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Thread started 14 Dec 2009 (Monday) 11:01
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7d AF System Summary

 
e02937
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Dec 14, 2009 11:01 |  #1

This is my compilation of information on the 7d's new autofocus system. The sources include theBH video (external link), manual (external link), canon's website (external link), various threads here on PTON. Hope it helps someone out.

This is a work in progress. Your feedback, corrections, and comments are welcome!

The basics:

Introduction
The 7d introduces a brand new focus system that is supposed to rival some of the most expensive Canon cameras of today and tomorrow. It has 19 focus points, all of which are cross point type for increased accuracy.

AF Modes
Still the classics:
One shot - for non-moving subjects
AI Focus - switches between one shot and servo when needed (if subject starts moving) (never use myself)
AI Servo - for moving subjects.

One neat thing right off the bat is that in Servo, the 7d does NOT require that you start focusing with the center point, you can select the point from which to start (assuming your are in a proper focus point selection mode).

Switching AF Modes
Switching AF modes is easy, just press the AF DRIVE button (top of camera) and then roll the top dial to change modes. Rolling the back dial will switch shooting modes (single shot, continuous high, low, etc).

AF Areas (page 87 of the manual):
By default, you have 3 AF area selections using the custom function III - 6 you can enable 2 more. I highly recommend doing so.

If I could have explained the areas better I would have but this does it well with pictures:
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Notes:
1) One thing to note is that the focus area is actually a little bit bigger than the box you see through the viewfinder. Enabling and using spot expansion will make the focus area about equal to the size of the box you are seeing.

2) Spot AF is great for focusing through things (like tree branches) or when you have a busy area and want to try and nail a specific focus area

3) AF expansion gives you the ability to pick a primary focus point but give the camera a bit more information. This differs from using zone af because you are still picking a primary focus point where as in Zone AF the camera is free to pick any of the points within the zone as the primary AF point.

4) Using C.Fn III 12, the camera can actually remember which AF area you used when horizontal and then vertical. That way when you switch to portrait mode it will go back to the last AF area you used in portrait. I generally leave my portrait AF area as spot AF since I am usually shooting someone's face.

How to toggle through Areas, and points quickly
Ok this button
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is your friends. You should be able to easily press it with your thumb. It's located on the back of the camera at the top right.

Once you press that you can do 2 things:
1) Press the M.fn button with your index (shutter) finger. It's located on the top towards the front. This will toggle your AF Area selection

2) You can use the dials on the top of the camera (by shutter button) and on the back (the one with the SET button) to move your AF point selection. This will work in every mode except in One-shot 19 point AF because all points are active. It will work however in servo 19 point as you can use it to pick the initial point before the camera starts tracking movement.

Please note that you can also use the custom functions to set the joystick to move the AF Point as well. This is C.Fn IV - 1. I like this because you don't have to press the AF button to switch points and since the joystick goes in all directions it's a bit faster.

One other note:
The AF-ON button, by default, performs the same function as pressing the shutter half way down. Many people have said that they have (using the custom function C.Fn IV - 1 gotten rid of the half shutter press doing anything) and exclusively use the AF-ON button (or another button mapped) to get more control. I think its personal preference but it can be nice to simple hold down that AF-ON button, for example, during Servo and then snapping away only when ready.

My thoughts (just that, my opinion)

On AF modes, forget about AI focus in my humble opinion and switch quickly between one-shot and servo when needed.

This is pretty much my opinion but I would say you can pretty much forget about about using 19 point AF. In my experience, it's often a bit erratic and picks different points causing you to really lose control over your focus. I am strongly considering disabling it entirely C. Fn. III - 6).

Similarly, I find that zone AF suffers from the same issue but it may have some merit. Overall, I've found it better to use a single AF point with expansion over zone AF particularly in Servo when tracking an object and trying to keep focus on say the face rather than letting zone af go where it pleases.

I've got my camera set to use the joystick to change AF points and find that i am almost always toggling between single point, single point expanded, or single point spot.

Single point for most shooting (and moving the point around), not center the recompose.

Expanded in Servo to give the best chance for a keeper

Spot AF for shooting in challenging situations like through tree branches or if there are a lot of subjects, of which one I am trying to isolate.


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Mosca
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Dec 14, 2009 11:16 |  #2

Nice summary, I just spent half a day coming to the same conclusions. I did my experiments by shooting the Christmas tree from across the room and picking out single ornaments to focus on, moving the points around and trying the various modes. I have to add that Spot AF can be scarily accurate shooting into tree branches.


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e02937
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Dec 14, 2009 11:25 |  #3

Mosca wrote in post #9195247 (external link)
Nice summary

Thanks much, I was just trying to compile all the information I've seen over the past weeks and months. It really is an excellent camera and spot AF is fantastic as you said!


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apersson850
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Dec 14, 2009 11:42 as a reply to  @ e02937's post |  #4

If you use the Q button to activate the Quick setting menu, and have the AF point selection mode highlighted, you can change the mode by rolling the rear dial. This is the only case where you can use anything else than the M-Fn button for this purpose.


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haknslash
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Dec 14, 2009 11:50 |  #5

Nice summary Merlin and thanks for posting.


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Dec 14, 2009 11:55 |  #6

Very nice summary. Very useful info and I've had my 7D a few weeks now. :)


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Dec 14, 2009 12:32 |  #7

I have only had the 7D for a short time, but I would like to add, when shooting fast moving sports (like Hockey) in AI Servo, I don't use single point, spot, or expanded point. I find it is too easy to lose the subject and focus on something in the background. I use either Zone AF or 19 Point Auto Select AF.

