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Autofocus Points manually selected, but incorrect?


I've been shooting with my T4i and 50mm 1.8 for almost a year now, and recently, it's autofocus is on the fritz. I manually select an autofocus point, but for some reason, it autofocuses on somewhere OTHER than the point I have selected.


Is this an issue with the body or with the lens?



Are you saying it's "missing" the focus... or are you saying it's actually using a different focus point.  Those would be two different issues.


If you open an image shot by the camera using the Canon Digital Photo Professional (it came with the software included with the camera), open the image, and then right-click to expose the pop-up menu.  There is option to show the focus points.  This will cause DPP to put an overlay on the image showing the familiar focus point pattern, but it will highlight the specific focus point (or points as the case may be) which the camera actually used.


This would help you determine if the camera is actually using the point you asked it to use.


It's entirely possible it's using the point you've asked it to use, but the shot is still coming out blurred - and there can be many reasons for this.


If the subject was moving and focus distance was changing, and if you were using the default "One Shot" drive mode, then the camera will focus until it gets a focus lock and then it will STOP focusing... even though the subject may still be moving.  In essence the camera is focused at the point where the subject used to be at time of focus.  So you'd want to make sure you are doing a test on a non-moving subject.


If you use AI Servo mode (normally used for action photography), then the camera WILL take the shot when you fully press the shutter button -- even if it did not have time to focus.  That mode take the photo WHEN you tell it to... no exceptions.  It's for photographers who want to capture the "decisive moment" and don't want the camera to lag at all.  You have to half-press the shutter to pre-focus before fully pressing the shutter if you want to ensure the sujbect is focused AND in that mode... the focus system remains active even after it locks focus (it keeps re-tweaking focus so that a moving subject will be continuously re-focused.)


And then you can get blur caused by camera or subject movement -- so it looks blurred but not for lack of focus.


There is also the possibility that the camera is missing focus... and testing needs to be done to determine if this is the case (you'd find and print a focus chart and do some testing... before going down that path, let's see if we can isolate another reason for your out-of-focus images.)  I'm somewhat fussy about how focus testing is performed because I want to eliminate any possibility of operator error to ensure that the only reason for a missed focus would have to be the equipment.

Tim Campbell
5D III, 5D IV, 60Da