7D -- "Zone AF" vs. "AF Expansion" for BIF

[David Stephens]

I just read this nice article at Canon's site about the 7D's various AF modes:
http://www.usa.canon.com/dlc/controller?act=GetArticleAct&articleID=3049

Toward the end they tout the Zone AF mode as being good for BIF, but I've been using the AF Expansion mode because it allows me to "aim" with the central focus point and gives me room around that for error. There's enough precision that I can attempt to focus on the head of the bird in flight. Also, if a bird lands within range, then I can put the focus point right on an eye.

Is anyone using the Zone AF mode? I don't see the advantage for BIF, or birds in general, but maybe I'm missing something. If I can, I want to "know" which point to expect to lock on, or at least get a very near by point.

BTW, I hadn't discovered the M.Fn button before this article and will start practicing toggling between AF modes quickly tonight. I'd been screwing around with menus on the back, which is not the way to go (too slow).

Thanks,

Dave

[Doug Brown]

I've been using AF point expansion around the center point. I haven't really tried the Zone AF system yet, but will probably play around with it at the Bosque in a couple of weeks.

[Michael Pancier]

I've tried both and found a significantly larger percentage of keepers using a/f expansion around center point

[Ed Cordes]

I have been using center as well as center with expansion and like both depending on subject and conditions. I have not yet used zone.

[John Blumenkamp]

Thought I'd share this... I experimented with the Zone AF on the 7D this morning, and see it will be useful when you have a subject against a uniform background, and the subject is still somewhat small in the frame. My thought is with single point AF the chosen point may move off a small subject, especially if the subject does not fill the AF point and the subject is fast moving. In such a case, the Zone AF should be helpful to keep a focus point on the subject. I realize center point w/ expansion may be doing the same thing, but for this scenario today the Zone AF worked well... and as the Canon write-up for Zone AF states, this mode focuses on the object nearest within the zone, while center point w/ expansion may seek to focus on distant objects (if in the frame)?

Here is an example of where the Zone AF seemed to work well this morning, with the original frame shown as noted, and my cropped and processed version. FYI... of the nine AF points in the center Zone AF, the point that locked focus was just right of the hawk's head, so I guess in this particular instance the Zone AF did not choose the object nearest, which would've been the head. ;-)

Thoughts?

[David Stephens]

Thought I'd share this... I experimented with the Zone AF on the 7D this morning, and think it may be useful when you have a subject against a uniform background, and the subject is still somewhat small in the frame. My thought is with single point AF the chosen point may move off a small subject, especially if the subject does not fill the AF point and the subject is fast moving. In such a case, the Zone AF may be helpful to keep a focus point on the subject. I realize center point w/ expansion may be doing the same thing, but for this scenario today the Zone AF worked well... and as the Canon write-up for Zone AF states, this mode focuses on the object nearest within the zone, while center point w/ expansion may seek to focus on distant objects (if in the frame)?

Here is an example of where the Zone AF seemed to work well this morning, with the original frame shown as noted, and my cropped and processed version. FYI... of the nine AF points in the center Zone AF, the point that locked focus was just right of the hawk's head, so I guess in this particular instance the Zone AF did not choose the object nearest, which would've been the head. ;-)

Thoughts?

Ok, that may be the whole point (pun intended). I lost a bunch of shots today using Point Expanded today because the camera often focused on a distant object (trees mostly). If Zone AF picks up the near object, most likely my bird, that would be better.

I'll try it again on my next outing with that in mind.

Thanks John.

Dave

[John Blumenkamp]

Ok, that may be the whole point (pun intended). I lost a bunch of shots today using Point Expanded today because the camera often focused on a distant object (trees mostly). If Zone AF picks up the near object, most likely my bird, that would be better.

I'll try it again on my next outing with that in mind.

Thanks John.

Dave


Hey Dave... I tried the Zone AF with harriers today just briefly, but they were near the ground and the lower points of the zone would pick-up the ground/grasses, which makes sense since they were slightly closer than the harrier. I'm planning to go out and shoot some more tomorrow, and will hopefully be able to experiment some more with subjects against a varied background, etc.

Let me know what you experience.

[arash_hazeghi]

Zone AF should only be used for acquiring small to medium subjects against uni-color BG, otherwise the camera will-in most cases randomly-choose one point within the zone for you, for example it will lock on BG when tracking against foliage or a tail instead of a head when the bird is large in the frame.
I only use manual single point AF to force the camera to focus where I want.

