7D MK2 AF Point Expansion Question....

I beleive the centre point have priority,,,and the out points kick in as a aid if centre point not focused. The AF manual has a good description of this. Also for the very situation you described you can set up you back buttons to give you instant centre spot focus.

it works extreamly well for picking out a bird surrounded by branches or twigs.

this is explained well on pages 20-28 of the 7dmkii AF guide. the 4 and eight pts are referred to as "assist" points.

Thanks Keekerseeker, I read the entire AF Manual and saw that the expansion points "assisted" the center point but wasn't sure if the center point would remain truly dominate but sounds like it will. Plan on setting up one the "Preset Cases" for perched hawks that I know will fly off (hopefully can grab a couple reasonably focused still shots) and I can continue tracking as it fly's off. I know I'm not good enough to follow with a single point so I plan on using the 8 point assist to start. Hope that seems reasonable.

It appears this camera is new too you. If so,,you will need to dry many different things,,your set-up options are close to limitless. Try different things read-re read the AF section and exp. The single point is not really meant for birds in flight there are many other settings for this.

I've shot more than 200,000 frames on my 7D2. Had it from the first month. Love it. Still can't figure out what the heck Canon meant when it spoke of those extra points being "assist" points. I've seen them come at this a few times in interviews, manuals, autofocus tutorials on their website. In none of them are they not deliberately obscure. At first I thought it was a Japanese/English issue, but as the American executives were interviewed, I realized they were deliberately not being explicit (or didn't know).

My own personal experience is very inconclusive, but here's my rough impression. I think of the focus point structure as being one continuum, moving from the small point, to the "normal" sized single autofocus point, to the 4-assist points and then the 8 assist points. In each case, the physical area of the autofocus point - acting as one autofocus point - gets a little bigger.

If this is indeed the case, then what it means is that there will be no differentiation between the different points in each of those modes, and they will merely employ the AF algorithm on a bigger or smaller area. Because the algorithm tends to prioritize the closest object, my sense is that any target whose focus might be thwarted by any object that is slightly closer than the subject needs to be shot with a smaller effective AF point.

If anyone actually knows better, I'd love to get conformation or contradiction.

Tig, that's pretty much what I was wondering. The claim is the expansion points "assist" the center focus point if the center point is having a hard time focusing or the subject moves away from the center point but the question is if one of the assist points also sees a closer object than one that is in the center focus point does it influence the accuracy of that main center focus point. I guess it may not matter depending on depth of field of the subject. Just wanted to know because I was planning on photographing a perched bird (more than likely in a tree as opposed to say a pole) and then continue tracking in AI servo when it takes off with the help of the expansion points. I didn't want to have to use just a single point for the perched bird and then shift to the expanded point mode. I just don't think I could be that quick and my skill level won't allow me to use just a single point to track. I don't think I can use zones in this case because from what I understand selecting zones will focus on the closest object to me. If I'm not approaching this in the right way I would appreciate any advice.

Some good video on YouTube with regard to the AF system, I'm using af servo with back button, 65 zone with center point as the initial point. Generally using case 2 or 4. I take my thumb off the back af button when the bird is at rest to lock single af.

from what I understand any of the assist points (if set) will automatically take over if the main point loses lock, so in that case it could focus on something entirely different in distance from the intended target, but as soon as the main point covers the intended target again the focus will switch back automatically.

A very complicated system that leaves many of us hoping we're using it to the best for our intended use!

Artistguy, you've expressed the crux of the question, which remains unanswered by Canon (and has proven difficult to suss out by experimentation)... which is...

When the "assisting" AF points find an object closer to the camera, do they become the prime focus point in all cases? (less desirable) Or, when the "assisting AF points find an object closer to the camera, does take over the AF role ONLY in the case that the center point has poor lock, such as when having a very low contrast subject. (more desirable)

In the first instance, the zone you've created just becomes one big, fat focus point. In the second instance, there is much more of a sophisticated set of behaviors dependent on the best information available to the camera at the moment.

My guess is that it does the less desirable variant of that, and I base that guess on the fact that Canon has seemingly deliberately obscured the language whenever it speaks on the topic.

