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Thread started 13 Apr 2009 (Monday) 19:29
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Is this AF scheme possible?

 
joe ­ mama
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Apr 13, 2009 19:29 |  #1
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The number one thing I looked forward to in a 5DII was a few more AF points that all worked as well as the center AF and more spread out. Ideally, I'd like to see a 7x5 grid of equally spaced AF points with custom order assignments. That's only 35 AF points which is considerably less than what other cameras offer.

However, the AF points on all FF DSLRs are all clustered near the center, so is there a technical reason that they can't be spread out? Also, is there a technical reason that all cannot function as quickly and accurately as the center AF? Surely, even if there are technical problems, they can be more spaced than how they are, and work better than they currently do.

Anyway, I hear great things about Nikon's AF, but I don't know how much better it really is. But if it is a lot better, I wonder why Canon's holding back. Is it because they can't, or because they won't?

Am I alone in thinking that even the old sensor of the original 5D with a killer AF system wouldn't be a huge hit? Anyway, I'm just saying. The 5DII looks like a great machine, and I'd have been falling all over myself to find some way to afford it if it had the AF I wanted. As things stand, I still want the 5DII, but I'm not in a huge rush. In fact, because of that, skipping a generation isn't as hard a thing to do as it would have been otherwise.


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Thalagyrt
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Apr 13, 2009 19:32 |  #2

Short version: distortion and falloff near the edges of the projected image makes phase shift detection less accurate the farther away from center your sensor is.


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jra
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Apr 13, 2009 19:45 |  #3

Thalagyrt wrote in post #7723859 (external link)
Short version: distortion and falloff near the edges of the projected image makes phase shift detection less accurate the farther away from center your sensor is.

That's the way I understand it also. That's why you don't see autofocus sensors towards the edge of the frame and the center point works the best.




  
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basroil
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Apr 13, 2009 20:13 |  #4

jra wrote in post #7723919 (external link)
That's the way I understand it also. That's why you don't see autofocus sensors towards the edge of the frame and the center point works the best.

Pretty sure the main reason is that it's cheaper to manufacture them the same size for all cameras rather than different sizes for each. Right now they have 1.6 crop and 1.3 crop ones, ff ones are just 1.3 crop sensors (at least in the 1ds line)


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Thalagyrt
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Apr 13, 2009 21:11 |  #5

That's another part of the reason, but technically at the edges the detection won't work as well either, so it's both really.


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Daniel ­ Browning
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Apr 13, 2009 21:41 |  #6

joe mama wrote in post #7723844 (external link)
However, the AF points on all FF DSLRs are all clustered near the center, so is there a technical reason that they can't be spread out?

Yes.

joe mama wrote in post #7723844 (external link)
Also, is there a technical reason that all cannot function as quickly and accurately as the center AF?

Yes.

joe mama wrote in post #7723844 (external link)
Surely, even if there are technical problems, they can be more spaced than how they are, and work better than they currently do.

Yes and yes.

joe mama wrote in post #7723844 (external link)
I wonder why Canon's holding back. Is it because they can't, or because they won't?

The fact that Nikon was able and willing to put their newest top-end AF into a midrange $1700 body (D300) indicates to me that using autofocus to segment markets (and increase margins!) is a marketing choice and not a technical one. (Similar to when Canon disabled MLU and ISO 3200 in the Rebel 300D until users found out how to enable them in firmware.)

joe mama wrote in post #7723844 (external link)
Am I alone in thinking that even the old sensor of the original 5D with a killer AF system wouldn't be a huge hit?

Perhaps you meant to say it *would* have been a huge hit? I don't think it would have been a huge hit, because the read noise, live view, resolution, and video features were very important to me. However, I do think a killer autofocus system would have been really great.

Compared to the 5D it has one feature that is very cheap to implement, but very very critical: microadjustment. Canon could probably sell a firmware upgrade to 5D1 owners with that feature for hundreds of dollars.

joe mama wrote in post #7723844 (external link)
Anyway, I'm just saying. The 5DII looks like a great machine, and I'd have been falling all over myself to find some way to afford it if it had the AF I wanted. As things stand, I still want the 5DII, but I'm not in a huge rush. In fact, because of that, skipping a generation isn't as hard a thing to do as it would have been otherwise.

I don't blame you. Hopefully the next successor wont be three years in coming.

basroil wrote in post #7724074 (external link)
Pretty sure the main reason is that it's cheaper to manufacture them the same size for all cameras rather than different sizes for each. Right now they have 1.6 crop and 1.3 crop ones, ff ones are just 1.3 crop sensors (at least in the 1ds line)

I don't think cost is the issue. The $4500 camera AF coverage is only slightly larger than the $500 camera. If cost was the only reason they don't make it bigger, then I would think they could put some of that $4000 to use making it bigger.

Thalagyrt wrote in post #7723859 (external link)
Short version: distortion and falloff near the edges of the projected image makes phase shift detection less accurate the farther away from center your sensor is.

That's correct; however, I don't think that's the reason the sensors are not there. If it was the reason, then it would unfairly penalize anyone who used lenses with no distortion or falloff (particularly the super teles). Even users with lenses that have distortion/falloff could simply elect to not use the outer AF sensors.

