How about a camera with only f/2.8 and f/2 AF sensors?

[joe mama]

Since a decent number of people have only f/2.8 and faster lenses, why not a variant of some existing models with f/2 and f/2.8 AF sensors instead of the f/2.8 and f/5.6 AF sensors currently used?

I'd like to say f/1.4 and f/2.8 AF sensors instead (2 stops apart, just like the f/2.8 and f/5.6 AF sensors currently used), but then the 85 / 1.8, 100 / 2, and 135 / 2 woudn't see any benefit, and going f/1.4 and f/2 AF sensors would exclude all the macro lenses, as well as many of the popular fast zooms.

Would such a move be unprofitable? That is, we have no choice but to get the bodies with the current AF sensors, so what's the incentive to go to the effort to give us a choice? Or might it not improve the AF speed/accuracy enough to maybe convince at least enough of the people Canon has lost to Nikon to come back and be a profitable move?

[EOS_JD]

Not sure I understand this. The sensors work for ANY lens faster than f2.8/f5.6 - so they will work fine with the f2 lenses. the problem arises when the lens has a smaller aperture than f2.8 for the centre and f5.6 for the other sensors.....

So feel happy the sensors are active for your fast primes :)

[joe mama]

Not sure I understand this. The sensors work for ANY lens faster than f2.8/f5.6 - so they will work fine with the f2 lenses. the problem arises when the lens has a smaller aperture than f2.8 for the centre and f5.6 for the other sensors.....

So feel happy the sensors are active for your fast primes :)


The point is that with f/2 + f/2.8 AF sensors instead of f/2.8 + f/5.6 AF sensors, one "should" get more accurate/faster AF with all your lenses, no? The downside being, of course, that you wouldn't be able to use any lens slower than f/2.8, which, unsurprisingly enough, is not a concern I have. : )

[EOS_JD]

No. You would only get the faster AF with lenses that can open to f2 or f2.8. Only your primes can open up to f2. So all with zooms would not be able to get the faster AF.

Currently we get the faster AF woth f2.8/f5.6 lenses. Making an f2 sensitivity wouldn't make these any faster as they already get the use of the faster AF points.

The 1D series AF points are sensitive up to f8 (now that would be better for all)

[midnight_rider]

The point is that with f/2 + f/2.8 AF sensors instead of f/2.8 + f/5.6 AF sensors, one "should" get more accurate/faster AF with all your lenses, no? The downside being, of course, that you wouldn't be able to use any lens slower than f/2.8, which, unsurprisingly enough, is not a concern I have. : )

Dont bird much do you?
I would have to be the poor guy that has to lug around the 600mm 2.8.
Currently any lens 2.8 or faster can utilize the cross points on the AF system. So your 1.4 prime does benefit from this.

[joe mama]

EOS_JD wrote:
No. You would only get the faster AF with lenses that can open to f2 or f2.8. Only your primes can open up to f2.


That was kinda my point.


So all with zooms would not be able to get the faster AF.


Why not? The AF sensors that were previously f/5.6 would now be f/2.8.


Currently we get the faster AF with f2.8/f5.6 lenses. Making an f2 sensitivity wouldn't make these any faster as they already get the use of the faster AF points.


I don't know what that means. An lens slower than f/2.8 will not be able to utilize an f/2.8 AF sensor.


The 1D series AF points are sensitive up to f8 (now that would be better for all)


It's "better" for those that don't shoot entirely at f/2.8 and below, which was the point of my post.


midnight_rider worte:
Dont bird much do you?


Not unless you count eating chicken and turkey as "birding". : )


I would have to be the poor guy that has to lug around the 600mm 2.8. Currently any lens 2.8 or faster can utilize the cross points on the AF system. So your 1.4 prime does benefit from this.


My f/1.4 prime can also utilize an f/5.6 AF sensor, too, but I wouldn't say it "benefits" from this. It seems that the point of my initial post is being missed by one and all -- I'm not saying for *all* DSLRs to have f/2.8 and f/2 AF sensors only, I'm saying if *some* DSLRs had *variants* that had such AF sensors. For example, a 5DIIX. Would there be enough of an advantage to the faster AF sensors that *some* people would want the *variant*, and would there be enough people purchasing said variant to make it a profitible move on the part of Canon?

