The Tamron 17-50mm F2.8 is a popular lens, cos its F2.8 but it dont really cost you a limb. But there are saying that this lens, depending on your luck may suffer from some front or back focusing problem.
So here to share a simple DIY to fine tune your Tammy if you got the guts and interest to DIY
Tools u need will be a tweezers and a small Philips screwdriver (those mini type)
the front of the lens is covered by a plastic ring which you need to remove using a tweezers, there is a small hole on the ring, insert your tweezers into the hole and pry up. the ring is secure in by 3 short protruding pin.
the ring once removed and you can see on of the plastic holding pin.
with the front ring removed u will see 3 metal screws, these screw hold the front glass element and lock the focusing position of the front glass. remove the 3 screw and gently remove the ring, while removing use a soft cloth to press down on the glass, so that the glass is not lifted along with the ring. (the fitting is quite tight, u dont want to lift the ring and the glass together and while lifting the glass fall out, so press the glass down and remove only the ring)
with the ring removed, u can turn the glass clock wise or anti clock to adjust the focusing position of your lens. turn abit either way, replace the front holding ring and secure the 3 screw. make sure to aline the dot on the front holding ring with the focal range indicating line (so that your hood position is correct) Test for the focusing problem and repeat the steps to find the sweet spot.