An added benefit to these two modes is that the little square (or squares) will turn black to show you which focus point was used. In the other modes, there is no focus confirmation while in AI Servo.

Between Zone AF or 19 Point Auto Select AF, I didn't really notice one mode being better than the other but it is nice to see the focus confirmation while in AI Servo.

Does anyone have any other thoughts on what is the best AF Area Selection for sports in AI Servo?

(Simulation of how focus is confirmed)

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Dec 14, 2009 12:38 |  #8

Great stuff! One other reason for not using 19 point auto-select focus - the camera will tend to focus on the closest high contrast area in the frame, which will often be the ground or something on the bottom half of the image, regardless of where the actual subject might be.


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e02937
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Dec 14, 2009 12:39 |  #9

msowsun wrote in post #9195706 (external link)
Does anyone have any other thoughts on what is the best AF Area Selection for sports in AI Servo?

I haven't shot a lot of sports but I think that Single Point with AF point expansion may be really good for sports shooting. It still gives you the ability to select one point for focus but also gives you some leeway if you don't quite nail the focus point on the subject.

It's certainly worth a shot. I would use the 19 point, just too erratic in my experimenting especially in a busy setting like sports.


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Lester ­ Wareham
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Dec 14, 2009 12:51 |  #10

Good summary well done.

Trying area AF with BIF the other day I found that although easier to keep track of the subject it locks to the nearest part of the target, so focus tends to be on the proximal wingtip; a problem if the exposure level limits you from being well stopped down.

Spot AF has the problem of keeping it on target unless the birds were small in the frame or the motion was very predicable.

Spot AF with expansion is probably the thing, but I ran out of time to try that.


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Dec 15, 2009 05:09 |  #11

msowsun wrote in post #9195706 (external link)
I have only had the 7D for a short time, but I would like to add, when shooting fast moving sports (like Hockey) in AI Servo, I don't use single point, spot, or expanded point. I find it is too easy to lose the subject and focus on something in the background.

In which case you want to set C.Fn III-1 to a slower value. That will lengthen the time between you losing the subject and the focussing switching to the background. It gives you a bit of time to re-acquire the subject.


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amd ­ is ­ the ­ best
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Dec 15, 2009 05:17 |  #12

I don't want to clutter up your thread, but maybe you guys would be able to shed some light on a possible issue I may be having. When on the center focus point (or single point) I notice that sometimes the point will turn red and beep together however sometimes the box will not light red. Is this normal?

It just seems to me that the 7D (mine anyway) isn't always accurate with the AF. I have done several tests comparing my 50D to my 7D and find that the 50D seems to AF more accurate.

I mostly notice this with my longer lenses 50 1.8, 85 1.8 and 70-200.


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Mike ­ R
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Dec 15, 2009 05:39 |  #13

Nice summary. I'll only add that you can set the camera to automatically switch the focus point when you turn the camera. It's a great feature when shooting sports.


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Dec 15, 2009 05:57 |  #14

msowsun wrote in post #9195706 (external link)
I have only had the 7D for a short time, but I would like to add, when shooting fast moving sports (like Hockey) in AI Servo, I don't use single point, spot, or expanded point. I find it is too easy to lose the subject and focus on something in the background. I use either Zone AF or 19 Point Auto Select AF.

An added benefit to these two modes is that the little square (or squares) will turn black to show you which focus point was used. In the other modes, there is no focus confirmation while in AI Servo.

Between Zone AF or 19 Point Auto Select AF, I didn't really notice one mode being better than the other but it is nice to see the focus confirmation while in AI Servo.

Does anyone have any other thoughts on what is the best AF Area Selection for sports in AI Servo?

(Simulation of how focus is confirmed)
QUOTED IMAGE

I always use Center spot for fast moving sports. I just dial down the AI servo tracking sensitivity. Just like the 1Series. Not all the way to slow, but in between default and slow.


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Dec 15, 2009 06:08 |  #15

merlin2375 wrote in post #9195144 (external link)
This is my compilation of information on the 7d's new autofocus system. ....

This is pretty much my opinion but I would say you can pretty much forget about about using 19 point AF. In my experience, it's often a bit erratic and picks different points causing you to really lose control over your focus. I am strongly considering disabling it entirely C. Fn. III - 6)....

Great compilation!

There is one thing I'd like to point out about using all 19 points. I avoid it like the plague, as I do with my 5D2 and as I did with cameras before it. But there's one situation where it will help. Let me explain.

When you take a fast lens like the 300/2.8 or 400/2.8 and add a 2X teleconverter, it becomes an f/5.6 lens of double the focal length (e.g., 600/5.6, 800/5.6, etc.). That works fine. Now, go ahead and add a 1.4X teleconverter between the 2X and the body and you will have an f/8 lens of even greater focal length, but the logic of the lens scheme will only report to the camera that the 2X teleconverter is present.

So in the case of the 400 mm f/2.8 lens with stacked 2X and 1.4X teleconverters, you now have an 1120 mm f/8 lens that the camera thinks is an 800 mm f/5.6 lens. Meaning, the camera will try to autofocus (note that metering is still correct since it works on the amount of light hitting the metering sensor and calculates exposure from that). Now, with the 5D and 5D2 and a reasonably bright and contrasty subject, this worked as long as you used the center AF point. Not so with the 7D. However, I found out that the stack WOULD work if I activated all 19 AF points in auto. The camera would autofocus, but would always choose an AF point 1 or 2 places away from the center AF point.

Some experimentation would probably reveal that the AF points 1 or 2 points away from center are slightly more sensitive than the center point, so manually selecting one might work also, but I didn't test that theory. BTW, this trick was passed on to me by a poster on another forum so it does seem to have credibility.


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7d AF System Summary
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