I'll take a stab in the dark and guess your answer would be a resounding "no". : )

[midnight_rider]

So if you wanted to shoot birds with this special market camera you would need one of these?
Click here (http://www.youtube.com/watch?v=K77UvaiHii0)

I see what you are saying but to be honest I think it is the most profitable to a company to never limit a camera this way. If a person were to ever want a UWA or a fisheye or a super tele they would have to get a different body.
Cool thought but AF is assisted by the amount of light a lens lets through. So by using a 1.2 lens you are getting better light to the AF system and the only thing slowing you down at that point is the lens AF system.
The 50mm f/1.2 and 85mm 1.2 for example are by nature slow in the Af department. However you will be hard presses to find a prime to be more consistently accurate than them. By forcing these lenses to focus faster you would most likely make the accuracy decrease.

[Panopeeper]

The AF speed has nothing to do with the max aperture diameter. The accuracy is higher with the larger aperture.

However, it is not necessary to increase the aperture. The 1Dxxx models prove, that the electronics can be more elaborate: the high accuracy (phase detection) focusing works already with f/4.

[joe mama]

So if you wanted to shoot birds with this special market camera you would need one of these?
Click here (http://www.youtube.com/watch?v=K77UvaiHii0)


Not at all. If you wanted to shoot birds, you'd either get closer and use a 400 / 2.8L IS, or not get one of these bodies with the faster AF sensors. Not unlike if you wanted 10 fps, you wouldn't get a 5DII.


I see what you are saying but to be honest I think it is the most profitable to a company to never limit a camera this way. If a person were to ever want a UWA or a fisheye or a super tele they would have to get a different body.


I am not talking about "limiting a camera". I am talking about creating another *version* of a camara, not unlike Canon did with s special version of one of the 1.6x bodies for IR work.

As for a fisheye, well, maybe some won't work, but the 15 / 2.8 FE and 10 / 2.8 FE (for 1.6x) would.


Cool thought but AF is assisted by the amount of light a lens lets through. So by using a 1.2 lens you are getting better light to the AF system and the only thing slowing you down at that point is the lens AF system.


This isn't the case. It's not the amount of light, but rather the geometry specified by the minimum f-ratio of the lens.


The 50mm f/1.2 and 85mm 1.2 for example are by nature slow in the Af department. However you will be hard presses to find a prime to be more consistently accurate than them. By forcing these lenses to focus faster you would most likely make the accuracy decrease.


100 / 2, 70 / 2.8 macro, 200 / 2.8L -- I know, I own those three plus the 50 / 1.2L.

[cdifoto]

I don't think you're really understanding the concept of the sensor "rating" Joe. No offense intended of course. However, the "f/2.8 or faster" means a lens that's f/2.8 or faster wide open will benefit with a faster and more accurate autofocus than anything with a smaller maximum aperture. If it's an "f/2 or faster" sensor, it'll be worse.

[joe mama]

I don't think you're really understanding the concept of the sensor "rating" Joe. No offense intended of course. However, the "f/2.8 or faster" means a lens that's f/2.8 or faster wide open will benefit with a faster and more accurate autofocus than anything with a smaller maximum aperture. If it's an "f/2 or faster" sensor, it'll be worse.


Could you elaborate? Are you saying that an f/5.6 AF sensor is faster and more accurate than an f/2.8 sensor, for example? In other words, an f/2 AF sensor would be slower and less accurate than a slower AF sensor since it would require a faster lens to operate? Thus, slower AF sensors are better than faster AF sensors in speed and accuracy? If so, why does Canon advertise f/2.8 AF sensors? For stupid people like me? : )

[cdifoto]

Could you elaborate? Are you saying that an f/5.6 AF sensor is faster and more accurate than an f/2.8 sensor, for example? In other words, an f/2 AF sensor would be slower and less accurate than a slower AF sensor since it would require a faster lens to operate? Thus, slower AF sensors are better than faster AF sensors in speed and accuracy? If so, why does Canon advertise f/2.8 AF sensors? For stupid people like me? : )
I don't know of any sensors advertised as f/5.6 or faster. However, if there were such a thing, it would (in theory) be better than an f/2.8 or faster sensor if an f/4 or f/5.6 lens was used, assuming everything else was equal (but when is it ever? ;)), because it would be sensitive "sooner" - ie on a cheaper, slower aperture lens. It probably wouldn't be much better with an f/2.8 lens or faster though.

