help with focus on Canon 5d mark iii

[Bushino]

the answer to my question is probably pitifully obvious so i acknowledge my limits 🙂

I'm having a terrible time focusing with my canon 5d mark iii.  i primarily use a 35mm 1.4L and an 85mm 1.2L, and use back button focus with ai servo mode.  i also usually manually select a single focus square. 

its a bit bizarre, but there are days where i constantly nail focus, but then there are days when i miss on 85% of my shots, but i dont think anything is changing (settings) between the shots.  the problem arises across a broad aperture range (meaning its not only if shooting at 1.2 or 1.4i don't understand what is happening, and hope someone can help.

the problem arises with both stationary and moving targets. 

i would greatly appreciate if someone could help me understand how to improve.  some questions include:

1.  is it possible there's a flaw with the camera itself? 

2.  how do i best use back button focus?  should i, for exe, hold down the focus button constantly, or only depress it long enough to focus, release, then snap?  what about for moving subjects?

3.  should i be using a different auto focus mode? 

greatly appreciate the help.

[TCampbell]

You should use "one shot" focus when your subject distance is not changing.  You should use "AI Servo" when your subject distance is changing.  

Canon doesn't actually recommend just leaving the camera in "AI Servo" mode (it seems like it would be a good catch-all focus mode, but they claim it's not.)  The issue with "AI Servo" is that the camera actually expects that the subject actually is changing distance and it's trying to evaluate that in order to use predictive focus.  When it turns out the subject is not really moving, they claim it can throw the camera focus accuracy.

When using back-button focus you can press and release the button at will.  That's actually the point of it... it was created for situations when you're tracking a subject, suddenly a distancting object enters the frame and you do not want the camera to switch focus to that other object... but then the object passes out of frame and you want to resume focus on your original subject.  So you could press-and-hold... and when the distraction shows up in frame just let go... until it's passed and then you can press-and-hold again to resume AI servo focus.

The 5D III has an amazing focus system.

Canon does have a great guide explaining how it works and how to adjust settings and tune it in great detail.  You can find it here:  http://learn.usa.canon.com/resources/articles/2012/1dx_guidebook.shtml

This same guide is titled for the 1D X but if you read the intro they explain that it's really for both the 1D X and the 5D III which have nearly identical focus systems.

There's a PDF in the lower-left area of that page that you'll want to download -- which is the 47 page guide on the focus system.

[Bushino]

Tim,

Hugely appreciated.  I'll experiment with one shot at the very least.  Very much appreciate your response.