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06-09-2012, 04:19 PM   #1
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Selecting focussing area-Centre v Auto

This is not just a Pentax beginners question but a complete beginners question. Could someone help me understand please what the difference is between Centre and Auto focusing . Why can't I simply use Centre every time to focus on the object I'm shooting? Am I not in a better position than my camera to know what I want to focus on? Thanks

06-09-2012, 04:26 PM   #2
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The auto focus point feature can work quite well, but most of the time I do as you do and use the center point and recompose. In my experience, the auto focus point system sometimes locks on the wrong thing. So I tend to leave the camera in center point AF mode.
06-09-2012, 05:08 PM   #3
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QuoteOriginally posted by TylerD Quote
can't I simply use Centre every time to focus on the object I'm shooting?
Yes you can. And lots of people, myself included, do. I think the multi-point focus is OK if you are taking a picture of a forest and do not really care which tree is in focus. I prefer to know which tree I am shooting at.

Which leads to another question, I always see everyone bragging/commenting on how other systems have more or better AF points. But I just do not understand all the excitement, Pentax has (11 points??) but I only use one of them so Pentax already has 10 times the AF points I would ever use.

Maybe I am totally missing something or misunderstanding the whole AF thing and I look forward to someone more knowledgeable than me providing a good explanation.
06-09-2012, 06:23 PM   #4
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I choose the focus point for each shot, as it eliminates focus errors caused by focus and recomposing. But other than that the other focus points are very useful for tracking movement.

But for still shots with longer lenses? One AF point would do you just fine.

06-09-2012, 06:32 PM   #5
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I grew up with center-weighted metering. (Actually I grew up with a hand-held meter, but my first and favorite in-camera meters were center-weighted.) And focusing meant zone-focus or pace-the-distance focus. Are indeed times when in-camera focus and metering aids are useful, when it's good to set a point, and concentrate on it, and be right there. But I mostly use MF lenses which force my K20D to use center-focus and -meter. I don't worry about focus-and-recompose rites. My goal is to nail the subject. Almost everything else can be finessed in PP. That includes composition. I've been composing in PP for many many decades. If a selected point allows nice in-camera compositions, great! If not, no problem. Cropping cures many ills.
06-09-2012, 06:51 PM   #6
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Not all images have the subject in the center, but you can focus with the center point and then adjust the composition. If you are limited in camera movement, say on a tripod, but your subject is near the side, that is when you can select a particular focus point on the side to use. If you are at an airshow where there is only a plane in clear sky, you could use the auto-select just fine as there is only one thing to focus on and the camera will find it.
06-09-2012, 07:55 PM   #7
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QuoteOriginally posted by jatrax Quote
Which leads to another question, I always see everyone bragging/commenting on how other systems have more or better AF points. But I just do not understand all the excitement, Pentax has (11 points??) but I only use one of them so Pentax already has 10 times the AF points I would ever use.
With a wider array of (more) focus points, spanning more of the frame, you can dial in the point you want more effectively to directly AF on the subject without it being centered. With Pentax, the nine cross type points are all clustered close together in the center, making it much less effective to purposely dial in another. The two on the outside aren't cross type, so they're only good to f./5.6, which limits their use and accuracy. I can see why some people get excited about having fiftysomething points to choose from, with some near the edges or corners of the frame. It's not enough to make me want to change systems though, it's just something I wish Pentax would catch up with - that and smaller points, the current ones are an aging system that they should have improved on awhile ago.
06-09-2012, 08:54 PM   #8
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QuoteOriginally posted by Philoslothical Quote
you can dial in the point you want more effectively to directly AF on the subject without it being centered.
So the only advantage of multiple focus points is that you can manually select which one to use? Wouldn't it be faster to use center point, focus, and recompose? I am really struggling to understand this. I assume I am over-simplifying or completely missing something. Surely there is more to all this auto-focus hype or is it really all marketing hype?

QuoteOriginally posted by Philoslothical Quote
smaller points, the current ones are an aging system
I'll agree 100% on smaller points, even trying to focus on the eye of subject sometimes it just grabs an eyelash or something.

06-09-2012, 09:19 PM   #9
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QuoteOriginally posted by jatrax Quote
So the only advantage of multiple focus points is that you can manually select which one to use?
No, I'm saying it's one advantage. It's supposed to be better for tracking moving subjects and such, too. Also, the points are going to be smaller when there are 50+ of them crammed into the same frame we use (or larger if it's FF, obviously, but the point holds), and smaller points are more likely to accurately grab what you're going for.

Focus and recompose has its limitations. When we focus on something, especially with a large aperture, it's a fairly narrow band that's in focus. Recomposing shifts the angle of that band, which can cause what you originally focused on to end up softer than it should be. This is really easy to visualize at macro magnifications, where the DoF is very thin and you're close to subject. A couple degrees can make a big difference there. At "normal" ranges, the effect is less prominent, but it can still happen if you're not careful.