Now that I've wasted so many minutes talking about it, I guess I have to design an experiment to actually determine this with a camera

OK, so I did the test, and the results were quite pleasing. It turns out that Canon does indeed do the more sophisticated interpretation of this, as shown by these results:

In all cases the 8 assist points were active, as well as the center point....

1) The center focus point was placed on a no-contrast background, and the right column of assistance points was on a clear object (a chess piece) about a foot closer to the camera. Result: the camera focused on the closer chess piece.

2) The center focus point was placed on a chess piece, while a second chess piece was placed a foot closer, in light with the right column of assistance points. Result: the camera focused on the chess piece that was further away, under the center focus point, and not on the closer one under the assist focus points.

I did a bunch of variants of this, and it was all consistent. So, I think we have our answer.

The language that we might use in the future to describe this and not get quizzical looks:

Assisting focus points are used only in the case that the designated central focus point lacks the information to accurately determine focus, such as when the subject under that focus point lacks enough contrast to determine its distance.

Colocam,,,what you perceive as a problem,,,has a very easy solution. As easy as pushing one button ,,but you have to do a custom setting to re-assign one of the re-assignable buttons. For example you can reassign the front depth-of-field button to instantly switch to one shot,,,and use another assignable button to switch to single point or spot single point when the bird is stationary,,,,it starts to fly,,,simply take your finger off the two buttons and it automatically goes back to your case set-up Al servo and what ever zones you have set it for. The switch back-and -forth takes about 1/10 of a second. And is the best thing since sliced bread !

Excellent work, thanks for this.....?...can you squeeze the word algorithm in a couple of times to make it official☺?

Thanks Tig for doing that test. That answered my question. Thanks Keekerseeker for the advice as well. I'll be trying a couple of customizations to see what works best for me. A lot of variables but I think that's one of the great features of the 7Dmk2 , being able to fine tune your setup for varying conditions and subjects....Now to get out later this afternoon and do some shooting...weather pending.

TigTillinghast said:

I've shot more than 200,000 frames on my 7D2. Had it from the first month. Love it. Still can't figure out what the heck Canon meant when it spoke of those extra points being "assist" points. I've seen them come at this a few times in interviews, manuals, autofocus tutorials on their website. In none of them are they not deliberately obscure. At first I thought it was a Japanese/English issue, but as the American executives were interviewed, I realized they were deliberately not being explicit (or didn't know).

My own personal experience is very inconclusive, but here's my rough impression. I think of the focus point structure as being one continuum, moving from the small point, to the "normal" sized single autofocus point, to the 4-assist points and then the 8 assist points. In each case, the physical area of the autofocus point - acting as one autofocus point - gets a little bigger.

If this is indeed the case, then what it means is that there will be no differentiation between the different points in each of those modes, and they will merely employ the AF algorithm on a bigger or smaller area. Because the algorithm tends to prioritize the closest object, my sense is that any target whose focus might be thwarted by any object that is slightly closer than the subject needs to be shot with a smaller effective AF point.

If anyone actually knows better, I'd love to get conformation or contradiction.

Click to expand...

+1

Could not agree more. I got the camera on the day it came out, my current shutter count is 430,000 but I still have no idea what those assist points really do. I've never been able to detect any obvious difference between center point and the expansion mode. My guess is that if there is really a difference it is probably extremely negligible.

If the expansion points are never active when focus is achieved by the center point, why would there be an option to choose single point focusing?

Yes, the expansion points assist when photographing flying birds but they take over sooner and more than you wish when photographing perched birds in trees.

That's the reason to switch to single point.

Possibly, the center point gets into trouble in busy environments and calls in the assisting expansions points. The effect is small most of the time but with a shallow depth-of-field and some disturbing branches it can be a problem.

If you read all of Canon's documentation the centre point is the primary AF point. The system will always choose the centre point unless there is not enough contrast then an outer assist point will kick in.

This is an easy test to perform in one shot. Find a low contrast target like a plain wall and keep moving the cam around and keep lifting and pressing the shutter button. Eventually one of the outer assist points will take over. You will always see the outer assist point illuminate first followed by the centre point. You will see both the centre and outer point illuminated after in the LCD or if you have it set up or in DPP. I believe it shows both the outer and centre point to illustrate the centre was the users primary choice.

