I have a canon EF-S 55-250 mm zoom lens. The canon web site says it is 88-400 mm full frame equivalent for the APS-C sensor. I have a recently purchase Tamron 18-270 mm Di II PSD zoom lens. The tamron web site says it is a 28-419 mm equivalent. I have a Canon 20D. When I use the Canon EF-S lens at 250 mm, the image is larger than the same picture taken at the same distance with the Tamron Di-II set at 270 mm. Why would this be the case? Is there a problem with the Tamron?
I've just been searching the web because I've got exactly the same problem. I'm using the Canon 600D with which I got the 55-250 as a kit lens. I just purchased the same Tamron and was surprised to see that there's a significant difference in the full zoom magnification with the Canon lens giving a larger image at 250mm than the Tamron at 270mm. Definitely not what I was expecting. Other reviews of the Tamron lens that I've seen clearly show the Tamron lens able to zoom in further than the Canon. If your lens is faulty then mine must be too, but you'd have to suspect something else is going in here which I don't understand.