Can someone explain to a nublet how exactly the AF points on a 5D MarkII Work

I have been doing some research lately due to the fact that I've started noticing weird focusing behavior while using the outer points.

What I've gathered.

Center Point - cross type, kicks into a better focus at 2.8? What the hell does that mean?

Outer Points - Have trouble focusing in low light, but then I read somethign about F 5.6? Again what the hell?

I did a test yesterday shooting at 2.8 on a 24-70l at 70mm.

Shot a birdfeeder focusing handheld at 1/1000 at a distance of about 25 feet.

Center focus was perfect.

Left most AF point was no where near the target and ended up focusing in a random place.

Right most AF point - blurry not in focus.

then I did a license plate at 70mm, 2.8 at about 2 feet.

Focused on a small screw.

Center - perfect
Left Most AF point - perfect
Right Mopst AF point - perfect


I dunno, I don't get it, can someone help explain to me why at distances the outer points become unreliable? Is it an issue with focus shift? My lens? The camera itself?

I'd suggest you read 5D MkII / 50D 'White Paper'. It should give a good run down of how the AF works on your camera. Try this:

http://learn.usa.canon​.com …11/eos_white_pa​pers.shtml

Regarding your focus test, I can imagine the number plate shots but it's impossible to comment on your birdfeeder ones without seeing the actual non-cropped shots.

You might want to read the manual also, especially about selecting focus points and depth of field (when to use a small versus large aperture.

Sounds more like user error. Are you saying that when you put the left, or right, focus points on the bird feeder they wouldn't focus?

crn3371 wrote in post #13851237
Sounds more like user error. Are you saying that when you put the left, or right, focus points on the bird feeder they wouldn't focus?

Yes. The bird feeder though was a good 25 feet away though, again shooting at 2.8 70mm.

It's almost like the left and right focus points take on a mind of their own when I try and use them past a range of 10 feet.

When a lens with a max aperture of f/2.8 or faster is mounted the focus point performs more accurately than the others.

With a lens having a max aperture slower than f/5.6 the AF will not work (unless the lens lies to the body).

Here is an example, I focused on her right eye in this pic with the left most Af point and it's blurry at her eye and sharp down in the bottom left of the pic?

http://farm8.staticfli​ckr.com …34570527_ca53ba​14dd_o.jpg

5Dii autofocus is just crap. It's something everyone that has a 5Dii just accepts.

Well then, just stick with center point and recompose when necessary.

h4ppydaze wrote in post #13851602
5Dii autofocus is just crap. It's something everyone that has a 5Dii just accepts.

^^ Tis true Every shot on my Flickr is blurry because of the crap focussing

Is there a way to lock both the focus and exposure when recomposing other that holding the shutter button?