Whats the point of having multiple focus point?


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android17

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When i got my DSLR, i was using multi segment / zone mode for autofocus. However the camera keeps selecting focal points that is not what I wanna focus on. As such i switched to spot focus and from then on I hv been using that.

Just wanna ask, whats the use of the multi segment AF mode? Is there any advantage compared to the old "center focus, half press and recompose" method or focus point selection?
 

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limwhow

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#3
When i got my DSLR, i was using multi segment / zone mode for autofocus. However the camera keeps selecting focal points that is not what I wanna focus on. As such i switched to spot focus and from then on I hv been using that.

Just wanna ask, whats the use of the multi segment AF mode? Is there any advantage compared to the old "center focus, half press and recompose" method or focus point selection?
Hello, android17. Thank you for raising this thread.
Allow to express my limited knowledge in this.

1. Using auto mode and allowing the camera to find its own AF point/s is the most convenient. Almost like going on 'A' mode and let it take control. Difficult to create the precise DOF that one may want.

2. Spot focus (meaning using individual AF points by scrolling through them) is great because the eye of the subject will always be tack sharp. Only disadvantage is - many cameras in the market do not have enough AF points spread evenly at the periphery of the view finder and thus often we are forced to choose an AF point that approximates the eye of our subject,

3. Semi-pro models of different Camps have this issue - their metering follows the centre AF and not any other perhipheral AF points. So you can achieve sharpness on the eye of the subject by using a peripheral AF point, you metering does not follow that selected AF point and thus you are met with exposure problem. Only pro series have metering (particularly spot metering) that follows the selected AF points.

Thus use the various selected AF points to keep recomposition down to a minimal. But be mindful about the metering issue and move the centre of the VF to the part that is to be metered, set the setting, and then move back and use that peripheral AF point to focus on the subjects eye.
Can get used to it one lah...
 

Rendell

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#4
But be mindful about the metering issue and move the centre of the VF to the part that is to be metered, set the setting, and then move back and use that peripheral AF point to focus on the subjects eye.
Learn something useful again.
Thanks Limwhow.
 

LenzLaw

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Multiple focus points allow u to keep a subject in focus, esp when they are not at the centre of the frame.

Lets say u are taking a full body portrait shot of a model, and u have only the centre focus point but want the model's head to be absolutely in focus. How are u going to change ur focus to the head of the model then? If u have only the centre point ur focus will always be on the subject at the middle of the frame. This is especially important if have a shallow DOF that might be due to a large aperture, as focusing on one part of a large subject (such as a person) might not mean that the rest of the subject will be in focus.

So, if u have many focus points, u will be able to focus on something that is not at the centre of the frame, a scenario that u will definitely encounter very often. IMHO thats the most impt reason for having many focus points.

BTW, I usually select my AF point manually and don't use multi segment/zone mode very often since I usually shoot at places that have many subjects that can potentially confuse the autofocus of the system. Multi segment/zone mode would be more useful if u are shooting with just one object in the frame i.e. a toddler running around in an empty room. In this case the chances of confusing the AF mechanism of the cam would be very much reduced.

HTH, just my 2 cents.
 

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night86mare

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#7
When i got my DSLR, i was using multi segment / zone mode for autofocus. However the camera keeps selecting focal points that is not what I wanna focus on. As such i switched to spot focus and from then on I hv been using that.
well, letting the camera selecting the af point is just plain silly if you ask me, i never really got the hang of it. the only time when i use that is when i have no choice, i.e. on my cheap slr which ONLY has predictive af.

no control, you let the machine decide.
 

android17

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#8
Thx everyone for the input.

