amfoto1 Loc: San Jose, Calif. USA

Not necessarily. It may not change as much as you might think. The reason is...

In addition to what other responses have pointed out (that focal length changes depending upon the distance being focused) focal length also is rarely exactly what the lens is labelled. There is some "fudge factor" allowed. If I recall correctly it's about 10%. For example, this would mean that an 18-270mm lens might actually be 19.8-243mm and still be labelled "18-270mm". This "error allowance" is as determined by certification groups such as JCII, etc. Usually manufacturers will take advantage of it by claiming wide is wider than it actually is and that telephoto is more powerful than it actually is, as in the above example.

I don't know that this is the case with that Tamron lens. Unless the manufacturer discloses the info in some way, precise optical measurements would need to be done to determine the true focal lengths of any lens.

Canon's lens patents typically show "real" focal lengths, which are interesting to compare with how the lenses end up being labelled. For example, the patent for the RF 100-400mm shows it is actually a 119.97-360.96mm focal length zoom. Note that this is an error factor of 10% at the longer end of the zoom range, but more like 20% at the shortest! So maybe the 10% fudge factor I mentioned above is no longer used. The patent for the RF 50mm f/1.8 is much closer to reality, which is 48.7mm (2.6% fudge factor). Incidentally, this also applies to the lens aperture. That 50mm's true max aperture is f/1.85 rather than the f/1.8 it's labelled. "Close enough", for all practical purposes.

Finally, I think internal focusing lenses tend to change focal length at close focus distances more dramatically than lenses that aren't internal focusing. That Tamron is not internal zooming, but it is internal focusing. An extreme example of the effect of IF design is the Canon EF 100mm f/2.8 USM Macro lens. While it is close to 100mm at infinity, because it's an internal focusing lens by the time it's focused to minimum distance for full 1:1 magnification (around 11" from film/sensor plane of the camera), reportedly it's true focal length has been reduced to around 70mm. This isn't a problem or an anomaly... it's just the way internal focusing lenses work. With this macro lens it is more extreme than most because the lens focuses so much closer to be able to render 1:1 magnification. That's almost 4X higher magnification than what's claimed for that Tamron zoom.

profbowman Loc: Harrisonburg, VA, USA

This is interesting data. Can you please share with us where we might find this on the internet? Or are these your own measurements? --Richard