Tamron 17-50mm f/2.8 front focus

My Tamron 17-50mm f/2.8 seems to front-focus by about 2cm on my 400D (whichever AF sensor is selected). This seems to be scene-dependent, since sometimes it will focus OK, but there are certain scenes where it will simply not focus on the subject in the sensor area. Otherwise, the lens is sharp and has good colours. Now this is the second copy I've tried of this lens (both built in Japan); should I mail this one back and try a third (which I'm not to keen on as it seems a bit like lens ping-pong), or send it into Tamron for calibration under warranty? Does anyone know where the UK service centre is and have any experience with them?

Since this is your second Tamron, my strong recommendation would be to perform a focus test of your camera, preferably with a number of different lenses, the faster, the better. I had a similar problem with my 350D and, after trying the 2nd or 3rd copy of the same Tamron, I found out it was actually the camera that was front-focusing. The front-focusing was just easier to notice with the Tamron but I found 11 other lenses also showed varying degrees of front-focusing with the kit lens being the sole exception for some mysterious reason. With my camera, the front-focusing also varied considerably according to light levels being particularly bad in low light.

http://www.focustestch​art.com/chart.html

I am using the original digital rebel, i think it is the 300d and my Tamron has the same thing.

I had the camera calibrated by Canon less than a month ago. I've tested it with my Canon EF 50mm f/1.8 II, which has a much higher hit rate for the correct focus (basically it performs as well as you'd expect that lens to), so I doubt it's the body. The Nifty has a little bit of error here-and-there (some front-to-back focus, but still within the DoF), whereas when the A16 mis-focuses, it does so consistently.

I ordered a 28-70 last year and sent it back for replacement as focus was way out (confirmed by rigorous focus testing) Replacement was better but at close distances was poor. Called the service centre and arranged to send it in (you need to specify what camera you have). It was returned quickly and was spot on. Details are as below:

Service Tel: 01628 799902. Service Fax: 01628 799912 service@intro2020.co.u​k
Intro 2020, Priors Way, Maidenhead, Berks SL6 2HP

Thanks for the info. Do they refund shipping costs?

Unfortunately they don't refund shipping costs. This is the uk service centre, it didn't occur that you might not be uk based! American one appears to be the link below - you might have to google tamron and service centre to find your nearest location if you are somewhere else.
http://www.tamron.com/​about/services.asp

No, no... I am UK-based. In fact, the service centre is about an hour's drive from me, but I can't spare the time during the week at the moment.

I just altered my test rig to something more appropriate (tripod mounted camera, black and white pattern hung vertically) and this copy of the Tamron now comes out as the most accurate lens in my bag! Strange. Perhaps it was me not quite using the AF system correctly. Anyway, with the same test I discovered that it's now my Nifty that front-focuses when AF-ing from the minimum focal distance... not sure it's worth sending that one back, though!