I have bought many…yes MANY Tamron SP 17-50mm F2.8 lens. Most are in Canon and Nikon mount, a few in Sony/KM mount.
Every unit I tested them myself for focus accuracy. From my previous experience, I found this lens is seriously having QC issue with focus accuracy. My most recent experience testing 6 units of this lens in Nikon mount that I ordered, 2 of them are having front focus problem. I have spent more than 2 hours in testing them, comparing against my own copy that is sharp with focusing.
What is front focus?
Front focus of a lens is when you focus on subject A, in the photo captured, the sharp focus patch is a few cm or even meters in front of subject A. Back focus simply means in actual photo the sharp focus path is a few cm or meters behind subject A.
When testing for front / back focus issue of a lens, one need to test with far subject as well as near subject. This is to make sure that the focus accuracy is not just for close-up shot but problematic on far/infinity subjects.
I use a few methods in testing focus. I find the focus test chart in the internet is not entirely accurate, especially if your DSLR focus sensor is not exactly in the middle or center of your viewfinder, then you might be in trouble. So, I prefer to use top to bottom or depth of field kind of testing technique. With this I can be sure that what I focus doesn’t get affected by the variation of focus sensor.
I will share with photos in future my testing technique.