Spurred on by the Megapixel thread I was testing my 28-300mm. In the end I put a ruler left right on a table and photographed it at 300mm . Width was 18 inches. I then did he same with my 300mm f4 prime ...width was 12 inches.
Now as far as I know primes don't breath so that means at 300mm and 10 ft my 300mm is 200mm !!!!
The implication is that if I change to another lens I don't need 300mm 200 will do ..mainly used for close ups of objects (flowers hands etc) at about 10 ft.
I wonder if the 24-120 breathes much as I never got on with that lens at 120mm?