Originally posted by audiophil But in my case, two different lenses set at 200mm, on the same body, gives me different results.
That is strange. I have the Takumar A 70-200 and the Pentax FA 18-250, which is nearly identical to the Tamron 18-250, but I haven't compared their coverage. I will do that right now. [I go outside and test the lenses, also a Pentax FA 100-300 and a Super-Takumar 200/4.]
I could not test at infinity focus because I can not
see infinity in this forest, but looking at 25m and 50m, all these lenses at 200mm seem to have the same field of view (FOV). If your Tamron can not get a smaller FOV than your old Takumar, it is possible that the Tamron isn't extending all the way out to 250mm.
Lens specifications say the FOV at 200mm should be 8 degrees, and at 250mm should be 6.5 degrees. If you are seeing a 1.5x difference, or about 12 degrees, that means the Tamron is only extending out to 135mm. A 300mm lens has a FOV of 5 degrees. I don't know if there was ever a Takumar zoom that went out to 300mm; I don't see any in the
lens database. Perhaps your Tamron is faulty or damaged. It should certainly reach a smaller FOV than your Takumar.
Bon chance!