Canon says it works exactly the same in AI Servo however we can't see any outer points illuminated. Here are my AI Servo tests. Two sets of match boxes with the same contrast - one behind the other. Cam on tripod and 4 point expansion. DPP does not show outer assist points but I could see their placement through the viewfinder.

Defocus before test



All 5 points on front target



I moved center point onto back target and kept right assist point on the front target. I could hold it there all day and it would maintain focus on the front target.



I moved all points off front target. System refocused on the back target. TS -2 had a delay. TS +2 was instant.



Final test. I placed the centre point on the back target while right assist was on the front. System focused using the centre point which I would expect since that is my primary AF point.



The above is easily tested. My final theory can't because you can't see the assist points in AI Servo. Some have referred to expansion as mini zone and the AF points flip around like in Zone AF. If so why didn't Canon just call it mini zone.

Since Canon said that AI Servo works exactly the same in A Servo as it does in One Shot I believe that in AI Servo the system will always try to use the centre point when tracking. If poor contrast threatens a loss of AF then an outer point will kick in. Once the centre point gets enough contrast to track it takes over again. That is my chosen primary point and that is what I would expect. If I didn't care about a primary point to track with then I would just choose zone AF.

It is possible the centre point in expansion is only a start point and it does flip around but I can't find any specific documentation. I have read everything I could find that Canon has put out to lead to my view on how it works in AI Servo. One on my bucket list. To sit beside Rudy Winston on a plane. I have this question for him and a few others. I would only need about a ½ hr. Poor guy

If the centre point is the primary and the system will always try to choose it first during initial AF acquisition for both one shot and AI Servo which can be backed up by documentation, why would this not apply to tracking?

Zee Char said:

If you read all of Canon's documentation the centre point is the primary AF point. The system will always choose the centre point unless there is not enough contrast then an outer assist point will kick in.

Click to expand...

For instance, with perched birds in trees there is often not enough contrast due to low light. The expansion points start working and can mess up the focus.

Zeee said:

This is an easy test to perform in one shot. Find a low contrast target like a plain wall and keep moving the cam around and keep lifting and pressing the shutter button. Eventually one of the outer assist points will take over. You will always see the outer assist point illuminate first followed by the centre point. You will see both the centre and outer point illuminated after in the LCD or if you have it set up or in DPP. I believe it shows both the outer and centre point to illustrate the centre was the users primary choice.

Canon says it works exactly the same in AI Servo however we can't see any outer points illuminated. Here are my AI Servo tests. Two sets of match boxes with the same contrast - one behind the other. Cam on tripod and 4 point expansion. DPP does not show outer assist points but I could see their placement through the viewfinder.

Click to expand...

Probably, the expansion points are built for AI Servo and tracking of moving subjects. They simply 'assist' when the center point misses the subject, as happens a lot with fast flying birds.

Zeee said:

The above is easily tested. My final theory can't because you can't see the assist points in AI Servo. Some have referred to expansion as mini zone and the AF points flip around like in Zone AF. If so why didn't Canon just call it mini zone.

Click to expand...

From my experience, in a zone mode all points are created equal. The zone points follow the subject wherever it is and stay there.

In expansion mode the center point has the focus priority. The expansion points hold the focus until the center point finds the subject again.

It is a more sophisticated way than using the Tracking Sensitivity in the AF-settings.

Because with Tracking Sensitivity the AF stays at the same distance until focus is acquired again, while the expansion points actively focus till the center point kicks in.

Zeee said:

Since Canon said that AI Servo works exactly the same in A Servo as it does in One Shot I believe that in AI Servo the system will always try to use the centre point when tracking. If poor contrast threatens a loss of AF then an outer point will kick in. Once the centre point gets enough contrast to track it takes over again. That is my chosen primary point and that is what I would expect. If I didn't care about a primary point to track with then I would just choose zone AF.

Click to expand...

As far as I know, you are right. Otherwise we would only have the zones.

Zeee said:

It is possible the centre point in expansion is only a start point and it does flip around but I can't find any specific documentation. I have read everything I could find that Canon has put out to lead to my view on how it works in AI Servo. One on my bucket list. To sit beside Rudy Winston on a plane. I have this question for him and a few others. I would only need about a ½ hr. Poor guy

Click to expand...

Excellent work, thank you.

Thanks for this.