AF points


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chjing

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Sep 11, 2006
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#1
hi, I have some queries about the 7 AF points of my 350D. Hope anyone can explain.
Before i ask anything, all of us know that we can use either all 7 AF points on, or choose any AF point on.

1. When using all 7 AF points on and shoot, i realised only a few or one AF point will blink. How does the camera choose these points? and wat happened to those not blink? meaning those areas not focused?

2. When using only 1 AF point, does it mean that the remaining 6 AF points not focused and comes out blur?

3. When shooting a person, use which mode(7 or 1)?

4. Is it true that if I use 1 AF point to shoot a person and align that point at the person's face, only the face will be focused, the neck, shoulder will be out of focused?

I am confused. Please help.
 

raptor84

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Dec 6, 2005
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#2
1. Normally the camera will select which points to use and will blink red once focus is confirmed. Supposedly what lines inside those dots will be in focus and what are not in those may not be in focus (depending on DOF)

2.When you chose what AF point to use then you would on focus on what ever is under that AF point so anything else in the scene may or may not but in focus again depending on the DOF. t

3. Its personal but most people perfer to use only 1 AF point to ensure that only what they want is in focus as the camera does not really know how to decide what your subject is.

4. Depends on your DOF again. Using a long lens with a large aperture and close distance will result in a very shallow DOF and sometimes may only render one eye in focus.

You can google around for articles explaining the concept of DOF :)
 

#3
personally, i really only use centre point focus. this is so i know exactly what i'm focusing on and can lock and move if i need to.

DOF can be adjusted using your aperture settings. if u have the Canon Eyes of EOS book, it explains aperture and its correlation with DOF well in there. if not, there are plenty of articles on the internet. don't forget wikipedia either.

centre point focus is just my personal preference and i think that tehre are many associated good/bad points with it.

i'd be interested to see everyone elses reply on how they use their focus points too. i could use a few good pointers (no pun intended =P)
 

raptor84

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#4
Because of my subjects I tend to use the center paoint more to avoid cutting off ears :D. Ther eare times when i do use the left or right point more so i do not have to keep re-composing though.
 

chjing

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Sep 11, 2006
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#5
Thanks for the explanation.
But if you only use 1 AF point, only the subject(eg face) will be in sharped, the rest not of the body not sharp. How to make the rest of the body sharp too? cos i dun think you want only the person's face to be focused.
 

raptor84

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#6
Thanks for the explanation.
But if you only use 1 AF point, only the subject(eg face) will be in sharped, the rest not of the body not sharp. How to make the rest of the body sharp too? cos i dun think you want only the person's face to be focused.
Like I mentioned, read up on DOF and how focal length, aperture and focus distance all affect it. The af points only hlep tell the camera what elements of the picture are in focus, its up to the photographer to control exactly how much else will be in focus.
 

#7
Which focus point the camera chooses depends on which options u might choose. For new cameras they have the face-priority, so the camera will focus on the face.

In my nikon camera, there is the option of choosing for "nearest object" or to fix the AF point you want.

The object of AF is jus to tell the camera which is the main subject of the photo and therefore that area has to be most in-focused as compared to the rest of the picture. As described by others, the rest of the areas are then determined by aperture, DOF..etc.
 

pRoLoS3r

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Nov 15, 2005
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#8
Since he/she is asking about AF points, Can I ask about something related to it too???

Nowadays D80 cameras come with 9 AF point, 400D 7 AF point, 350D 5 or 7 AF point and the D70 is 5 AF point if i not wrong.

What is the differences between the numbers of AF point??

Do they make focusing more accurate or?? If someone dont mind, can enlighten me what is the point of having of more AF points and how it works?

