I have the Tamron 18-400, had the 16-300 sent it back, vowed not to buy another Tamron, but decided to get the 18-400 after reading reviews. Have had good and poor results, find the 18-50mm range not very sharp for landscape type images, but OK for closer work. Have a problem with the lens not focusing @ 400mm every now and then, have to zoom back to around 100mm to get focus again, AF or M does not matter. Sent it to Tamron for repair, but lens was sent back with a note that there was nothing wrong with it. Also have the Canon 70-300, which makes for me anyway a better walk around lens.

First thing to do is check whether you need to tune or not. I am familiar with Nikon dslrs, but not others, so this is what I have done for my 4 Nikon bodies, 2 DX, 2 FX. Live View uses a different method of focusing than AF mode (contrast (slow) vs phase (faster)). Live View shows the best the camera and lens can do (if you use good technique). So, I take images of a resolution chart in LV and AF. If they match, congratulations, you have confirmed AF is fine. If the AF image is softer, then you might want to tune.

Next thing to do is determine is whether your AF system is focusing in front of or behind the intended focus point. This can be done by shooting a page of text, focusing in the center of the page, like a newspaper page, at 30 to 45 degrees, i.e. front (closer to lens) of the page lower than the camera lens and back of the page higher than the camera lens. Look at the image to see if the text is in focus in front of or behind the focus point.

My dslrs have a AF fine-tune feature where you can move the AF point forward or backward. Nikon's only have one adjustment per lens, but I thought I heard Canons have two adjustment points per lens. In any case read your manual. For Nikon, go to the AF fine tune menu item and you will see a -20 to +20 adjustment range. By default it's at 0. A negative number moves the focus point towards the lens and a + value move the focus point away from the lens. I usually move 5 units at a time. It's trial and error, but with time and patience you will walk it in. So AF fine-tune is in the camera body and only has one (2 Canon) adjustment points per lens. For modern Sigma and Tamron lenses which use the usb dock or tap-in, you can tune at multiple points in the lens itself. For the 18-140mm in this post, there are 24 different points to tune, 3 different distances and 8 different zoom factors.

Important. When taking test shots use a sturdy tripod, remote release, aperture wide open (shallow DOF), turn off VR (if any), AF mode still (not continuous), single focus point, always take at least 3 shots. Before taking every shot, manually move the focus ring off a bit so you are forcing the system to re-focus before every shot. AF is a random process. It does not go back to the exact same point every time, which is why you need to take at least 3 shots and compare them.

Mike, I went to Africa in June with my 70D and only this lens. I know I took a chance with just one lens but I didn't want to lug others or even change lenses in the middle of a safari.

I tweaked the focusing at each of the lengths using the Tamron Tap-In Console which worked very well. I was very happy to have the 18-400 length whether I was in Cape Town shooting panoramic of Table Mountain or getting up close to animals in the bush. Look up the last few posts by me to see some of the pictures I took with the lens. I was very happy with it's performance. One note, I am not a professional photography so I take pictures just to make me happy.

Mike, I went to Africa in June with my 70D and only this lens. I know I took a chance with just one lens but I didn't want to lug others or even change lenses in the middle of a safari.

I tweaked the focusing at each of the lengths using the Tamron Tap-In Console which worked very well. I was very happy to have the 18-400 length whether I was in Cape Town shooting panoramic of Table Mountain or getting up close to animals in the bush. Look up the last few posts by me to see some of the pictures I took with the lens. I was very happy with it's performance. One note, I am not a professional photography so I take pictures just to make me happy.

Serj - the 16-300 is a different animal entirely. That one is a VC PZD. This one is a VC HLD. So, it uses an entirely different AF mechanism than the earlier Tamron - apart from knocking out the lower 2mm and adding 100mm to the top. Pricewise - they're approximately the same - around $650 … but, to be honest, I have seen the earlier model at around $495 - which is a $155 difference!!! … What I'm trying to find out, here - is whether that difference makes sense to invest - which it might - if it's significantly better … ok?