They advertise f/2.8 sensors to sell you f/2.8 and faster lenses. Advertising an f/5.6 sensor would neither sound impressive and nor give anyone other than a photographer who needs the shutter speeds an incentive to ditch the kit lens in favor of something expensive.

[joe mama]

I don't know of any sensors advertised as f/5.6 or faster. However, if there were such a thing, it would (in theory) be better than an f/2.8 or faster sensor if an f/4 or f/5.6 lens was used, assuming everything else was equal (but when is it ever? ;)), because it would be sensitive "sooner" - ie on a cheaper, slower aperture lens. It probably wouldn't be much better with an f/2.8 lens or faster though.

They advertise f/2.8 sensors to sell you f/2.8 and faster lenses. Advertising an f/5.6 sensor wouldn't be impressive and wouldn't give anyone other than a photographer who needs the shutter speeds an incentive to ditch the kit lens.


Excuse me for being a bit slow. Let me rephase my question. Consider a 100 / 2 (one of the fastest and most accurate lenses I know of). If it were used on three different hypothetical bodies that were otherwise identical, except one had only f/2 AF sensors, another only f/2.8 AF sensors, and a third with only f/5.6 AF sensors, on which body would the focus be the fastest and most accurate, and by how much?

[Panopeeper]

They advertise f/2.8 sensors to sell you f/2.8 and faster lenses
The limitation is not arbitrary but technical. The role of the maximum aperture is, that the image projected by the two halves of the lens shows greater difference with larger apertures; the aim of the focusing is to achieve equal image in the two halves. This is the same principle as the split focusing screen.

[cdifoto]

Excuse me for being a bit slow. Let me rephase my question. Consider a 100 / 2 (one of the fastest and most accurate lenses I know of). If it were used on three different hypothetical bodies that were otherwise identical, except one had only f/2 AF sensors, another only f/2.8 AF sensors, and a third with only f/5.6 AF sensors, on which body would the focus be the fastest and most accurate, and by how much?
I doubt there'd be any difference. You're talking about a fast lens that maxes out all three bodies spec-wise. In a way it's like asking whether a BMW, M-B, or Ford would be better in stop-and-go traffic. All three meet the minimum requirement - a brake pedal and 5mph capability. Pick the one with the best AC and radio.

[joe mama]

I doubt there'd be any difference. You're talking about a fast lens that maxes out all three bodies spec-wise. In a way it's like asking whether a BMW, M-B, or Ford would be better in stop-and-go traffic. All three meet the minimum requirement - having to hit the brakes right after accelerating to 5mph.


OK. I'm going to have to investigate this further, then. I was under the impression that it would make a significant difference, well, at least "significant" inasmuch as there's a "significant" difference between how an f/4 lens focuses in low light vs and f/2 lens.

[cdifoto]

OK. I'm going to have to investigate this further, then. I was under the impression that it would make a significant difference, well, at least "significant" inasmuch as there's a "significant" difference between how an f/4 lens focuses in low light vs and f/2 lens.
It's all theory at this point, but I don't see how having a crane that can lift up to 50 tons is better than a crane that can lift up to 25 tons when you're lifting a 5 ton object.

It basically boils down to asking yourself: how can a sensor that is only extra-sensitive at f/2 and larger be better than a sensor that is extra-sensitive at f/5.6 and larger?

[joe mama]

It's all theory at this point, but I don't see how having a crane that can lift up to 50 tons is better than a crane that can lift up to 25 tons when you're lifting a 5 ton object.

It basically boils down to asking yourself: how can a sensor that is only extra-sensitive at f/2 and larger be better than a sensor that is extra-sensitive at f/5.6 and larger?


I thought the AF accuracy was supposed to be something like 1 "DOF unit" (whatever that is) for regular AF sensors and 1/3 of a "DOF unit" for high precision AF sensors. Thus, I thought the f-ratio at which the AF sensor became effective would have something to do with the size of the "DOF unit".

So while a high-precision f/5.6 AF sensor might be more accurate than a regular f/2.8 AF sensor, a regular f/2.8 AF sensor would be more accurate than a regualr f/5.6 AF sensor. As to how it would affect AF speed, I have no idea one way or another.

