5d mkII vs 50D in terms of AF

[briansquibb]

@briansquibb
So in terms of the 17-40, I would assume then you use center AF point to shoot? If that's the case then how can you insure that all subjects will be in focus? Like short & tall subjects as you indicated, given that the AF is not that advanced.....

Not sure what context you are talking about here - most images have OOF subjects, it is only the one which you are interested in which are important.

If you are talking about a wedding group - focus on the ground in front of the nearest person and then have sufficient DOF dialled in.

Tall and short people are all in the same plane so will be in focus

 

[wickidwombat]

WOW!!
You guys have me all confused about the 5DII, I'm this close to pulling the trigger on purchasing this body and now I'm not so sure. I shoot events and weddings; AF accuracy is extremely important to me. If the AF is that bad then why do I see so many wedding photographers using the 5DII? I should just stick with the 7D then if this is the case, sure I lose out of the FF but AF accuracy is very important. I also understand that noise is much better on a FF body, but that's what Lightroom is for.....

just use the center point and f2.8 lenses or faster and you'll be fine

 

[briansquibb]

WOW!!
You guys have me all confused about the 5DII, I'm this close to pulling the trigger on purchasing this body and now I'm not so sure. I shoot events and weddings; AF accuracy is extremely important to me. If the AF is that bad then why do I see so many wedding photographers using the 5DII? I should just stick with the 7D then if this is the case, sure I lose out of the FF but AF accuracy is very important. I also understand that noise is much better on a FF body, but that's what Lightroom is for.....

just use the center point and f2.8 lenses or faster and you'll be fine

I think F/4 as well?

 

[Meh]

@briansquibb
So in terms of the 17-40, I would assume then you use center AF point to shoot? If that's the case then how can you insure that all subjects will be in focus? Like short & tall subjects as you indicated, given that the AF is not that advanced.....

@RonQ I recall you asked a similar question the other day in another thread. It might just be your choice of words but your question about short/tall subjects being in focus if only using a center AF point suggests that you might think that by using multiple focus points that the camera can independently achieve focus at each point but that is not the case (I apologize if I'm just misunderstanding your comment). The camera can only focus at one distance at a time and every object that is at that distance from the camera will be in focus. A tall person standing vertically (assuming the camera is held perpendicular to the ground) will be in focus from head to toe (more or less) because their whole body is aligned on the focal plane. It doesn't matter whether you only used the center AF point, an off center point, or many points.

 

[RonQ]

@briansquibb
So in terms of the 17-40, I would assume then you use center AF point to shoot? If that's the case then how can you insure that all subjects will be in focus? Like short & tall subjects as you indicated, given that the AF is not that advanced.....

@RonQ I recall you asked a similar question the other day in another thread. It might just be your choice of words but your question about short/tall subjects being in focus if only using a center AF point suggests that you might think that by using multiple focus points that the camera can independently achieve focus at each point but that is not the case (I apologize if I'm just misunderstanding your comment). The camera can only focus at one distance at a time and every object that is at that distance from the camera will be in focus. A tall person standing vertically (assuming the camera is held perpendicular to the ground) will be in focus from head to toe (more or less) because their whole body is aligned on the focal plane. It doesn't matter whether you only used the center AF point, an off center point, or many points.

It's OK Meh, I'm just trying to learn from others.... I recently had a similar experience where my lens was wide open 2.8 shooting in AP and only the foreground subject was in full focus, all others were not. I was so upset that I kicked myself several times trying to figure out what, if anything, I did wrong..... Once I changed to A-DEP the next day trying to take a similar shot with different subjects, everyone was in focus. However, the only difference between the two were one was shot at sunset and the other (test subjects) were shot in full sunlight.
Yehh, I'm aware it could have also been my camera and that's a possibility, I took it into the Canon Service center (about 2 blocks) from me and they swore nothing was wrong... However, 2 days later, I shot a family for portraits and I had a similar issue..... I was so frustrated that I went to manual focus and finished the job. I took it back to Canon and told them that this was unacceptable, it took them 8 days to get the camera back to me indicating that they had to change out the FA and that it was causing inaccurate focus, they changed it out and did some electrical adjustments and since I have not had any clients to shoot, I'm assuming it's back to normal but still curious as to how other photographers are shooting.

 

[QNeX]

just use the center point and f2.8 lenses or faster and you'll be fine

I think F/4 as well?

Well, with f/4 you get only horizontal sensitivity, vertical line has
sensitivity with f/2.8 or brighter lenses only.

In other words, with f/4 lens you won't have a cross center focus point,
you'll get plain old horizontal f/5.6 point.

 

[skitron]

just use the center point and f2.8 lenses or faster and you'll be fine

I think F/4 as well?

Well, with f/4 you get only horizontal sensitivity, vertical line has
sensitivity with f/2.8 or brighter lenses only.

In other words, with f/4 lens you won't have a cross center focus point,
you'll get plain old horizontal f/5.6 point.

Now that is a fly in the ointment for my plan to buy a 70-200 f4 IS...

 

[Loswr]

just use the center point and f2.8 lenses or faster and you'll be fine

I think F/4 as well?

Well, with f/4 you get only horizontal sensitivity, vertical line has
sensitivity with f/2.8 or brighter lenses only.

In other words, with f/4 lens you won't have a cross center focus point,
you'll get plain old horizontal f/5.6 point.

Briansquibb is correct. The 5DII's center AF point is an f/5.6-sensitive cross-type point, with additional f/2.8 sensitivity in a single orientation.

 

[QNeX]

Briansquibb is correct. The 5DII's center AF point is an f/5.6-sensitive cross-type point, with additional f/2.8 sensitivity in a single orientation.

I've had an impression that a single line can be either higher precision (f/2.8), but will only work with f/2.8 or faster lesnes, and that is the reason why xxD bodies (IIRC starting with 40D) have center point that is composed of horizontal and vertical lines sensitive to f/5.6 and additional diagonal lines sensitive to f/2.8.

I did look for more authoritative source and found this:
http://cpn.canon-europe.com/content/education/infobank/autofocus/autofocus.do

EOS 5D Mark II AF system
(...)
The centre AF point has a special hybrid design. With f/2.8 or faster lenses, focusing is a two-step process. First, the f/5.6-sensitive cross-type sensor components are used to focus. When focus is almost achieved, a switch is made to the f/2.8-sensitive vertical line-sensitive sensors for high-precision focusing.

Additionally, the centre AF point’s f/5.6 sensors each have two lines with individual pixels arranged in a zigzag pattern, making a total of four lines for horizontal line-sensitive focusing at the centre.

With this improvement, the centre AF point can do a better job of detecting difficult-to-read, low-contrast subjects, improving the overall performance of the autofocusing system.

So, I stand corrected. Thanks for pointing this out!

 

[Loswr]

xxD bodies (IIRC starting with 40D) have center point that is composed of horizontal and vertical lines sensitive to f/5.6 and additional diagonal lines sensitive to f/2.8.

I think the reason for that arrangement is so that the center AP point is acting as cross type with all lenses for the entire focusing process.