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10-27-2016, 02:16 PM   #1
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Not finding a good tutorial on using AF.C correctly

After re-reading the Perfect focus everytime thread recently I remembered some lingering questions I had regarding AF.C (specifically with AF Select and Expanded Area AF settings). I did some online searches and now understand how autofocus works at a sensor level, and situations where it's useful (sports, kids, etc..). I still have some questions about using it in practice, though.

I know that focus depends on distance and depth of field, so obviously AF.C is great for keeping focus when the subject in the selected AF point moves in the Z coordinate... away from or closer to the lens. How does it work for lateral (X,Y) movement? In theory there's no change in focus distance if the camera moved in parallel to match the subject, but since there's usually angular change between subject and camera then there is probably some change in focus required as well (either more or less depending on how the subject is moving). Am I correct in assuming that AF.C will keep the focus as the subject moves left to right as well?

If so, does it only track the subject as long as it's kept on the selected AF point? If the subject moves to the right or left of that AF point, I'm assuming focus is lost.

When Expanded Area AF is enabled, does it keep focus only on the slected point and the surrounding (pink) points? Beyond that, focus is again lost?

Generally speaking, is the photographer supposed to maintain framing after focusing and keep the subject centered on the selected AF point at all times? Are you supposed to either pan the camera or zoom the lens to keep the framing while AF.C is active?

I appreciate your help on this. It's one of the last few technical aspects of my camera that I still don't understand. It might be helpful to explain using a scenario, such as a dancer walking off-stage (left-right movement) and a race car going around a bend (X,Y, and Z change) where maybe some zooming is also required to keep the composition as the car moves further away.



10-27-2016, 02:43 PM   #2
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QuoteOriginally posted by lightbox Quote
After re-reading the Perfect focus everytime thread recently I remembered some lingering questions I had regarding AF.C (specifically with AF Select and Expanded Area AF settings). I did some online searches and now understand how autofocus works at a sensor level, and situations where it's useful (sports, kids, etc..). I still have some questions about using it in practice, though.

I know that focus depends on distance and depth of field, so obviously AF.C is great for keeping focus when the subject in the selected AF point moves in the Z coordinate... away from or closer to the lens. How does it work for lateral (X,Y) movement? In theory there's no change in focus distance if the camera moved in parallel to match the subject, but since there's usually angular change between subject and camera then there is probably some change in focus required as well (either more or less depending on how the subject is moving). Am I correct in assuming that AF.C will keep the focus as the subject moves left to right as well?

If so, does it only track the subject as long as it's kept on the selected AF point? If the subject moves to the right or left of that AF point, I'm assuming focus is lost.

When Expanded Area AF is enabled, does it keep focus only on the slected point and the surrounding (pink) points? Beyond that, focus is again lost?

Generally speaking, is the photographer supposed to maintain framing after focusing and keep the subject centered on the selected AF point at all times? Are you supposed to either pan the camera or zoom the lens to keep the framing while AF.C is active?

I appreciate your help on this. It's one of the last few technical aspects of my camera that I still don't understand. It might be helpful to explain using a scenario, such as a dancer walking off-stage (left-right movement) and a race car going around a bend (X,Y, and Z change) where maybe some zooming is also required to keep the composition as the car moves further away.

What camera are you using? I wonder about this too. I have the K-5 and I doubt it knows what my subject is
10-27-2016, 02:58 PM   #3
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QuoteOriginally posted by jack002 Quote
What camera are you using? I wonder about this too. I have the K-5 and I doubt it knows what my subject is
I've got the K-50. There may be more advanced AF in the newer cameras with more and better AF points and low light focusing, but I think the principles should be the same for all of us, hopefully.
10-27-2016, 03:59 PM - 2 Likes   #4
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QuoteOriginally posted by lightbox Quote

If so, does it only track the subject as long as it's kept on the selected AF point? If the subject moves to the right or left of that AF point, I'm assuming focus is lost.

When Expanded Area AF is enabled, does it keep focus only on the slected point and the surrounding (pink) points? Beyond that, focus is again lost?

Generally speaking, is the photographer supposed to maintain framing after focusing and keep the subject centered on the selected AF point at all times? Are you supposed to either pan the camera or zoom the lens to keep the framing while AF.C is active?
You are correct on every point, Lightbox.

Sports and wildlife shooting involves real panning skills ... practice, practice, practice.

A monopod becomes useful.

10-27-2016, 04:10 PM - 1 Like   #5
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QuoteOriginally posted by clackers Quote
You are correct on every point, Lightbox.
Wow... that never happens.