[Duncan Frenz]

I get what you're trying to say Joe, but I think you are confused. Here is a direct quote for the 50D from Canon's website:
The EOS 50D has a high-precision 9-point wide area AF that uses cross-type points at f-stops of f/5.6 or faster, enabling the camera to focus faster, more accurately, and in difficult lighting situations. AF sensitivity is a stunning EV-0.5 to EV18. Adding to this AF performance, the EOS 50D incorporates a diagonally mounted cross-type sensor that is sensitive to both vertical and horizontal lines at f/2.8, perfect for enhanced operation in dim light.

With regard to that which is in red refers to the 9 point AF which utilizes a cross-type sensor at f/5.6 or faster. With regard to the text in blue, it indicates added sensitivity via a diagonal cross type sensor enabled via using a f/2.8 or faster lens. Anything slower would result in less speed and accuracy in both cases and would be further aggravated if your idea were implemented.

To achieve what I believe you are asking, would be to increase the range which the sensors in question could be enabled, i.e. f/8 or higher; not make it more restrictive. To make the requirements more stringent would only make it worse for everyone that had less than the best of glass offered.

Point in case; I am considering the EF 70-200 f/4L and 70-200 f/2.8L. Beyond the obvious attributes such as better light gathering, DOF, etc., I want the f/2.8 for the added sensitivity with the center point. The f/4 will not enable this feature. If Canon were to extend the sensitivity from f/2.8 to f/4, my choice would be easier; I would have the same benefit as that of the f/2.8 regarding AF benefits. To implement your idea, it would make it even more important to purchase faster glass to gain better AF attributes and marginalize a large group of consumers that cannot or do not want to pay for faster glass, but would want to have the benefit of the added sensitivity.

Adding diagonal points to all of the points at f/2.8(while still having cross type sensitivity @ f/5.6) would be beneficial, but I think what you are asking would be moving in the wrong direction.

[joe mama]

I get what you're trying to say Joe, but I think you are confused. Here is a direct quote for the 50D from Canon's website:
The EOS 50D has a high-precision 9-point wide area AF that uses cross-type points at f-stops of f/5.6 or faster, enabling the camera to focus faster, more accurately, and in difficult lighting situations. AF sensitivity is a stunning EV-0.5 to EV18. Adding to this AF performance, the EOS 50D incorporates a diagonally mounted cross-type sensor that is sensitive to both vertical and horizontal lines at f/2.8, perfect for enhanced operation in dim light.

With regard to that which is in red refers to the 9 point AF which utilizes a cross-type sensor at f/5.6 or faster. With regard to the text in blue, it indicates added sensitivity via a diagonal cross type sensor enabled via using a f/2.8 or faster lens. Anything slower would result in less speed and accuracy in both cases and would be further aggravated if your idea were implemented.

To achieve what I believe you are asking, would be to increase the range which the sensors in question could be enabled, i.e. f/8 or higher; not make it more restrictive. To make the requirements more stringent would only make it worse for everyone that had less than the best of glass offered.

Point in case; I am considering the EF 70-200 f/4L and 70-200 f/2.8L. Beyond the obvious attributes such as better light gathering, DOF, etc., I want the f/2.8 for the added sensitivity with the center point. The f/4 will not enable this feature. If Canon were to extend the sensitivity from f/2.8 to f/4, my choice would be easier; I would have the same benefit as that of the f/2.8 regarding AF benefits. To implement your idea, it would make it even more important to purchase faster glass to gain better AF attributes and marginalize a large group of consumers that cannot or do not want to pay for faster glass, but would want to have the benefit of the added sensitivity.

Adding diagonal points to all of the points at f/2.8(while still having cross type sensitivity @ f/5.6) would be beneficial, but I think what you are asking would be moving in the wrong direction.


I hear what you're saying and appreciate the post. So, the bottom line is that slower AF sensors do not take a hit in accuracy/speed, but instead offer the same accuracy/speed to slower lenses? And the "hype" of the cross-sensitive f/2.8 sensor is not that it is f/2.8, but that it is a "cross-sensitive", and that what would make the AF better is if it could be cross-sensitive at f/4 or slower, since it would then work with more lenses?

This makes sense to me.

