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I don't get it why camera manufacturers don't put an af point where the intersection of the "rule of thirds" lie?

Shooting would be a lot better

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I am not 100% sure what you mean. Looking at the viewfinder of my 500D, it looks like EOS 500D view finder via Bob Atkins

(Via Bob Atkins)

If I take the liberty to add "thirds lines", it looks like this:

EOS 500D view finder + "Rule of thirds" lines

Unless you mean "100% spot on!", this looks very much like spots in the intersections of thirds lines. (My handiwork is a bit shoddy on the second picture, I have to admit as well.)

  • Yup. Remember ur camera's view finder is not 100% so ur a bit further that ur masterpiece sketch. I have a 600d. I think we see the same. What more if u have a full frame like 6d. Not sure where af points on 5d my or others. If u use the technique "af focus and re frame" (forgot the right term for u lose ur focus. Annoying. Need to move ur camera parallel to the initial focus to keep it. Grrrrrr. Awkward position. Happy to know I'm not the only one. Any advise to overcome this? – wayne Jan 23 '14 at 12:11
  • So what you really mean to ask is: Why does the camera not focus on what the AF indicators are indicating? Why the AF points are exactly where they are I do not know. I have to admit I never thought about it as they are "close enough" to 3rds for my purposes. I suppose the questions linked to in the comments up there can give more/better information; reframing and potentially "focus, then disable AF" for sequences of images with the same focal plane is what I usually do. – Cornelius Jan 23 '14 at 12:18
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    Thanks Cornelius. Just broadcasting my thoughts. Hope that in a few years someone will make a camera where I want the af to be. – wayne Jan 23 '14 at 12:27
  • This doesn't hold for full-frame camera's. In general their focus points are grouped closer to the center compared with a crop body. – mmumboss Jan 23 '14 at 15:58
  • My answer specifically relates to the EOS 500D, challenging "why do they not …" as some, as it seems, do. I quite happily will say that many other cameras might not have this; but the question as asked at the time of my writing was rather open-ended. – Cornelius Jan 23 '14 at 16:37
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As others have noted, there often are focus points near to the rule of thirds intersection points, but they aren't always precisely in that location. There might be some consumer demand for it, but there are two major reasons why not, both answered, I think, by my answers to What is the “Rule of Thirds”? and What is the 'Golden Ratio' and why is it better than the 'Rule of Thirds?'. They are:

  1. First, the actual rule as originally stated is about division of lines and areas, not about where you would place a point of interest or something to focus on. It's about dividing the frame between ground and sky, not what goes at the intersections. So, having a focus point there isn't generally relevant.
  2. Second, even if you do care about the intersections, there is nothing actually magical about the ¹⁄₃ point, or for that matter the golden section. Despite popular belief, there's no evidence that there is any actual aesthetic benefit to particular numbers, or for that matter that anyone used this rule before the modern day. Off-center composition works (because it generally is more dynamic — centered images and even divisions tend towards stability), but mathematical precision isn't the reason.
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    I have to challenge this: "or for that matter that anyone used this rule before the modern day." E.g. see Mona Lisa or Google for "golden ratio use in painting" (without quotes). Nice thoughts and links though. – TFuto Jan 23 '14 at 15:02
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    Re:"no evidence that there is any actual aesthetic benefit to particular numbers" - well, a large of numbers of successful logos suggest there is some advantage of using them, e.g notice golden ratio in the Apple logo :-) – TFuto Jan 23 '14 at 15:06
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    @TFuto Yes, it's a popular myth, but unsubstantiated by evidence other than questionable line-drawing (which usually disregards the actual dimensions of the painting, or exact location of features, and and makes flexible claims about what is good enough). The Apple logo is actually a good example of this — see it debunked here: fastcodesign.com/1672682/… – mattdm Jan 23 '14 at 15:28
  • Good points! Well, I actually do not have Mona Lisa here, must have left it in my other pants ;-), but I do understand why the lines are at those locations if they truly represent golden ratio. I will check that out. I will need more time for the Apple logo argument (especially because it is on Quora), as the logo changed a lot over time. You might be right with this though. – TFuto Jan 23 '14 at 15:46
  • There's also the little matter of "which thirds?" I shoot at 3:2 because that's the format the camera gives me, but I rarely shoot for 3:2 — it's just not a shape that works as an adequate container for many of my pictures. – user2719 Jan 23 '14 at 22:47
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You will have to observe that different camera makes have different amount and type of autofocus points.

Most entry level DSLR cameras have only one cross-type autofocus points in the middle, and people usually focus with that and then recompose even though there are additional focus points available.

More serious DSLR cameras have more focus points, but again, many people use the center focus point to focus because the optical axis of the lens provides the most incoming light there, so the center focus sensor gets the most light - so the low-light performance is the best.

In general, the other focus points are used for continous focus.

Now, the situation changes if your camera is full frame semi-pro. Your most sensitive sensors are usually on the middle vertical line, and going away from those the sensors get less and less sensitive. Again, use center to get best focus and recompose unless you use continous focusing.

And the big pro cameras (e.g. D3S) have a matrix which is all equally sensitive. There, you will have focus points at rule-of-third positions. Still, consider that the closer you get to the optical axis of the lens, the more light is coming in.

I think I answered your question, but do not hesitate to ask in a comment.

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