Thanks, clackers!
10-27-2016, 05:15 PM - 1 Like   #6
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On the K-5II when shooting sports - I set the AF mode to AF.C - disable AF for shutter half-press in the camera menu - and keep my thumb on the AF button on the back of the camera. This will eliminate shutter lag when using AF.C and I end up with more "keepers".
10-27-2016, 05:27 PM   #7
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QuoteOriginally posted by lightbox Quote
After re-reading the Perfect focus everytime thread recently I remembered some lingering questions I had regarding AF.C (specifically with AF Select and Expanded Area AF settings). I did some online searches and now understand how autofocus works at a sensor level, and situations where it's useful (sports, kids, etc..). I still have some questions about using it in practice, though.

I know that focus depends on distance and depth of field, so obviously AF.C is great for keeping focus when the subject in the selected AF point moves in the Z coordinate... away from or closer to the lens. How does it work for lateral (X,Y) movement? In theory there's no change in focus distance if the camera moved in parallel to match the subject, but since there's usually angular change between subject and camera then there is probably some change in focus required as well (either more or less depending on how the subject is moving). Am I correct in assuming that AF.C will keep the focus as the subject moves left to right as well?

If so, does it only track the subject as long as it's kept on the selected AF point? If the subject moves to the right or left of that AF point, I'm assuming focus is lost.

When Expanded Area AF is enabled, does it keep focus only on the slected point and the surrounding (pink) points? Beyond that, focus is again lost?

Generally speaking, is the photographer supposed to maintain framing after focusing and keep the subject centered on the selected AF point at all times? Are you supposed to either pan the camera or zoom the lens to keep the framing while AF.C is active?

I appreciate your help on this. It's one of the last few technical aspects of my camera that I still don't understand. It might be helpful to explain using a scenario, such as a dancer walking off-stage (left-right movement) and a race car going around a bend (X,Y, and Z change) where maybe some zooming is also required to keep the composition as the car moves further away.

I have the K30 (same guts as the K50) and K3II. I have actually found more success using center spot focus, while using the 'perfect focus every time' technique. I actually have a couple of entries on the last two pages of the thread "Perfect Focus Everytime" that could help + the newest entry from another member may be helpful as well.
https://www.pentaxforums.com/forums/38-photographic-technique/236970-perfect-...everytime.html

10-27-2016, 09:05 PM   #8
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I don't know what camera you are using, but I used the AF-C for my first real application relatively recently on a K-1. I didn't use the "SEL" for the focus points, but instead used the full 33 autofocus sensors. I also set the focus to the shutter button, contrary to my normal usage, which is usually in non-tracking situations. This was because I figured it would get uncomfortable having to hold the back button down all they time during a two-hour dance performance. If I was going to be constantly tracking, and my fingers would be on the shutter button anyway, it seemed like this would work out better. The tracking worked pretty great, but I also was able to use a semi-reasonable aperture so there would be a good amount of leeway with the depth of field. (I was using a 50-135 and apertures from f/4-8 or so -- I went up to 11 at one point -- and I tried to mostly keep it around 5.6 or higher.

Using the 33 AF points allows the subject to move away from the current AF point, and it will still be detected and refocused on. This handles the horizontal movement you were concerned about.
10-27-2016, 09:53 PM - 1 Like   #9
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QuoteOriginally posted by leekil Quote
I don't know what camera you are using, but I used the AF-C for my first real application relatively recently on a K-1. I didn't use the "SEL" for the focus points, but instead used the full 33 autofocus sensors. I also set the focus to the shutter button, contrary to my normal usage, which is usually in non-tracking situations. This was because I figured it would get uncomfortable having to hold the back button down all they time during a two-hour dance performance. If I was going to be constantly tracking, and my fingers would be on the shutter button anyway, it seemed like this would work out better. The tracking worked pretty great, but I also was able to use a semi-reasonable aperture so there would be a good amount of leeway with the depth of field. (I was using a 50-135 and apertures from f/4-8 or so -- I went up to 11 at one point -- and I tried to mostly keep it around 5.6 or higher.

Using the 33 AF points allows the subject to move away from the current AF point, and it will still be detected and refocused on. This handles the horizontal movement you were concerned about.
When you do that, Leekil, it's not actually tracking.

It's just focusing on whatever is closest to the camera of the 33 points. This may or may not be your intended target. It could be an adjacent dancer or a prop instead of the subject holding the prop.

It's why we have the SEL modes.

Last edited by clackers; 10-28-2016 at 12:08 PM.
10-28-2016, 04:01 PM   #10
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QuoteOriginally posted by clackers Quote
When you do that, Leekil, it's not actually tracking.

It's just focusing on whatever is closest to the camera of the 33 points. This may or may not be your intended target. It could be an adjacent dancer or a prop instead of the subject holding the prop.