[macroimage]

Hi Joe,

There might not be room in the bottom of the mirror box for a f/2 sensor. The f/2.8 sensor uses twice the baseline of the f/5.6 sensors already and you can see how much space it takes. I imagine that you are wanting a larger baseline than the f/2.8 sensor for greater accuracy. Especially on a 1.6x crop camera, there probably wouldn't be enough room. Full frame maybe though. It also may not work great in lower light due to vignetting issues. The f/2.8 sensors are pretty good. Usually the limitation is variations in focus from the lens. If you couldn't autofocus below EV2.0 rather than EV-1 would you accept that tradeoff?

[joe mama]

Hi Joe,

There might not be room in the bottom of the mirror box for a f/2 sensor. The f/2.8 sensor uses twice the baseline of the f/5.6 sensors already and you can see how much space it takes. I imagine that you are wanting a larger baseline than the f/2.8 sensor for greater accuracy. Especially on a 1.6x crop camera, there probably wouldn't be enough room. Full frame maybe though. It also may not work great in lower light due to vignetting issues. The f/2.8 sensors are pretty good. Usually the limitation is variations in focus from the lens. If you couldn't autofocus below EV2.0 rather than EV-1 would you accept that tradeoff?


What you are saying is in conflict with what was said above -- that slower AF sensors are superior. So naturually, I'm all confused. Don't worry about it, though, as it is my natural condition. : )

As for the tradeoff, no, I would not accept it. However, I thought that Canon could focus down to "only" -0.5 ev, whereas Nikon could hit -1 ev. Now that I think about it, Nikon is supposedly the AF king, but they don't even have f/2.8 AF sensors, do they?

Anyway, like I said, if confusion could take physical form, I'd be the model. I'm off to sleep now, so I'll check back in the morning. Type real slow, though. When people type too fast, it's hard for me to read it. : )

[Duncan Frenz]

I hear what you're saying and appreciate the post. So, the bottom line is that slower AF sensors do not take a hit in accuracy/speed, but instead offer the same accuracy/speed to slower lenses? And the "hype" of the cross-sensitive f/2.8 sensor is not that it is f/2.8, but that it is a "cross-sensitive", and that what would make the AF better is if it could be cross-sensitive at f/4 or slower, since it would then work with more lenses?

This makes sense to me.
Yes. The benefit of the center point is only possible because technology limits it to f/2.8. If they could make it this sensitive at f/22 they would. Well, marketing might not let that happen, because then you would have one less reason to buy faster lenses.

[EOS_JD]

I hear what you're saying and appreciate the post. So, the bottom line is that slower AF sensors do not take a hit in accuracy/speed, but instead offer the same accuracy/speed to slower lenses? And the "hype" of the cross-sensitive f/2.8 sensor is not that it is f/2.8, but that it is a "cross-sensitive", and that what would make the AF better is if it could be cross-sensitive at f/4 or slower, since it would then work with more lenses?

This makes sense to me.

The AF senors are themselves NOT slower.
They allow slower lenses to take advantage of the faster sensitive AF points.

You are starting to get it though :)

[jacuff]

Yeah I don't think you quite get it based on the posts I've read. There are essential two sensors in your AF sensor. There is a sensor (the center and sometimes more) that has double precision with lenses that are f/2.8 or faster. This is what you've been calling a f/2.8 sensor. Than the rest of the points have regular precision with lenses that are f/5.6 or faster. This is what you've been calling an f/5.6 sensor. You gain NOTHING by lowering those numbers.

The ultimate would be a double precision f/5.6 sensor with normal precision f/11 sensors. This would give you fast and more accurate autofocus with virtually every Canon EF lens and allow the use of a 2x teleconverter and still maintain autofocus. You gain nothing by having f/1.4 double precision and f/2.8 regular precision sensors. You would be limiting the camera to Autofocus with lenses f/2.8 or faster. Hence it would be a very unprofitable move.

[Pixel9ine]

I think the majority of the confusion in this thread stems from a question of nomencalture - assigning names like "f/5.6 sensors" or "f/2.8 sensors" - in this context, it is not a question of speed but of sensitivity.

Most amateur photographers come to see "faster" lenses as better - hence, an f/2.8 lens is faster (or better) than an f/4 lens, or f/1.4 vs. f/2.8, and so on. Hence, it might be natural to think of anything with a smaller f-stop as inherently "better."