So having answered Joe's question ("Yes"), I think I'll use this opportunity to answer a separate, but somewhat related question (;)): What is the technical reason that the AF sensors can't be closer to the edges? Joseph S Wisniewski explained it succinctly in the following post (external link):

Joseph S Wisniewski wrote:
="Joseph S Wisniewski"]
"The main mirror is partially silvered in the center, and a little mirror underneath it, called the secondary mirror, diverts the light downward into the AF sensors. This little mirror swings up and lays flat against the back of the main mirror when the main mirror is up. Because the secondary mirror has to fit under the main mirror and be angled 90 degrees to the main mirror, there is only enough room to cover about 1/3 of the height of the mirror, vertically."

As I understand it, the limitation comes down to the fact that we want to use the viewfinder and focus at the same time. If we had no viewfinder, then it could have AF sensors much further out. It's only because we want to do both at the same time that we're forced to live with AF sensors clustered in the center.

For additional reading, I recommend this post about how autofocus systems work (external link), specifically to answer why the extra light coming through an f/1.4 lens does not help the autofocus system.

Hope that helps,


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Thalagyrt
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Apr 13, 2009 21:44 |  #7

Dan, I think your post needs to be a sticky somewhere. That was very well explained.


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basroil
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Apr 13, 2009 23:25 |  #8

Daniel Browning wrote in post #7724578 (external link)
I don't think cost is the issue. The $4500 camera AF coverage is only slightly larger than the $500 camera. If cost was the only reason they don't make it bigger, then I would think they could put some of that $4000 to use making it bigger.

From a consumer's standpoint that may be true, but sure as hell isn't true on the manufacturing end. Last time samsung upgraded a facility for a larger pane of glass used in LCD screens, they spent $5 BILLION on it. Same goes to any sensor manufacturing system. Even if the cost isn't that high, it'll be no less than 50-100 million to make the right equipment for the job, which includes getting new wafers, new lithography machines, either retrofits to existing machines for manipulating the larger die or entirely new machines if the current ones build more than one type (but roughly the same die size), you'll need to train the engineers to design for the new size, retrofits to testing machines or getting new ones, etc, etc. Every little change takes time and money, which in the end results in a lot of money one way or the other. Also, for the price of an electronic piece on the market (consumer pieces, not specialty stuff), the actual cost to produce something is generally 30-50% of the cost. Another 20% for R&D, minus anything for at cost parts (like xbox 360 was and ps3 just bearly still is), then stick a healthy dose of transportation (~10%) and markup (~20-30%). But guess what, canon doesn't get that last 30% (well, they don't get much after r&d costs generally). Of course, the numbers can vary a lot depending on the actual costs (new ipod shuffle is about 40% parts, 10% r&d, 50% markup, transport, etc, but ALL the money goes to apple (minus a bit of markup for non-apple store sales))


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Daniel ­ Browning
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Apr 13, 2009 23:45 |  #9

basroil wrote in post #7725134 (external link)
From a consumer's standpoint that may be true

I think the problem with your theory is that the AF is already different. If Canon literally used the same autofocus (same coverage, etc.) on a range of cameras, like Nikon, then they would reap the benefit of all the manufacturing costs you described. But they don't. The $4500 is very different from, but has only slightly larger coverage than the $500 camera. So they're not saving cost there. If cost scaled with area, then why does the cheap camera have only slightly less coverage area?

Again, I think it's due to the fundamental technological limitations of viewfinder+PDAF. Manufacturers would build AF sensors out to the edge of the viewfinder, even if it cost an additional $1,000, but it's not not physically possible.


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joe ­ mama
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Apr 14, 2009 01:10 |  #10
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Thanks for the great replies! What I'm missing is why APS-C AF schemes are more spread out than FF schemes. I didn't find an explanation as to why that is the case. Not saying it wasn't given, just that I didn't find it. : )


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podoco
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Apr 14, 2009 07:38 as a reply to  @ joe mama's post |  #11

Daniel - the post with the AF post on dpreview is a great read. It made me realise I was making the wrong assumptions myself!

joe mama - you take fantastic shots already - you don't need a 5DII! :)

Reading Daniel's post, I would actually favour some AF points that were set at more sensitive points than f/2.8. But, I doubt that day would ever come considering zooms are only 2.8 at best :mad:




  
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Apr 14, 2009 09:53 |  #12

Canon keeps the high level AF in the 1-series cameras at this point. I can understand why they do that. This is one of the main reasons I think that the 1D Mk III is the best all around camera that Canon makes - more so than the 5D Mk II. The 1D is just a more versatile photographic tool and can be used to "get the shot" in more varied shooting situations from my view.

Having now used the 1-series and its auto focus system for a while now, it would be hard to go back to anything less......




  
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Daniel ­ Browning
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Apr 14, 2009 09:57 |  #13

joe mama wrote in post #7725652 (external link)
Thanks for the great replies! What I'm missing is why APS-C AF schemes are more spread out than FF schemes.

The AF sensors can only cover areas where they are given any light. If the mirror doesn't give them any light, then there can be no AF sensors at all. The method used to give AF sensors light is to make part of the the mirror semi-transparent, put a mirror behind it, and reflect light down to the AF sensors.

