LesWurse wrote:
I'm slowly learning to use my newish Canon 7DMII with a Tamron 18-270mm zoom lens. On several occasions I found my AF shots out of focus. Only recently I spotted a message in my view finder saying "Turn on autofocus switch". Up until then I just made sure the lens switch was set to AF. Now I'm thinking I also have to press the "AF-On" button on the top right of the camera body when I am preparing to shoot. Yes? No?
No, in all cases Canon EOS DSLRs and EF/EF-S lenses... the only switch to turn AF on and off is on the lens itself. The "AF-On" switch on the camera body
activates AF (as well as metering and, if the lens has it, image stabilization). It serves the same purpose as a half-press of the shutter release button (unless you've set up Back Button Focusing, described below, in which case
only the AF-On button activates AF.)
The AF of your lens is
probably working (see note at the end of this rather long response
). The occasional shot where you have missed focus... Are they mostly fast shooting and/or quickly moving subjects?
The Tamron 18-270mm uses their "PZD" focus drive system. I've seen this described as a small ultrasonic motor that drives a gear set that focuses the lens. That sounds suspiciously like Canon's slower focusing (and cheaper) "micro motor" lenses. If subjects are moving, or even if you shoot too quickly, that may be cause of missed focus. I.e., the lens' focusing system just isn't fast enough!
Canon USM or "ring-type ultrasonic" autofocus lenses are the fastest to acquire focus and the best at tracking movement/maintaining focus. I shoot fast-action sports a lot (and other things less frequently) with a pair of 7DII and make a point of using USM lenses. All mine are Canon, but AFAIK Sigma "HSM" and Tamron "USD" have similar design and performance. (Note: There are
a few specialized USM lenses that are not as fast focusing. Macro lenses, for example. Also, ultra large aperture lenses such as the 50mm f/1.2L and 85mm f/1.2L. All these use "long throw" focus designed to emphasize accuracy over speed, because the potentially very shallow depth of field that close macro distances and large apertures often calls for extra precise focus.)
It also probably doesn't help that the Tamron 18-270mm is an f/3.5-6.3 lens. A lens with larger constant aperture such as Canon's 70-200mm f/4 IS USM would provide more light for the AF to work with and make for better AF performance. An f/2.8 or faster lens would be even better. Heck, even the Canon EF 100-400mm L IS USM II that I use is 2/3 stop faster at f/5 up to 312mm (and a third stop faster at f/5.6 after that). That's actually the
slowest focusing lens I use for sports (tho it's fast enough that I rarely see any missed focus images with it). 500mm f/4, 300mm f/4, 300mm f/2.8, 70-200mm f/4 and 70-200mm f/2.8 I use a lot to shoot sports and active wildlife are all faster focusing. So are some other lenses I use less frequently for sports (10-22mm, 24-70/2.8, 135/2. 85mm f/1.8, 50mm f/1.4). But ALL of those are Canon USM... no STM and
especially no micro motor (except for one macro lens that I don't use for sports: Tamron 60mm f/2).
It also depends upon how you set up your camera's AF. For sports/action you need to be using AI Servo. One Shot is for stationary subjects only. It focuses, then stops. If the subject moves and distance between you and it changes, focus will be off. NEVER use AI Focus.... it's not really a focus mode at all. It's automation where the camera is supposed to detect whether or not the subject is moving, then select the correct focusing mode. Sometimes it works, sometimes it doesn't.
For sports/action, a lot of people like to use Back Button Focusing. This separates the focus function from the shutter release button. By default your camera is set up so that you can either half press the shutter release or press the AF-On button with your thumb, to activate AF. When it's set to AI Servo, focus will run continuously as long as you maintain pressure on either button, constantly updating focus on moving subjects. It will generally work with stationary subjects too, but can be problematic if you use "focus and recompose" and some other techniques with stationary subjects. When you do focus & recompose, BBF is necessary so that you can stop AF and prevent it from mis-focusing.
If you want to try BBF, it's easy to set up on 7DII. Just go to the button customization screen in the menu and
unassign AF from the shutter release button (set it to "Meter", which allows it to start metering and IS, but not AF). That's all there is to it. The AF-On button is already set up to start AF when you press it and stop AF when you release it.