No so with AF sensors; remember, they are not "slow" or "fast". To say that an AF point operates at f/5.6 and larger (or, f/2.8 and larger for the centre point) means that, at a minimum, that autofocus sensor requires at least f/5.6 in order to function. Use a lens with an aperture smaller than that and your camera will totally lose the ability to autofocus.

Similarly, the centre AF point can have added sensitivity - but in order to take advantage of it, you'll need an extra 2 stops of light - hence f/2.8.

Example: Have you ever tried to write by candlelight? It's possible, but in order to write more accurately and without mistakes, you need more light - the camera's centre AF point is just like that. It needs more light in order to give you that extra precision.

Now, if AF sensors were made to work only at f/1.4 or f/2.8 like you originally suggested, that would basically mean that the sensors would be doing the same amount of work, but would need more light to do it - making them less efficient.

True progress would be AF sensors that are just as sensitive while needing less light - hence the 1D's ability to autofocus with a lens as slow as f/8.

But, like someone said above, this is all moot... like debating whether a crane can lift 50 tons or only 25, when all you need to lift is 5.

[johnj2803]

To say that an AF point operates at f/5.6 and larger (or, f/2.8 and larger for the centre point) means that, at a minimum, that autofocus sensor requires at least f/5.6 in order to function. Use a lens with an aperture smaller than that and your camera will totally lose the ability to autofocus.



sorry but this confused me a lot :D newbie here... so you mean to say my XS when placed with a lens with f2.8 will not autofocus?

[jacuff]

sorry but this confused me a lot :D newbie here... so you mean to say my XS when placed with a lens with f2.8 will not autofocus?

2.8 is a larger aperture than 5.6. Look at the sizes of the holes in this diagram... http://en.wikipedia.org/wiki/File:Aperture_diagram.svg So when a 2.8 lens is placed on your XS, it will autofocus still. It won't autofocus if you stick an EF Extender II on a 70-200mm f/4L lens, since the new aperture rating will be f/8.

[macroimage]

The issue with the higher sensitivity sensors isn't from having two more stops of light at f/2.8. If that were true, then simply brighter light would allow autofocus at smaller f-stops.

At f/2.8 there are rays at larger annular angles so that you can measure the phase shift. At f/5.6 the light must go through the smaller aperture and therefore the rays are in a tighter cone and there is less phase shift. As the aperture gets smaller, there isn't much left to measure a phase shift. The limitation is due to the acceptance angle of the AF sensor, not the amount of light. If the wide open aperture is too small, the sensor won't receive any light rays to work with at any brightness level that have a measurable phase shift.

[RDKirk]

At f/2.8 there are rays at larger annular angles so that you can measure the phase shift. At f/5.6 the light must go through the smaller aperture and therefore the rays are in a tighter cone and there is less phase shift. As the aperture gets smaller, there isn't much left to measure a phase shift. The limitation is due to the acceptance angle of the AF sensor, not the amount of light. If the wide open aperture is too small, the sensor won't receive any light rays to work with at any brightness level that have a measurable phase shift.

That is part of it. The difference his makes is that the high-precision mode is not a matter of sensitivity, but the fact that when an f/2.8 or faster lens is mounted (and interrogated by the camera), the camera switches on an additional set of arrays set wider apart than the normal-precision sensors.

Those wider-set arrays are not useable except with the faster lenses.

More light does permit faster focusing calculation than dim light, and focusing calculation speed is one of Canon's considerations. This is the primary reason the camera switches off autofocusing entirely when the lens reports a maximum aperture of f/5.6 or less with the consumer cameras--at that point autofocusing speed would fall below satisfactory levels for most people too much of the time.

The 1-Series cameras have an additional cpu to speed autofocusing, so they maintain adequate focusing speed down to f/8.

[EOS_JD]

sorry but this confused me a lot :D newbie here... so you mean to say my XS when placed with a lens with f2.8 will not autofocus?

Yes it will be able to AF. The sensors will alow AF at any lens with a maximum aperture of down to f5.6. (f2.8 is larger than f5.6 just so you are aware)

[joe mama]

jacuff wrote:
Yeah I don't think you quite get it based on the posts I've read. There are essential two sensors in your AF sensor. There is a sensor (the center and sometimes more) that has double precision with lenses that are f/2.8 or faster. This is what you've been calling a f/2.8 sensor. Than the rest of the points have regular precision with lenses that are f/5.6 or faster. This is what you've been calling an f/5.6 sensor. You gain NOTHING by lowering those numbers.