I understand the advantages of focus point selection, what I mean is the multi af selection whereby the camera selects a few zones automatically for you. I have never see the point of setting my camera to use this af mode, thus wanted to ask whats the use of it (besides landscape)
 

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night86mare

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#9
I understand the advantages of focus point selection, what I mean is the multi af selection whereby the camera selects a few zones automatically for you. I had never see the point of setting my camera to use this af mode, thus wanted to ask whats the use of it (besides landscape)
er, i don't think there's really a use for it in serious photography.. i think it's really more for people who just adopted the camera, to help them "walk" and slowly get used to having more control over a camera instead of what they're used to in a P&S.. which as the name suggests, just p&s, let the camera do everything for you.

there is a high rate of error. selecting the specific zone is already hit and miss enough as it is already, let alone giving your camera (which isn't exactly the smartest pencil in the box) free reign to choose as it sees fit.

i think the only use for it is to increase sellability by camera manufacturers. if the camera is easy to use, more auto functions, hey ho presto, more people will buy it.
 

daredevil123

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#10
It is useful to capture fast moving subjects... eg. kids, birds. I know the birding community uses AF tracking a lot. But usually the AF tracking systems in lower models really CMI.

And about using other AF points apart from the center one... This really comes into play when using very thin DoF, especially in portrait work. If you do a slight adjustment of focusing with center AF point and recompose, and the DoF is extremely thin, when you recompose you actually change the distance between you and the intended target of focus. With a razor thin DoF, your shot will now be OOF. See here for more info:
http://blog.duncandavidson.com/2008/09/focus-and-recompose-exposed.html
 

daredevil123

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#11
Hello, android17. Thank you for raising this thread.
Allow to express my limited knowledge in this.

1. Using auto mode and allowing the camera to find its own AF point/s is the most convenient. Almost like going on 'A' mode and let it take control. Difficult to create the precise DOF that one may want.

2. Spot focus (meaning using individual AF points by scrolling through them) is great because the eye of the subject will always be tack sharp. Only disadvantage is - many cameras in the market do not have enough AF points spread evenly at the periphery of the view finder and thus often we are forced to choose an AF point that approximates the eye of our subject,

3. Semi-pro models of different Camps have this issue - their metering follows the centre AF and not any other perhipheral AF points. So you can achieve sharpness on the eye of the subject by using a peripheral AF point, you metering does not follow that selected AF point and thus you are met with exposure problem. Only pro series have metering (particularly spot metering) that follows the selected AF points.

Thus use the various selected AF points to keep recomposition down to a minimal. But be mindful about the metering issue and move the centre of the VF to the part that is to be metered, set the setting, and then move back and use that peripheral AF point to focus on the subjects eye.
Can get used to it one lah...
We have spot metering, center weighted and evaluative (matrix for Nikon) available even on the lower models of most DSLR brands. Even the Nikon D3000, and my old D40x has those options. In the canon camp, I think only the 1000D has no spot metering but every other Canon DSLR has it. Spot metering follows your AF point. Center weight is the center of the frame. Evaluative is auto.

By default, the final exposure values are calculated and applied at the point when the picture is taken and you press the shutter fully. So if you use spot metering by half-press and recomposing, the exposure is actually calculated at the af point, right when the picture is taken (which is no longer on your point of focus). So take note of that. If you want to lock the exposure values at that particular spot where you half-press, you need to either 1) turn on the option of AE-L with half press shutter, or 2)hold your AE-L button when recomposing.

The same principle applies to center weighted and evaluative. The exposure values are re-calculated and applied right when the picture is taken. Use the AE-L if you want to apply the exposure values during halfpress.
 

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ortega

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#12
great for long tele lenses, where the background is really creamy and the subject is sharp
in this case the machine will be better than the human

i couldn't get the hang of it until i tried it with a super tele :)

sports, birds, safari ... ...
 

limwhow

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#13
... Spot metering follows your AF point. Center weight is the center of the frame. Evaluative is auto.

By default, the final exposure values are calculated and applied at the point when the picture is taken and you press the shutter fully. So if you use spot metering by half-press and recomposing, the exposure is actually calculated at the af point, right when the picture is taken (which is no longer on your point of focus). So take note of that. If you want to lock the exposure values at that particular spot where you half-press, you need to either 1) turn on the option of AE-L with half press shutter, or 2)hold your AE-L button when recomposing.

