Can't focus/recompose with my 5D MkII

[photoflurry]

I've had my 5D MkII for about 2 1/2 years now, and a few months ago it inexplicably quit locking focus for me. Nothing happened with the camera, have been using it like always - no damage, etc. Just started realizing that when I focused on an area, then re-composed, it wasn't holding the original focus point. So I turned on the focal point setting so I could see during and after a shot where the focus had been. Any time I focus/recomposed, the focus was never on the original point of focus. It happens with all of my lenses, at any camera setting. 

I've read my manual thoroughly and can't find any settings that may be causing this, so I'm really hoping it isn't a camera malfunction. Thoughts?

Also, I am no longer able to set my ISO at 100. The lowest I can set it is 100. 

With both of these issues, I welcome and greatly appreciate any thoughts, suggestions, or similar experiences.

[Skirball]

Which focus mode are you shooting in?  For that matter, which shooting mode are you using.  Some of the automated shooting modes use AI servo, which will select a new focus point at the time the shot is taken.

[photoflurry]

Using autofocus exclusively. I don't use any of the totally automatic shooting modes. I sometimes use the Programmed Auto mode, and do a lot of aperture priority and manual. It occurs in every shooting mode. I'm not using AI Servo. 

[Skirball]

You're using one-shot (not AI Focus)?

[photoflurry]

yes - one shot, not AI Focus

[Skirball]

Well, if that's the case, it's not a setting that I know of.  Barring that, my first suspect would be user error.  But if you've been doing focus/recompose for 2+ years without issue and it suddenly won't do it, then I guess I'd wonder about a device malfunction.  Maybe the button won't hold a half-push anymore?  It wouldn't hurt to do a factory reset, though I don't expect that to do anything.

I don't understand your issue with the ISO.  You can you can't set to ISO 100, but the lowest you can go is ISO 100?

[photoflurry]

Yes, I keep wishing it's just a setting, or something I'm doing wrong, but it's looking more and more like it has to be some malfunction. Unfortunately, I can't be without my camera for as long as it would take to send it in and have it evaluated. 

Maybe I'll try your idea on the factory reset, but I agree - not likely it will fix it.

As for the ISO thing, it was solved in another online Canon users forum. Turns out it was the highlight tone priority setting - apparently I had somehow inadvertently turned it to "enable" rather than "disable," and this setting doesn't allow for any ISO under 200. So at least THAT was an easy fix 🐵

[Skirball]

Well, maybe you'll get lucky.  If some of the custom settings were changed, maybe something was inadvertantly changed for focus as well.  There are quite a few custom settings for focus in there, give it a reset and see what happens.

[hsbn]

Have try to check the setting where AUTO FOCUS is removed from the Shutter button? Do you use back button focusing?

[photoflurry]

I don't know what that is - can you give me a bit more information about that setting?  Thanks!