathing - some slight, some are absolutely terrible, like the Nikon 70-200mm F2.8 VR, and the 28-300mm, both of which are effectively 135mm lenses when zoomed out to their max and focused at their minimum focus distance.

Here is one post regarding the otherwise very fine Tamron 90mm F2.8 Macro. Not a great lens if you want to do focus stacking by adjusting focus. For very close work it is easier and more reliable to use a focus rail, and move the entire camera/lens assembly to achieve an accurate stack with no breathing.

Mustanger Loc: Grants Pass, Oregon USA

thanks op & everyone. I never even heard of this phenomenon before. Very interesting reading the links I have a small understanding of what is going on and need to digest it a bit. What a vast hobby we have...love the challenge!