QuoteOriginally posted by jatrax Quote
I'll agree 100% on smaller points, even trying to focus on the eye of subject sometimes it just grabs an eyelash or something.
Or even a nose. Hell, I remember one thread where someone kept having focus stolen by a mic in front of a singer (which he then interpreted as front focusing, but that's beside the point - pardon the pun). The few large points we're all used to suck. I suspect that Nikon's reputation for superior AF when compared to Pentax is less due to wizardry in the system itself, and more to do with the AF sensors' size, placement and quantity. I could be mistaken on that though.
06-09-2012, 11:41 PM   #10
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Why not use both?

I use center point for when I'm composing shots carefully and where I want to pick my subject, multi point for birding - except when they're perched and 'pick a point' for when i'm using the tripod.
06-10-2012, 12:35 AM   #11
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QuoteOriginally posted by jatrax Quote
So the only advantage of multiple focus points is that you can manually select which one to use? Wouldn't it be faster to use center point, focus, and recompose? I am really struggling to understand this. I assume I am over-simplifying or completely missing something. Surely there is more to all this auto-focus hype or is it really all marketing hype?
.
Not for moving subjects! I photograph a lot of trains, typically want the smolebox in focus, but rarely will this be in the centre of the shot for compositional reasons. Now I could crop, but I would prefer to compose in camera and get the highest quality image

Bittern startles Pheasant | Flickr - Photo Sharing!

06-10-2012, 08:53 AM   #12
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QuoteOriginally posted by StephenHampshire Quote
Not for moving subjects! I photograph a lot of trains,
So on this shot you selected the focus point you wanted to use? By using the menu thingy to move the focus point?

On that shot I would have used center point, locked up the smoke stack then rotated the camera slightly to compose and shot. I started out with manual focus so this AF stuff doesn't come naturally. I keep thinking I am missing something or doing something wrong. (and I probably am)

Really cool shot by the way!

QuoteQuote:
multi point for birding
Ok, I can understand that especially for BIF as the bird is likely the only thing with enough contrast and using center point would be difficult. But if something is stationary or large enough to be able to put the center point on the spot you want to focus on, then what is the advantage of 11 points (or 50 points)? Doesn't that just mean the camera has more opportunity to pick the wrong thing to focus on? Sorry, this is making my head hurt.
06-10-2012, 08:59 AM   #13
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QuoteOriginally posted by jatrax Quote
On that shot I would have used center point, locked up the smoke stack then rotated the camera slightly to compose and shot.
The band of focus is thinner on the camera side, so with the train coming toward you, the extra time to recompose would probably have thrown off the focus. It takes enough time for the AF to lock to be a pain sometimes, especially when being approached by the subject.

@StephenHampshire - is that a pheasant in front of the smoke plume? Nice touch.
06-10-2012, 09:13 AM   #14
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QuoteOriginally posted by jatrax Quote
So on this shot you selected the focus point you wanted to use? By using the menu thingy to move the focus point?

On that shot I would have used center point, locked up the smoke stack then rotated the camera slightly to compose and shot. I started out with manual focus so this AF stuff doesn't come naturally. I keep thinking I am missing something or doing something wrong. (and I probably am)

Really cool shot by the way!



Ok, I can understand that especially for BIF as the bird is likely the only thing with enough contrast and using center point would be difficult. But if something is stationary or large enough to be able to put the center point on the spot you want to focus on, then what is the advantage of 11 points (or 50 points)? Doesn't that just mean the camera has more opportunity to pick the wrong thing to focus on? Sorry, this is making my head hurt.
As the train is approaching at 25mph, and I am taking a series of shots , moving the camera is not an option, so I selected a right of centre point on the K20D. In that particular shot there was probably enough depth of field to use centre point, but I find selection easy even with the camera held to my eye (although my K5 is NOT as easy, a backwards ergonomics step IMHO)
I never quite trust the auto focus select, it is easily fooled by large areas of brightness with low contrast- such as the exhaust plume from a steam loco, and the typically black smokebox can sometimes cause problems too. Of course in SEL mode it is perfectly easy to use centre spot, and I have the OK button set up to drop the selected point back to centre.

@Philosophical, Yes it is a pheasant, didn't even see it when I was shooting, a bit distracted by 90tons of loco and 180 tons of carriage, not to mention the rather splendid sound effects! On processing the sequence I got a couple of the bird taking off!
06-10-2012, 10:35 AM   #15
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QuoteOriginally posted by Philoslothical Quote
The band of focus is thinner on the camera side, so with the train coming toward you, the extra time to recompose would probably have thrown off the focus.
Ok, that makes sense to me and possibly explains why I never get clear shots of moving objects. But honestly I rarely shoot anything that is not sitting still anyway.

So in the train example would you use AFS or C? If using AFS it seems to me that it would not matter if you selected a focus point because the focus would be off almost instantly as the train moved forward.

QuoteQuote:
I never quite trust the auto focus select, it is easily fooled by large areas of brightness with low contrast- such as the exhaust plume from a steam loco,
Which is why I use center point, every time I have tried the multi-point the camera locks up on something besides what I want it to. I will have to try the select function, I just assumed it would be too slow to use in the real world and dismissed it as nothing but a gimmick.
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