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?????

[neuroanatomist]

Let me amend/correct a previous statement - it's not that images using the center AF point will be 2x sharper than images with other points. What I'm trying to say is that images shot with the center AF point and a fast lens will be more likely (on average) to achieve sharp focus.

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

So, there are focus problems, and also metering problems? Sounds like there are odd things going on, or perhaps just issues of interpretation?


You say the images are 'darker' but that's a function of metering, not AF. If the AF sensor is 'pooched' that would have no effect on metering (they are different sensors). If you're set to evaluative metering, that's linked to the selected AF point in the sense that metering calculations are weighted to that area (since the camera assumes that's where your main subject is), but that's independent of the AF sensor - the camera 'tells' the AF sensor which of the 9 points to use, and independently tells the metering system which of the 35 TTL metering zones to pay more attention to. So if you're using evaluative metering, and if the area at the selected AF point is brighter than other areas, the overall exposure will be less. For a graphical example, imaging shooting a white wall with a dark door off-center - if you select an AF point on the white wall, the door will be a bit underexposed, while if you select an AF point on the door, the wall will be a bit overexposed.


Regarding the lack of sharpness, have you checked the focus calibration of your lenses (e.g. using one of the many available printable charts, like this one)? It may be that your camera and/or lenses have front/back focus issues. With your XSi, if there are front/back focus issues, Canon will need to correct them (higher end bodies have a feature called AF Microadjustment to calibrate the AF for each lens - that's one reason I moved from a T1i to a 7D).


As for hunting for focus, is there sufficient light? If you're indoors with a slower lens, is your AF assist working (pop-up flash or IR pattern from Speedlite)? That can be turned off in the C.Fn settings, and in the 'creative' modes (Av, Tv, M) I think the flash will not pop-up for AF assist (not sure on that one). Also, in AI-Servo mode the AF assist is disabled.


Overall, I think you have two options. 1) Perform some carefully controlled testing to try and determine your specific issue(s), or 2) contact Canon Service for assistance.

[elmo_2006]

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<p class="MsoNormal" style="margin: 0in 0in 10pt;"]<span style="font-family: Calibri; font-size: small;"]@neuroanatomist...
<p class="MsoNormal" style="margin: 0in 0in 10pt;"]<span style="font-family: Calibri; font-size: small;"]Thanks for your insight in this matter.
<p class="MsoNormal" style="margin: 0in 0in 10pt;"]<span style="font-family: Calibri; font-size: small;"]I always shoot in raw, Av, spot metering mode and constantly use the centre focus point.
I also where possible shoot @ f2.8 or @ f4 where my lenses permit. I have also performed numerous tests to check for back/front focusing issues across all my lenses and suffice to say, they are not experiencing this issue.
As for taking photos, I have peformed numerous tests and for whatever reason, the images that are taken with the outer focus points, the images are underexposed, where as the same image taken with the centre focus is in my eyes perfect. Also, when using any other focusing sensor other than the centre one, the camera tends to hunt more often with a 50% chance of lock, it's actually really frustrating.
I have taken plenty of portraits before and I have never seen this problem before hence this thread.
Instead of wasting anymore of your time or others, I will take your recommedation and send my camera out for servicing to Canon. I would also like to take this time to say thank you for your quick and courteous responses.
<p class="MsoNormal" style="margin: 0in 0in 10pt;"]<span style="font-family: Calibri; font-size: small;"]I'm happy that there is a forum where ppl are willing to help each other out by providing valuable insight and opinions without insult.
<p class="MsoNormal" style="margin: 0in 0in 10pt;"]<span style="font-family: Calibri; font-size: small;"]Thank you
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[peety3]

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I always shoot in raw, Av, spot metering mode and constantly use the centre focus point.
<p class="MsoNormal" style="margin: 0in 0in 10pt;"]<span style="font-size: small; font-family: Calibri;"]I also where possible shoot @ f2.8 or @ f4 where my lenses permit.
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For the sake of mention, AF and metering don't care what aperture you've set, they only care what aperture the lens offers wide open (as the aperture is wide open during AF and metering).

[elmo_2006]

Thanks peety3, regardless there is something strange going on with my camera and I as well as a buddy of mine cannot figure out what the heck is happening when taking photos. It is the 'weirdest' thing. Sometimes it works fine and other times, the thing has a mind of its own. I've gone as far as resetting all the settings to no avail. There are 3 things I have yet to perform:


1. Reflash the firmware


2. Have it checked/serviced by an authorized Canon depot


3. Throw it at the wall in a fit of rage and damn the gods.


I think I'll settle for option 2.


[:P]