The same principle applies to center weighted and evaluative. The exposure values are re-calculated and applied right when the picture is taken. Use the AE-L if you want to apply the exposure values during halfpress.
Hello, daredevil123.
Thank you for clarifying ths point for me.
Please correct me if I am wrong. What you are saying is - whichever camp, Nikon or Canon, that we are using, when we set to Spot-metering, the metering point will follow the AF point that we have selected.
I would be most happy to know of this! I have always understood that only bodies like the 1D series have this property (of metering following the AF points), and not the 5D series. Maybe I am wrong after all.
Once again, thank you for pointing it out.
 

NaCha

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Oct 15, 2009
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#14
What you are saying is - whichever camp, Nikon or Canon, that we are using, when we set to Spot-metering, the metering point will follow the AF point that we have selected
From my experience with Canon 40D, the spot-metering is based on the centre AF point and does not follow the selected AF point. Pretty sure of that because I actually tested it out. I think the manual (or was it 40D Field Guide) stated that too.
 

daredevil123

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Oct 25, 2005
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#15
Hello, daredevil123.
Thank you for clarifying ths point for me.
Please correct me if I am wrong. What you are saying is - whichever camp, Nikon or Canon, that we are using, when we set to Spot-metering, the metering point will follow the AF point that we have selected.
I would be most happy to know of this! I have always understood that only bodies like the 1D series have this property (of metering following the AF points), and not the 5D series. Maybe I am wrong after all.
Once again, thank you for pointing it out.
From my experience with Canon 40D, the spot-metering is based on the centre AF point and does not follow the selected AF point. Pretty sure of that because I actually tested it out. I think the manual (or was it 40D Field Guide) stated that too.
NaCha, wow if what you say is true, I would think Canon's spot metering system is severely handicapped. But I am now looking into this, to see if this is true.

As for Nikon, I am very sure that Nikon's spot metering area follows the selected AF point. I've used it many times. This is true even for the cheapest entry level DSLRs like D40, D40x, D60, D3000. It is stated plainly in the manual. The manual also point out an important note: If the lens is a non-CPU one, or if "Auto-area" is selected for the AF area mode, selecting spot metering will force the camera to meter the center focus point only.
 

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roger8

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Sep 20, 2005
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#16
I have a simple question:

If only the focussed point i.e. in a 2D flat plane is in focus, then for multi-point focus, which plane is in focus? ;)
 

daredevil123

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#17
OMG OMG! you guys are right. Canon's spot metering (at least for the 50D), meters on center AF point only. See this:

Spot metering linked to AF point: All Nikon digital SLR cameras have this feature built-in, and the D300 is no exception. With the camera in spot metering mode, the currently-active AF point is the spot meter, while on the Canon 50D, the spot meter is always at the center AF point only.
http://www.dpnotes.com/canon-50d-vs-nikon-d300-compared/

As for Nikon, the mid level models (d90) and up, you can adjust the area size of the center weighted metering. You can make it a spot (6mm) as well. I remember this option is not available on D40x or D60, so probably not available on the D3000. D5000 I am not sure.
 

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limwhow

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#18
NaCha, wow if what you say is true, I would think Canon's spot metering system is severely handicapped. But I am now looking into this, to see if this is true.
Daredevil123, thanks for checking it out. Like NaCha, I have also tested my AF points with the metering on my 5DMkII. Also doesn't follow. But really would love to get to the bottom of this, for Canon cameras.
 

daredevil123

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#19
Daredevil123, thanks for checking it out. Like NaCha, I have also tested my AF points with the metering on my 5DMkII. Also doesn't follow. But really would love to get to the bottom of this, for Canon cameras.
Dude, I am sorry to tell you, you are right. 5D series' spot metering is in the center of the frame with a 6mm area.

I still can't believe it that Canon did not do AF point spot metering for models as high end as the 5Dmk2. That is terrible. I mean, it can't be that expensive to do right? Seeing how Nikon can even incorporate it into their cheapest DSLR - D40 (even though it only had 3 selectable points). I am still in shock...

Thanks for pointing it out. I guess we all learnt something today.
 

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limwhow

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#20
Dude, I am sorry to tell you, you are right. 5D series' spot metering is in the center of the frame with a 6mm area.
Haha.. Only 1D series has it. I'm not sure if the 7D has it or not.
But in this department, Nikon certainly has done exceedingly well. Upz for Nikon.
 

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