It's why we have the SEL modes.
Ahhhh... now I get it the complete picture! Here goes:

Each AF point in the grid just allows the camera to determine the distance to the closest "thing" in front of that AF point (based on contrast information). The camera samples the values and selects the AF point with the closest object as fast as it can focus.

In SEL mode, it's just trying to keep that one AF point in focus regardless of where the subject might move to in the frame. It doesn't follow the subject, really. It has no clue what the subject is. The photographer needs to follow the subject.

....

I was being thrown off by the term "tracking." (Subject) tracking implies that subjects are tracked as they move (presumably across the frame), so I thought that maybe the AF point moved across the grid as the subject does. Now I know. It only does that when not in SEL mode, and it's not following the subject. It's following whatever is closest to any of the AF points.

Perhaps it should be called "closest object detection"

See now I feel like I can make things happen, because I understand what the camera is doing. Thank you!
10-29-2016, 12:01 AM   #11
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QuoteOriginally posted by lightbox Quote
Ahhhh... now I get it the complete picture! Here goes:

Each AF point in the grid just allows the camera to determine the distance to the closest "thing" in front of that AF point (based on contrast information). The camera samples the values and selects the AF point with the closest object as fast as it can focus.

In SEL mode, it's just trying to keep that one AF point in focus regardless of where the subject might move to in the frame. It doesn't follow the subject, really. It has no clue what the subject is. The photographer needs to follow the subject.

....

I was being thrown off by the term "tracking." (Subject) tracking implies that subjects are tracked as they move (presumably across the frame), so I thought that maybe the AF point moved across the grid as the subject does. Now I know. It only does that when not in SEL mode, and it's not following the subject. It's following whatever is closest to any of the AF points.

Perhaps it should be called "closest object detection"

See now I feel like I can make things happen, because I understand what the camera is doing. Thank you!
You can really see demonstrations of this in the Canon/Nikon tracking example videos on YouTube.
10-29-2016, 12:35 AM   #12
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QuoteOriginally posted by clackers Quote
When you do that, Leekil, it's not actually tracking.

It's just focusing on whatever is closest to the camera of the 33 points. This may or may not be your intended target. It could be an adjacent dancer or a prop instead of the subject holding the prop.

It's why we have the SEL modes.

Doesn't it actually track once it has first achieved focus, but the initial point that is "in focus" is chosen by whatever algorithm that the camera uses to focus in AF-33? After that, it tracks what was under that point, once the object moves to another focus point?
10-29-2016, 01:27 AM   #13
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QuoteOriginally posted by leekil Quote
Doesn't it actually track once it has first achieved focus, but the initial point that is "in focus" is chosen by whatever algorithm that the camera uses to focus in AF-33? After that, it tracks what was under that point, once the object moves to another focus point?
Nup ... who told you that?

10-29-2016, 08:44 AM   #14
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QuoteOriginally posted by Kevwaly Quote
You can really see demonstrations of this in the Canon/Nikon tracking example videos on YouTube.
That might be why I missed them (stopped paying attention to CaNikon materials). Thanks, I'll look for them.

*edit: do you know of any that specifically talk about the low-level operation of the AF system? I've seen a few videos where they advertise the different modes and show some examples where they'd be used, but that doesn't help my understanding of what's happening at the hardware / software level. Call me pedantic, but that kind of information helps me.

The best I saw so far was an article by Cambridge In Colour that talked about how the AF sensor points work in terms of how they detect. It also talked about vertical vs. cross-type points, lighting conditions, and ease of focusing on horizontal vs. vertical lines and patterns. It wasn't until this thread though that I understood about how the camera goes about choosing what's in focus when all the AF points are active, and how that changes as things move relative to each other in the frame.

Last edited by lightbox; 10-29-2016 at 09:19 AM.
10-29-2016, 01:55 PM - 1 Like   #15
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QuoteOriginally posted by lightbox Quote
That might be why I missed them (stopped paying attention to CaNikon materials). Thanks, I'll look for them.

*edit: do you know of any that specifically talk about the low-level operation of the AF system? I've seen a few videos where they advertise the different modes and show some examples where they'd be used, but that doesn't help my understanding of what's happening at the hardware / software level. Call me pedantic, but that kind of information helps me.

The best I saw so far was an article by Cambridge In Colour that talked about how the AF sensor points work in terms of how they detect. It also talked about vertical vs. cross-type points, lighting conditions, and ease of focusing on horizontal vs. vertical lines and patterns. It wasn't until this thread though that I understood about how the camera goes about choosing what's in focus when all the AF points are active, and how that changes as things move relative to each other in the frame.
It's been too many moons since I've looked into those videos. I wish my gift was photographic memory for this wonderful craft of photography... considering all the info needed to absorb.
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