How many users set the focus point and use AF.S/AF.C?

aleonx3 · 11-21-2011, 03:55 PM

The OP brought up a good question relating to focus point(s) used that I was burning to ask so many times before, but never quite understand it. How does the camera nail the focus when so many focus point is used? This is of course if the user let the camera chooses whatever is the 'optimal' focus distance within the frame, that is if the user uses 'full' auto (dumb idea anyway), I can understand that the camera will choose the proper DOF (aperture) to accommodate, BUT what happen if the user select aperture mode? What are your thoughts?

Here is an example (taken from another user); here the lens is 16-35mm - result is that nothing in focus.

Attached Images

joe.penn · 11-21-2011, 04:26 PM

Originally posted by aleonx3

How does the camera nail the focus when so many focus point is used?

By the highest number of matching points:

Body Set For 5-Point
Focus At 31ft -> 3 out of 5 possible contrast points detected
Focus At 31.5ft -> 4 out of 5 possible contrast points detected
Focus At 32ft -> 1 out of 5 possible contrast points detected

Focus is set trapped at 31.5ft (second one above) and focus confirmation is given...

(PS: That is in theory)

--

aleonx3 · 11-21-2011, 04:48 PM

Originally posted by joe.penn

By the highest number of matching points:

Body Set For 5-Point
Focus At 31ft -> 3 out of 5 possible contrast points detected
Focus At 31.5ft -> 4 out of 5 possible contrast points detected
Focus At 32ft -> 1 out of 5 possible contrast points detected

Focus is set trapped at 31.5ft (second one above) and focus confirmation is given...

(PS: That is in theory)

--

That seems to make sense... however, there are 19 points there (7D, sorry not talking about any of my camera here - just a general question), what about the other points (and why are they not showing up)? There is a least one contrast point a few feet from that cluster, how does it get counted here. I don't see any thing is focus in this sample picture; which is the main reason why I ask. I always thought many focus points is an advantage.

joe.penn · 11-21-2011, 04:59 PM

Originally posted by aleonx3

however, there are 19 points there

I was only using 5 as an example - 19 points would be the same, the higher the matching contrast points the better the match thus traps the focus there.

Originally posted by aleonx3

There is a least one contrast point a few feet from that cluster, how does it get counted here.

Look at the photo, where it counted appears to be in the "line of focus" and thus the contrast point was detected.

..

Tonto · 11-21-2011, 08:11 PM

I use Spot Focus and Centre Metering. I recompose the shot for focusing and use the Exposure lock button if there is a problem with the histogram after chimping-sometimes I will change to spot metering depending on subject. Hardly ever use Autofocus but when I do I will change the parameters away from Spot and let the camera to decide the exposure

aleonx3 · 11-21-2011, 08:45 PM

Originally posted by joe.penn

I was only using 5 as an example - 19 points would be the same, the higher the matching contrast points the better the match thus traps the focus there.

Look at the photo, where it counted appears to be in the "line of focus" and thus the contrast point was detected.

..

Thanks for the explanation; I understand what you are saying to some degree, however, I still don't quite get it. Since it is not Pentax, so I can not understand. If I let the camera to use multi-focus point to auto-focus, I will see a couple of points (most of the time it only shows one) where the camera selected - and the focus are usually locked-on pretty good. The example I have shown you is very bad, the photo is very soft and not sharp at all; that's why I use this example to see what algorithm is used in the camera to achieve focus. Although at this point, I don't know how the user can stand this kind of photos from a supposedly good camera/lens (I would have bailed and switch to another brand).

ducdao · 11-22-2011, 07:29 AM

Originally posted by joe.penn

Are you keeping the same exact (or near exact) distance between the camera and the subject after focusing? Example, after focusing, when you re-frame while half pressing shutter to hold focus, are you:

A: Moving Left/Right
B: Pivoting the subject while keeping the same distance
C: Moving Forward/Backward any?

