After going through two Sigma 18-250 lenses in two years, I decided to give a Tamron 18-270 f6.3 Di II VC PZD a try. Tamron promised improved AF performance with a new ultrasonic AF drive and offered a five year warranty, which was definitely something I could use after spending over $800 on a Sigma 18-250 designed to last only 50K shots. The new lens was also much smaller and lighter, which made it easier to carry it around, given that sometimes I spend hours with a camera in my hand.
Every new lens takes some time to get used to and with a Tamron 18-270 on a Canon body this time was a few weeks until my muscle memory adapted to the opposite direction in which zoom and focus rings rotated. Less obvious, but more important were the technical parameters of the lens, which took me a long while to figure out and work around one way or another.
One immediate difference with the Sigma 18-250 I noticed was the chromatic aberration (CA) pattern. While it was easy to correct CA produced by the Sigma 18-250 with the CA plug-in in GIMP, CA produced by the Tamron 18-270 took more elaborate tweaking for each image with visible CA.
One day I was shooting an event at a restaurant with very limited light and used the IR AF assist beam of an external 430EX flash mounted on the camera to focus. After reviewing the shots later that night I found most of them were front-focused quite a bit. At first I blamed how the AF system was using the IR focusing pattern, but after a series of experiments it turned out that the Tamron 18-270 was front-focusing on the wide end, approximately between 18 and 50 mm. Starting at approximately 70 mm and all the way to 270 mm, focusing worked as expected.
Once I figured out this little problem, I started to add +10 AF micro adjustment (MA) every time I had to shoot wide angle. Sometimes it presented quite a challenge or was simply impossible because the subject was moving towards me too quickly. I did my best by making a shortcut to the AF MA function in my EOS 7D and switched between +10 when shooting wide angle and disabling AF micro adjustments when shooting 70mm and above.
After a few months of using the lens, I noticed that zooming became very stiff. Upon closer look it turned out that the rubber zoom ring warped slightly and extended evenly onto the body of the lens, creating extra friction. It was inconvenient to zoom at first, but then I realized that this malfunction took care of the zoom creep problem and I just learned to live with it and I never had to touch the zoom lock switch again.
When Spring came along, I started shooting more in manual focus and learned about another deficiency of this lens - the manual focus ring didn't move smoothly, but instead moved in little jumps that can only be noticed if the focus ring was moved gently and in small amounts. This became a major problem for all shots where I needed to focus manually.
After about a year and a half auto focus started to fail more and more and I noticed that the focus ring was often stuck around the 7m mark on the focusing scale. When in this position, the lens wouldn't focus at all until I jogged the ring slightly. Given my experience with consumer grade lenses, it wasn't much of a surprise and it was time to give the five year warranty a try. This is how the lens looked a couple of days before I wrapped it up.
I sent the lens to Amplis, which is an affiliate of Tamron's in Canada, and described not only the AF problem, but also asked them to look into AF front focusing and the jerky focus ring. My first surprise was a response from Amplis that the lens was dropped and cannot be fixed under warranty. I suppose they never saw one of their lenses after about 50-60 thousand shots before. I explained that the lens wasn't ever dropped or even bumped against anything and that it was just used extensively, as a lens should be. After a few calls and emails, Amplis apologized and acknowledged that their technician made a mistake. They also told me that the AF was totally shot and needed to be replaced.
I got my lens back after a few more days, which was much quicker than a few weeks it took Sigma to fix the Sigma 18-250. My first check was the movement of the focus ring in MF, which remained as jerky as it was and may be even got a bit worse, probably because of the new parts they put in, which indicated to me that the problem was not with just my copy of the lens, although this may be just my interpretation.
My second test was focusing at a wide angle. The lens was still front-focusing and required +10 adjustment. "Oh, well", I thought to myself, but in a few days I noticed that I'm getting more out of focus shots on the long end now. After doing a series of tests I figured out that the lens needed about +10 micro adjustment across the entire zoom range now, which actually made my life easier, since I didn't need to switch AF micro adjustment on and off all the time now.
After getting back the lens, I debated with myself whether I should switch to some other lens while I still could recover some money for this one, but in the end decided to use it a bit more because at the time there was not much alternative for anyone who wants a super-zoom lens and shoots a lot, which given the build quality of these lenses does require an extended warranty. However, things may change with Sigma's introduction of a 4-year warranty on all of their products purchased after July 1st, 2013.
Tamron 18-270 is not a bad lens and allowed me to capture some great shots, but it also requires quite a bit of skill to work with and a camera that supports AF micro-adjustments. I suspect this lens will work well for somebody who only posts non-cropped images on social sites, but for those who push their camera and the lens to capture better pictures, the extra 20mm is just not worth it to justify the nagging feeling of disappointment when you see out-of-focus shots you just captured and can't help but wonder if it turned out better if the focus ring moved smoothly or if you picked the right amount of AF micro adjustment.
If you decide stick with the Tamron 18-270 for whatever reason, make sure to test AF accuracy at all major focal lengths and at those distances that you shot most of the time. Remember that most AF tests and AF micro-adjustments done at home from a couple of meters from the wall are useless. Pick a high-contrast target outside (e.g. a traffic sign), make sure your shutter speed is over 1/1000 and take a series of shots at +10, -10 and 0 AF micro-adjustment, taking at least three shots in each setting and refocusing between each shot. Delete the least focused shots while looking at them at a 100% on your camera's LCD screen, but leave those that just slightly different - the amount of AF micro-adjustment you need is between those numbers.
At the end of November in 2013 I started to notice that the AF is becoming unresponsive at times. It wasn't as bad as it was in June of 2013, but I started to miss shots and that was the end of Tamron for me. On December 6th, 2013 I picked up the new Sigma 18-250, which was much smaller than my original Sigma lens and was covered by a warranty for as long as seven years in Canada. I switched the Tamron with the Sigma right at the store and, aside pushing the zoom and focus rings in the wrong direction for a few weeks, couldn't be happier to be able to focus with confidence again.