On Saturday I referred to a possible front-focus issue with the Tamron SP 150-600/5-6.3 Di VC USD megazoom on my Canon 6D. I did some testing and found out that the focus problem is consistent when using the small focus points, whereas the central point is spot on. I even compared the results to the Canon 300/4 but the fixed lens performed perfectly. As a side note, when I compared image quality between the zoom at 300mm (where it’s f5.6) and the fixed lens, the zoom lost only by a small margin so that’s nice.
The problem disappears when the subject is close and is less evident on the short end of the lens when the subject is far.
I contacted Cyberphoto (where I bought the lens) and asked if this behaviour is normal or if there’s a problem with the lens. They spoke with Tamron who said that this behaviour can be expected because the small focus points are not able to cope with the small aperture. The Tamron is f6.3 at the long end but transmits the aperture as f5.6 to the camera, in order to make the camera use AF, but my experience says that the camera is not completely fooled by it. According to Canon, the central focus point (which is the only cross type point) on the 6D will AF at f8 while the small points only work up to f5.6. So on paper, the focus should be ok with the small points until the lens is zoomed beyond f5.6 (=beyond 400mm), but in practice this doesn’t seem to be happening.
My test results kind of confirm what the Tamron rep said. If the problem is reduced when using a short focal length, then it could well be thanks to the aperture being bigger at the shorter focal lengths. Or it could also be the bigger DOF that compensates for the front focus.
I knew already when I got the 6D that the focus system is the camera’s single biggest failure but I haven’t really had a reason to complain about it until now. But like I said, AF is accurate when using the central focusing point, so I will just have to deal with it. 20 MP is nice to have, there’s room to crop for the best composition!
If any potential Tamron megazoom buyers are reading this and they intend to use the lens on a Canon 6D, then I think it’s good to have this limitation in mind when deciding on which lens to get! Maybe do some testing before buying to find out if your camera/lens combo suffers from the same shortcoming.
Today’s picture was taken in the morning with the Tamron @ 600mm, using the central focus point – focus is spot on! And then just crop until happy.