If "A", there should be no issue as long as the actual distance is the same for the line of focus ***
If "B", there should be no issues as pivoting means no matter where you re-position you would be the same distance as originally focused ***
If "C", there's your issue *** (that is if you are having this issue)

---

99% of time is A, the other 1% is B. So I guess I'm good

LazyLonghorn · 11-22-2011, 08:58 AM

I am trying to understand all this but I find myself in a fog right about now. It is all coming too fast to absorb

ducdao · 11-22-2011, 09:33 AM

Originally posted by LazyLonghorn

I am trying to understand all this but I find myself in a fog right about now. It is all coming too fast to absorb

You need to practice, the more the merrier.

Once you actually try you will absorb faster.

audiobomber · 11-22-2011, 09:36 AM

Originally posted by LazyLonghorn

Could someone please direct me to a good tutorial on the various focus modes available to the K-r user, I am anxious to learn and short on experience. Knowing what, when, and where to use each would sure help me out a lot. Thanks.

This is not specific to the K-r, but the principles are used by all dlsr's: Understanding Camera Autofocus

dragonfly · 11-22-2011, 09:51 AM

Originally posted by borno

I set a back button for AF on my bodies and use servo AF (continuous) that way it focuses when I want it to and I can manual focus whenever I want to. I mostly do wildlife or scenery type stuff, but it is also good for sports. Once you get used to back button AF it is very nice.

I do the same: use the af/if button for focus and disable focus on the shutter button. With constant focus I don't have to keep pressing the focus button to achieve accurate focus- I just hold it down until I'm satisfied or need a new focus reading.

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11-21-2011, 03:55 PM	#31
aleonx3 Pentaxian Join Date: Jul 2007 Location: Brampton, Ontario Photos: Gallery \| Albums Posts: 3,880	The OP brought up a good question relating to focus point(s) used that I was burning to ask so many times before, but never quite understand it. How does the camera nail the focus when so many focus point is used? This is of course if the user let the camera chooses whatever is the 'optimal' focus distance within the frame, that is if the user uses 'full' auto (dumb idea anyway), I can understand that the camera will choose the proper DOF (aperture) to accommodate, BUT what happen if the user select aperture mode? What are your thoughts? Here is an example (taken from another user); here the lens is 16-35mm - result is that nothing in focus. Attached Images

11-21-2011, 04:26 PM	#32
joe.penn Veteran Member Join Date: Mar 2011 Location: Maryland (Right Outside Washington DC) Photos: Gallery \| Albums Posts: 2,902	Originally posted by aleonx3 How does the camera nail the focus when so many focus point is used? By the highest number of matching points: Body Set For 5-Point Focus At 31ft -> 3 out of 5 possible contrast points detected Focus At 31.5ft -> 4 out of 5 possible contrast points detected Focus At 32ft -> 1 out of 5 possible contrast points detected Focus is set trapped at 31.5ft (second one above) and focus confirmation is given... (PS: That is in theory) --

11-21-2011, 04:48 PM	#33
aleonx3 Pentaxian Join Date: Jul 2007 Location: Brampton, Ontario Photos: Gallery \| Albums Posts: 3,880	Originally posted by joe.penn By the highest number of matching points: Body Set For 5-Point Focus At 31ft -> 3 out of 5 possible contrast points detected Focus At 31.5ft -> 4 out of 5 possible contrast points detected Focus At 32ft -> 1 out of 5 possible contrast points detected Focus is set trapped at 31.5ft (second one above) and focus confirmation is given... (PS: That is in theory) -- That seems to make sense... however, there are 19 points there (7D, sorry not talking about any of my camera here - just a general question), what about the other points (and why are they not showing up)? There is a least one contrast point a few feet from that cluster, how does it get counted here. I don't see any thing is focus in this sample picture; which is the main reason why I ask. I always thought many focus points is an advantage.

11-21-2011, 04:59 PM	#34
joe.penn Veteran Member Join Date: Mar 2011 Location: Maryland (Right Outside Washington DC) Photos: Gallery \| Albums Posts: 2,902	Originally posted by aleonx3 however, there are 19 points there I was only using 5 as an example - 19 points would be the same, the higher the matching contrast points the better the match thus traps the focus there. Originally posted by aleonx3 There is a least one contrast point a few feet from that cluster, how does it get counted here. Look at the photo, where it counted appears to be in the "line of focus" and thus the contrast point was detected. ..