[John Blumenkamp]

Zone AF should only be used for acquiring small to medium subjects against uni-color BG, otherwise the camera will-in most cases randomly-choose one point within the zone for you, for example it will lock on BG when tracking against foliage or a tail instead of a head when the bird is large in the frame.
I only use manual single point AF to force the camera to focus where I want.

Arash... has Zone AF locked on background foliage for you even when the foliage is a considerable distance behind the subject?

[arash_hazeghi]

Arash... has Zone AF locked on background foliage for you even when the foliage is a considerable distance behind the subject?

Yup, IME it usually grabs the contrasty BG unless the bird is very large/close to cover the entire zone, at which point it picks whatever point on the bird it "thinks" is closer. I use manual point selection or manual point + expansion in all cameras that I own since I don't want the camera to "think" for me.

With 7D I exclusively use single point and sometimes expansion point but only against sky (use ring of fire to switch between modes).

Example with expansion


http://ari1982.smugmug.com/Animals/Avian/kesterel-wings-down/701984474_tMYMH-L.jpg

[John Blumenkamp]

Yup, IME it usually grabs the contrasty BG unless the bird is very large/close to cover the entire zone, at which point it picks whatever point on the bird it "thinks" is closer.

Eh, then so much for the concept/description as given by Canon... ;)

"Most importantly, it (Zone AF) lets the camera focus upon whatever is nearest within that area..."

[David Stephens]

I'm glad I asked.

With a few more hours of training, maybe I'll be able to shift between single-point, single-point expanded and AF Zone within the fraction of second as I figure out what the BG will be.

With mallards it's really hard to keep the single-point on the bird, much less the eye. In checking my images tonight, most were single-point expanded, I checked the focus point and many cases I had the center point on the eye and the camera chose another part of the bird, or even some sky under the bird's head. I'm thinking that single-point may be the way to go, but I'll try the other modes a little more before I give up.

With the single-point expanded, it seemed like the AF was locked on as I tracked the bird, but then when I took the images up to 100%, many were OOF. With the single-point, if you get off the bird a little, then you might get blue sky and the camera gets totally lost. My keeper rate wasn't any better with expanded vs. single-point. I ws fooled into thinking I was in focus, but I wasn't.

Oh well, practice, pratice, practice...

Dave

[arash_hazeghi]

Eh, then so much for the concept/description as given by Canon... ;)

"Most importantly, it (Zone AF) lets the camera focus upon whatever is nearest within that area..."


Hey John,

I have seen the AF page on Canon website, unfortunately there isn't much quantitative technical information in it. Unless numerical data, in particular the distance, velocity and tracking angle for the subject are given, it cannot be determined in what conditions the camera can correctly identify the "closest" subject and focus on it.

The phase detect AF system primarily works on contrast so most likely, from objects of similar high contrast, camera tries to pick the closest one, and if you are lucky enough your intended subject might be one of these objects ;)

As an engineer I usually disregard manufacturer marketing information and advertisement material, unless they show me a curve with horizontal axis being distance to sensor and vertical axis being velocity and relative angle of the object camera can successfully track within a given tolerance. Otherwise, I will stick to my own field results :)

[John Blumenkamp]

Hey John,

I have seen the AF page on Canon website, unfortunately there isn't much quantitative technical information in it. Unless numerical data, in particular the distance, velocity and tracking angle for the subject are given, it cannot be determined in what conditions the camera can correctly identify the "closest" subject and focus on it.

The phase detect AF system primarily works on contrast so most likely, from objects of similar high contrast, camera tries to pick the closest one, and if you are lucky enough your intended subject might be one of these objects ;)

As an engineer I usually disregard manufacturer marketing information and advertisement material, unless they show me a curve with horizontal axis being distance to sensor and vertical axis being velocity and relative angle of the object camera can successfully track within a given tolerance. Otherwise, I will stick to my own field results :)


Thanks Arash... I'm like you, and having a degree in engineering and practicing for many a year now, I'll generally go for the field results. But it's still interesting to see if things perform as advertised. ;)

I went out some this weekend, and found myself staying with manual point AF, as well as spot AF at times. For me the Zone AF will be utilized as we discussed - for smaller subjects against a uniform background.