Canon 9-Point Auto Focus Points...

[elmo_2006]

Query...


Are the outer focus point's/sensors on a Canon Rebel Xsi just as sharp as the centre focus point? The majority of the time I shoot with thecentre focus point, but as of late I have been shooting more often with the outer focus points for portraits wherebyangling the camera in the vertical position. I have noticed that the images are not as sharp compared to the photos taken with the centre focus point. This is where I wish I had the 7D as all points are cross-type.


Someone has confirmed that ther centre focus point would be at at least 2x sharper if not more than the outer focus points but was wondering if anyone here can confirm this.


Also...my focus screen seems to be, for lack of a better term, scratched (removed a couple of times for cleaning)and would like to know if this may potentiallycause issues with focus lock?


As per usual, I thank thee for any opinions/recommendations.

[neuroanatomist]

Someone has confirmed that ther centre focus point would be at at least 2x sharper if not more than the outer focus points but was wondering if anyone here can confirm this

Correct, but t depends on the lens. The center AF point operates at a higher precision only when a lens with a max aperture of f/2.8 or faster is being used. It's a little better than 2x precision (the f/5.6 AF points are accurate within one depth of field, the high-precision center AF point with an f/2.8 or wider lens is accurate within one-third of the depth of field). Note that it doesn't matter what the aperture is set to, only the max aperture of the lens matters.

Also...my focus screen seems to be, for lack of a better term, scratched (removed a couple of times for cleaning)and would like to know if this may potentiallycause issues with focus lock?

Shouldn't, if it's the focus screen that's actually scratched. The AF sensor is below the mirror, the focus screen is above it (just below the pentamirror).

[elmo_2006]

Cool, so generally speaking, if I'm using a lens that is either f2.8 or f4, the centre focus images would be far sharper than the outer focus points?


What I'm trying to grasp is at the idea that the majority of images tend to be sharper at centre focus point over the outer focus points. As of late, I'm shooting more portraits than landscapes and it is just recently that I'm noticing this focusing issue.


Hope this helps...

[neuroanatomist]

Cool, so generally speaking, if I'm using a lens that is either f2.8 or f4, the centre focus images would be far sharper than the outer focus points?

No. An f/4 aperture is smaller than f/2.8. The lower the f-stop, the larger the aperture. So, if you're using a lens that is f/2.8, f/2, f/1.4, etc., you are activating the high-precision center AF point, but if you're using a lens that is f/3.5, f/4, f/5.6, etc., then all the focus points including the center one are operating at 'low' (f/5.6) sensitivity.


I've also read that some cameras had an issue where the outer points were less accurate even when using a slower (e.g. f/4) lens. Hopefully someone else can chime in.

[elmo_2006]

It seems that I'm seeing this phenomena occur when using an f4 lens, it seems to hunt for a focus lock more often than not and is quite aggrevating and I'm not sure what to do. It occurs even when using an f2.8 lens and one of the outer focus points. The camera is at least 2 years old, I'm debating if I should have Canon look at it.

[elmo_2006]

I have thisbad feeling that the focus sensor is pooched and I'm wondering if it's actually worth sending it into Canon for repair/replacement.


It seems that the when I use one of the outer focus sensors, the images come out darker than the remaining points....


Any ideas?????