11-21-2011, 08:11 PM	#35
Tonto Veteran Member Join Date: Apr 2010 Location: Wellington Posts: 969	I use Spot Focus and Centre Metering. I recompose the shot for focusing and use the Exposure lock button if there is a problem with the histogram after chimping-sometimes I will change to spot metering depending on subject. Hardly ever use Autofocus but when I do I will change the parameters away from Spot and let the camera to decide the exposure

11-21-2011, 08:45 PM	#36
aleonx3 Pentaxian Join Date: Jul 2007 Location: Brampton, Ontario Photos: Gallery \| Albums Posts: 3,880	Originally posted by joe.penn I was only using 5 as an example - 19 points would be the same, the higher the matching contrast points the better the match thus traps the focus there. Look at the photo, where it counted appears to be in the "line of focus" and thus the contrast point was detected. .. Thanks for the explanation; I understand what you are saying to some degree, however, I still don't quite get it. Since it is not Pentax, so I can not understand. If I let the camera to use multi-focus point to auto-focus, I will see a couple of points (most of the time it only shows one) where the camera selected - and the focus are usually locked-on pretty good. The example I have shown you is very bad, the photo is very soft and not sharp at all; that's why I use this example to see what algorithm is used in the camera to achieve focus. Although at this point, I don't know how the user can stand this kind of photos from a supposedly good camera/lens (I would have bailed and switch to another brand).

11-22-2011, 07:29 AM	#37
ducdao Pentaxian Join Date: Dec 2009 Location: Montreal/Vermont Posts: 2,160	Originally posted by joe.penn Are you keeping the same exact (or near exact) distance between the camera and the subject after focusing? Example, after focusing, when you re-frame while half pressing shutter to hold focus, are you: A: Moving Left/Right B: Pivoting the subject while keeping the same distance C: Moving Forward/Backward any? If "A", there should be no issue as long as the actual distance is the same for the line of focus * If "B", there should be no issues as pivoting means no matter where you re-position you would be the same distance as originally focused * If "C", there's your issue *** (that is if you are having this issue) --- 99% of time is A, the other 1% is B. So I guess I'm good

11-22-2011, 08:58 AM	#38
LazyLonghorn New Member Join Date: Nov 2011 Posts: 9	I am trying to understand all this but I find myself in a fog right about now. It is all coming too fast to absorb

11-22-2011, 09:33 AM	#39
ducdao Pentaxian Join Date: Dec 2009 Location: Montreal/Vermont Posts: 2,160	Originally posted by LazyLonghorn I am trying to understand all this but I find myself in a fog right about now. It is all coming too fast to absorb You need to practice, the more the merrier. Once you actually try you will absorb faster.

11-22-2011, 09:36 AM	#40
audiobomber Pentaxian Join Date: May 2008 Location: Sudbury, Ontario Photos: Albums Posts: 6,644	Originally posted by LazyLonghorn Could someone please direct me to a good tutorial on the various focus modes available to the K-r user, I am anxious to learn and short on experience. Knowing what, when, and where to use each would sure help me out a lot. Thanks. This is not specific to the K-r, but the principles are used by all dlsr's: Understanding Camera Autofocus

11-22-2011, 09:51 AM	#41
dragonfly Veteran Member Join Date: Jun 2009 Location: on the wall Photos: Gallery Posts: 715	Originally posted by borno I set a back button for AF on my bodies and use servo AF (continuous) that way it focuses when I want it to and I can manual focus whenever I want to. I mostly do wildlife or scenery type stuff, but it is also good for sports. Once you get used to back button AF it is very nice. I do the same: use the af/if button for focus and disable focus on the shutter button. With constant focus I don't have to keep pressing the focus button to achieve accurate focus- I just hold it down until I'm satisfied or need a new focus reading.