The center 1/3rd of the mirror is partially transparent, and has a secondary mirror behind it reflecting light down to the AF system (which, itself, is composed of yet another mirror, lens, and sensor).

What you can see is that there is not any room to make the secondary mirror any bigger. Even if they made the entire primary mirror partially transparent, there's no room to make the secondary mirror bigger to catch all that light. The only way to make more room is to have a longer back focus distance (more room between the exit pupil of the lens and the sensor).

Nikon's back focus distance is longer than Canon, which may be part of the reason why they have slightly more AF coverage. But it also makes fast wide angle lenses have more expensive designs (more rear elements for retrofocus). Perhaps that's why they don't have a 24mm f/1.4 yet, because it would be more expensive than even the Canon version.

It really helps to look at a diagram. The Canon white paper for the 1Dm3 has some good ones on page 18 if you can track it down. (PM me for a copy if you can't find it.)

EDIT: It comes down to the fact that we only have one source of light (the lens) and 3+ purposes for it: autofocus, viewfinder, sensor. We can't have viewfinder and sensor at the same time, so we share them by having a mirror (one or the other, never both). We can't have the viewfinder and autofocus at the same time, but we can give one of them a tiny sliver of light with a secondary mirror. The viewfinder would be incredibly dark with that tiny amount of light, so we give it to the autofocus (the AE meter may also need more light than that, I don't know). Now that we have live view, I think it would be cool:

* Replace the optical viewfinder with autofocus that covers the entire sensor
* Make the entire mirror partially transparent
* Use a top notch electronic viewfinder (liveview)
* Use the electronic viewfinder and full-sensor phase detect autofocus at the same time!
* The mirror only goes up for taking an actual picture (and even that is optional).

I'm sure I'm not the first person to think of this. I'm looking forward to it some day.


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Yohan ­ Pamudji
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Apr 14, 2009 12:05 |  #14

joe mama,

You're a man after my own heart.

joe mama wrote in post #7723844 (external link)
The number one thing I looked forward to in a 5DII was a few more AF points that all worked as well as the center AF and more spread out. Ideally, I'd like to see a 7x5 grid of equally spaced AF points with custom order assignments. That's only 35 AF points which is considerably less than what other cameras offer.

However, the AF points on all FF DSLRs are all clustered near the center, so is there a technical reason that they can't be spread out? Also, is there a technical reason that all cannot function as quickly and accurately as the center AF? Surely, even if there are technical problems, they can be more spaced than how they are, and work better than they currently do.

The technical reasons for not spreading AF points throughout the frame have been addressed already. But although spreading points all throughout a frame might not be technically feasible, at least the coverage of the 1Ds AF sensors is obviously possible and would've been welcome for the 5D and 5DII. I'm not talking about number of AF points but rather how much of the frame they cover. In particular that diamond layout is a disaster--good for 1.6x crop where there are points very close to the rule of thirds intersections, but terrible for FF where those same points are much further away from the rule of thirds intersections. Focus-recompose + FF + large aperture = asking for trouble.

I'm not nearly as ambitious as you with your 35-point grid. I would've been ecstatic with an 11-point system all cross-type. 9 points in a grid with corners at rule of thirds points, and 1 point on either end (left and right) of said grid.

Anyway, I hear great things about Nikon's AF, but I don't know how much better it really is. But if it is a lot better, I wonder why Canon's holding back. Is it because they can't, or because they won't?

Why can't they? Dismissing silly assertions from Canon execs that an all cross-type sensor array would've made the 5DII too large (:rolleyes:) there's really no reason other than cost cutting and product segmentation. In other words, it's not that they can't; they just won't, not so far anyway. They continue to jealously guard 1-series AF for only 1-series, while Nikon have trickled down their equivalent AF sensor layout all the way down to D300 level. That's been their strategy so far and it'll be interesting to see how long they can hold out in the face of Nikon's willingness to trickle down pro features all the way to prosumer level to the point where it feels wrong to call the D300 a prosumer instead of a pro camera.

Am I alone in thinking that even the old sensor of the original 5D with a killer AF system wouldn't be a huge hit? Anyway, I'm just saying. The 5DII looks like a great machine, and I'd have been falling all over myself to find some way to afford it if it had the AF I wanted. As things stand, I still want the 5DII, but I'm not in a huge rush. In fact, because of that, skipping a generation isn't as hard a thing to do as it would have been otherwise.

5D + better AF would've been amazing. 5DII + better AF would've been even more amazing. I was actually ready with cash in hand for a 5DII but ended up not getting one primarily because of the AF. At $2700 I don't think it's justified to have an AF system inferior to the 50D that goes for closer to $1000, but that's just me. Obviously many feel differently, as the 5DII has apparently sold very well so far.




  
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Apr 14, 2009 12:18 |  #15

For me the AF system isn't terribly important, if the center point is good that will cover me for most things. That being said I still think the AF system in the 5D2, while fine for most of what I would use the camera for if I had it, is a mistake and Canon should have put something better in it.


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Is this AF scheme possible?
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