The ultimate would be a double precision f/5.6 sensor with normal precision f/11 sensors. This would give you fast and more accurate autofocus with virtually every Canon EF lens and allow the use of a 2x teleconverter and still maintain autofocus. You gain nothing by having f/1.4 double precision and f/2.8 regular precision sensors. You would be limiting the camera to Autofocus with lenses f/2.8 or faster. Hence it would be a very unprofitable move.


Now I think I get it! However, per RDKIrk's post here:


RDKirk wrote:
That is part of it. The difference his makes is that the high-precision mode is not a matter of sensitivity, but the fact that when an f/2.8 or faster lens is mounted (and interrogated by the camera), the camera switches on an additional set of arrays set wider apart than the normal-precision sensors.

Those wider-set arrays are not useable except with the faster lenses.

More light does permit faster focusing calculation than dim light, and focusing calculation speed is one of Canon's considerations. This is the primary reason the camera switches off autofocusing entirely when the lens reports a maximum aperture of f/5.6 or less with the consumer cameras--at that point autofocusing speed would fall below satisfactory levels for most people too much of the time.

The 1-Series cameras have an additional cpu to speed autofocusing, so they maintain adequate focusing speed down to f/8.


So, high precision AF sensors require a wider set of arrays and thus also require faster lenses? That is, it is not possible to make high precision AF sensors (of the same basic design) at f/5.6? Could the arrays be spread even further apart making them even more precise when lenses f/2 or faster are on the camera?

Also, Nikon reportedly has better AF than Canon. Why is this? Do they use a different AF sensor design, or is it the processing power in-camera?

[johnj2803]

Yes it will be able to AF. The sensors will alow AF at any lens with a maximum aperture of down to f5.6. (f2.8 is larger than f5.6 just so you are aware)

Thanks... :D i still dont get why it is still an issue for the XS not being f2.8 sensitive.

[Daniel Browning]

There is a lot of misinformation and many misconceptions in this thread. I suggest the following reading material for a quick introduction about how autofocus *really* works:

http://doug.kerr.home.att.net/pumpkin/Split_Prism.pdf

Also check out the animations on this page:

http://www.nikon.com/about/technology/core/software/caf/index.htm

And see the 1D Mark III white paper, which includes this diagram (also in many Canon autofocus patents):

http://thebrownings.name/images/camera/Canon_Autofocus_Diagram.jpg


Since a decent number of people have only f/2.8 and faster lenses, why not a variant of some existing models with f/2 and f/2.8 AF sensors instead of the f/2.8 and f/5.6 AF sensors currently used?


I don't think the removal of f/5.6 AF sensors is a marketable option because, as you stated, it would completely disable autofocus with f/4 and f/5.6 lenses. The furor that would erupt over Canon releasing a camera that doesn't autofocus with over half of its lenses would be enormous. Like Nikon's removal of the autofocus motor in the very low end DSLR (which did not even affect the most popular lenses), it will cause bad PR no matter *how* many disclaimers and warnings they put on the camera; particularly because the biggest sellers are the f/3.5 and slower zooms.

Even if Canon could live with that, the more important issue is probably that the market of photographers that only use f/2.8 and faster lenses is so very much smaller that I don't think Canon could reach the volume necessary for a reasonable price (i.e. less than $10,000). As much as I would love it, you and I are in a tiny minority.

Similarly, f/2.8 cannot be removed (to do f/2 and f/5.6 only) because the use of f/2.8 lenses dwarfs f/2 shooters by a very wide margin, just as f/2.8 is dwarfed by f/3.5 and slower. The only possible option would be to add a third sensor for f/2.

The f/2 AF sensors would be great thanks to the increased accuracy and utility in low light; however, I can identify at least two problems. First of all, lens aberrations. The f/2.8 AF sensors only look at a tiny portion of the f/2.8 ring of glass, which, in turn, is only a portion of the total glass in the lens. (This is clearly demonstrated in the above diagram.) On the other hand, when we look at an f/2.8 image, we're seeing a convolution of the *entire* f/2.8 ring of glass, and, more importantly, the *entire* aperture of the lens (from the center f/32 all the way up through f/4). So even if the f/2.8 ring is somewhat bad, the rest of the lens can make up for it. To see how bad the outer ring is would require an aperture stop that was opaque in the center and clear around the outside edge.