Thanks. :D
 

Splutter

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Dec 28, 2003
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#9
Generally if you set to multiple AF points, the subject with the most contrast will be locked on. This doesnt mean that only that subject will be focused though. It just means that whatever subject is on the same focal plane (eg, same distance from your sensor) will be in focus as well. Generally I choose the AF point nearest to the thing I want to be in focus. For instance when shooting general portrait, I enable the right most AF point such that when the cam is in portrait orientation, the AF point will be somewhere near my subject's eye. I try to avoid the focus and recompose type of shooting unless I'm shooting wider angles.

pRoLoS3r bro!

Having more AF points just means that you can have a wider selection to choose from so that the AF point is near to what you want to be in focus. Of course if you set to enable all AF points, it'll mean that the chances of having something in focus is higher. This is more applicable for sports or simple scenes with 1 subject and 1 background where you can just point your camera and shoot and usually the subject nearest to you will be in focus.
 

Jul 17, 2005
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Clementi
#10
i use a 350D too and i fully utilise all AF points. i never stick to just the centre point alone. reason being that the 'focus-recompose' method will fail horribly if you use it with lenses like the 85/1.8 where the DOF is really really thin.

the failings of the AF system in the 350D is that only the centre point works well in low light.
 

pRoLoS3r

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Nov 15, 2005
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Yio Chu Kang, Singapore
#11
Thanks for your replies. :D. I think i roughly get it le.

i use a 350D too and i fully utilise all AF points. i never stick to just the centre point alone. reason being that the 'focus-recompose' method will fail horribly if you use it with lenses like the 85/1.8 where the DOF is really really thin.

the failings of the AF system in the 350D is that only the centre point works well in low light.
Isaiahfortythirtyone, can u kindly enlighten me abt the failure on the focus-recompose method?

Maybe u can gimme some links to read on? or a key word to search on???

Just pondering.. AF focusing uses the image it receives to process when the lens is at its wide open??
means, the AF detection uses the image tt occurs at 1.8 if u use a len 50mm f1.8 and AF detection at 2.8 if the lens is 17 - 40L f2.8?

Thank you for ur help.
 

raptor84

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Dec 6, 2005
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#12
Yes all the lenses are wide open until you press the shutter then its electronically stopped down. The focus-recompose method is where you use one afpoint to lock focus then shift your camera to re-compose the scene while maintinign the focus lock on your subejct. The problem occours when your DOF is no narrow that a slight shift renders the subject OOF...
 

Splutter

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Dec 28, 2003
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#13
Yes AF is done when the lens is wide open. That's why bigger aperture lenses can AF easier, though there are trade offs at times.

When you focus at say 15cm and recompose, the subject to sensor plane may not be 15cm anymore. For macro, this is extremely critical since the focal point usually has a threshold of less than 1cm. This means that anything out of the 14cm - 16cm range will be out of focus.
 

unseen

Senior Member
Dec 14, 2004
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#15
i use a 350D too and i fully utilise all AF points. i never stick to just the centre point alone. reason being that the 'focus-recompose' method will fail horribly if you use it with lenses like the 85/1.8 where the DOF is really really thin.

the failings of the AF system in the 350D is that only the centre point works well in low light.
XD I think that's a failing in almost all of Canon's dSLRs...
 

#17
Hi

I have a few questions, please help.

1. wat if the point that i intend to focus on don't not blink after i lock focus? what must i do to make the camera lock onto the desired spot.

2. wat is the correct way of focusing, in terms of camera setting when i need to take a photo of a group of people.

Thanks for helping.

Keith :)
 

cantaresg

New Member
Feb 23, 2007
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Woodlands
#18
Hi

I have a few questions, please help.

1. wat if the point that i intend to focus on don't not blink after i lock focus? what must i do to make the camera lock onto the desired spot.

2. wat is the correct way of focusing, in terms of camera setting when i need to take a photo of a group of people.

Thanks for helping.

Keith :)
Generally I'd use the focus/ recompose technique since I seldom use a narrow DOF anyway. So in this case you can do the same too. Use the focal point closest to the point you intend to focus on, lock the focs and recompose. That way, you should be able to ensure a reasonable focus.

If you have a group of people, it will be better to use a smaller aperture to ensure that everyone is in focus.
 

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