It also makes a difference which of the camera's focus patterns you use. The most accurate is to use Single Point/Manually Selected... but that requires you to keep the chosen AF point right on the subject where you want the camera and lens to focus. So it's more work for you. The two Expansion modes that the 7DII offers can be useful at times when it's difficult to keep a single point on a subject. Those start with a single point that your select, but allow the camera to switch to using one of the adjacent points if necessary, if you fail to keep the first AF point right on the subject (4-point Expansion allows use of the points to right and left, above and below the selected one. 8-point does the same, plus allows the four diagonal points, so all points surrounding the selected one can be used). The two Zone Focus patterns also can be used, in certain situations. Zone (15 points) and Wide Zone (25 points) are best used in situations where subject is not close to other, possibly distracting elements... such as a bird flying in a clear blue sky or in front of distant clouds with little detail. Both the Zone modes enable the camera to start with any of the enabled AF points and switch freely among them as it sees fit. (They are sort of like reduced size All Points/Auto Selection, which leaves everything up to the camera.) Finally, Spot Focus mode is a single point, but it's smaller than usual for high precision uses.... but it's slower so I don't recommend it for rapidly moving subjects. I would use it to focus on a bird sitting in a tangle of branches, for example... but not for the same bird in flight.
There are some other tweaks you can do to the AF system. For example I have 7DII's set to prioritize focus accuracy versus shutter release. I don't have any use for missed focus shots, and would rather give the camera time to achieve focus even if a slight delay in shutter release occasionally costs me a shot. I also usually set the camera to
slow how quickly focus jumps from one object to another, in case I don't keep the AF point right on the subject or the subject passes behind some obstruction momentarily (however, when shooting subjects that change direction erratically, I may dial this back up faster... particularly if there are no obstructions to deal with).
Especially with larger aperture lenses and closer distances, it also can help to stop the lens down a bit from wide open to increase depth of field. That makes focus accuracy a little less critical (plus some lenses are sharper stopped down, than they are wide open). The lens remains wide open during focusing, but stops down very momentarily during the exposure.
In the end, there are a lot of factors involved in autofocus: the lens, the camera and the photographer themselves being the primary ones that are in your control. Subject detail and contrast, ambient lighting conditions, atmospheric clarity are others that are largely out of your control. 7DII itself has an extremely capable, high performance AF system... but you need to choose lenses carefully, set up the camera appropriately, and practice, practice, practice!
Personally with my 7DIIs I only see 1 or 2 percent of my shots miss focus on average, even shooting fast sports action. In other words, I get 98 to 99% of shots focused well enough that they're usable for prints and other purposes. I use Back Button Focus all the time (have done so for 15+ years). That allows me to use AI Servo as my default with both moving and stationary subjects (only occasionally use One Shot or Live View for certain situations). Further, I use Single Point much of the time even tho it's extra work for me. Occasionally I use Expansion Points, rarely Zone and
never All Points/Auto. Though generally only with stationary subjects, I also sometimes use Spot Focus and find it quite useful.
One other thing I'd note.... 7DII doesn't have a whole lot of built in automation, such as you can find on some of the more entry-level, less pro-oriented models. But it does have the Green Box "Super Auto" mode. I NEVER use that because it forces camera to use AI Focus and All Points/Auto. (It also forces a lot of other things, such as only allowing you to shoot JPEGs, automating the built-in flash and more.)
Finally, I don't know why your camera would give you a warning to "turn on AF". If the AF is already turned on at the lens (which is the only place you can turn it off or on, with ANY Canon camera), it may be a minor lens compatibility issue... those can happen with third party lenses, in particular. It also may indicate that the switch on the lens isn't working. I'd just turn it on and off a dozen times or so, since those types of switches are often designed to be self-cleaning (maybe there's some oil or oxidization in the switch).
It also may help if you clean the lens' electronic contacts.... Which is quite easily done. I recommend using a couple drops of isopropyl alcohol (cheap "rubbing" alcohol works fine) to dampen a clean cloth, which you then use to carefully wipe the contacts on the rear of the lens. Those are usually gold-plated to prevent oxidization... So don't use anything abrasive that might scratch and damage the plating.
If the problem continues, you might contact Tamron to see if that's a known issue and a fix for it.
Hope this helps!