So, again, the f/2.8 autofocus sensor does not look at the entire lens, just one tiny portion of the very worst part of the lens. But even so, there are enough lenses with well corrected aberrations that autofocus works well enough for f/2.8 autofocus.

Second, there is necessarily an increase in cost and space required thanks to the additional and wider secondary imaging forming lens and AF sensor. I don't know how significant the cost would be, or how the additional parts would affect the design given the amount of space available for the AF module, but if it increases cost by 5% or makes the camera slightly larger, and only 1% of the market will utilize the feature, then the manufacturer may decide it's not worth it. Even when features cost nothing, but are used by less than 1% of the market, Canon will not bother to implement them (e.g. raw histogram, auto iso in manual, etc.) Money is made by catering to the majority, and Canon exists to make money, not cameras.

As for all the misinformation and misconceptions in this thread, I think Panopeeper and macroimage have done a nice job of correcting them, but I'll repeat their statements for emphasis:

Panopeeper: "The limitation is not arbitrary but technical. The role of the maximum aperture is, that the image projected by the two halves of the lens shows greater difference with larger apertures; the aim of the focusing is to achieve equal image in the two halves. This is the same principle as the split focusing screen."

macroimage: "The issue with the higher sensitivity sensors isn't from having two more stops of light at f/2.8. If that were true, then simply brighter light would allow autofocus at smaller f-stops. At f/2.8 there are rays at larger annular angles so that you can measure the phase shift. At f/5.6 the light must go through the smaller aperture and therefore the rays are in a tighter cone and there is less phase shift. As the aperture gets smaller, there isn't much left to measure a phase shift. The limitation is due to the acceptance angle of the AF sensor, not the amount of light. If the wide open aperture is too small, the sensor won't receive any light rays to work with at any brightness level that have a measurable phase shift."

And I may as well add a few comments of my own:

I doubt there'd be any difference.

I kindly think you are mistaken. With the 100mm f/2 lens, the an f/2 af sensor would measure a greater phase shift than an f/2.8 af sensor, which in turn would meausure a greater phase shift than the f/5.6 AF sensor.


To implement your idea, it would make it even more important to purchase faster glass to gain better AF attributes and marginalize a large group of consumers that cannot or do not want to pay for faster glass


Agreed.


If Canon were to extend the sensitivity from f/2.8 to f/4, my choice would be easier...if they could make it this sensitive at f/22 they would.


It's not a matter of "sensitivity". The f/2.8 sensors physically point to completely different parts of the lens. An f/4 AF sensor, no matter *what* the technology, will never measure the phase shift of the f/2.8 af sensor because it doesn't accept light from that angle. While f/4 AF sensors would be better than f/5.6, and I can understand your desire for them, but they'll never be as accurate as f/2.8.


The ultimate would be a double precision f/5.6 sensor with normal precision f/11 sensors. This would give you fast and more accurate autofocus with virtually every Canon EF lens and allow the use of a 2x teleconverter and still maintain autofocus.


No, the ultimate would be antigravity pants and world peace, but I'm not holding my breath for those either. :)


You gain nothing by having f/1.4 double precision and f/2.8 regular precision sensors. You would be limiting the camera to Autofocus with lenses f/2.8 or faster. Hence it would be a very unprofitable move.

The fact you have missed is that the precision is fundamentally and directly tied to the f-number by the very nature of phase detect autofocus. As Panopeeper mentioned, electronics are an important element, but no amount of electronics are going to make an f/11 AF sensor anywhere near as accurate as the current f/5.6 sensors.

I think the majority of the confusion in this thread stems from a question of nomenclature - assigning names like "f/5.6 sensors" or "f/2.8 sensors" - in this context, it is not a question of speed but of sensitivity.


No; it's not a question of sensitivity, but of geometry.


Most amateur photographers come to see "faster" lenses as better - hence, an f/2.8 lens is faster (or better) than an f/4 lens, or f/1.4 vs. f/2.8, and so on. Hence, it might be natural to think of anything with a smaller f-stop as inherently "better."


This is a good point. When all you have are f/2.8 AF sensors, having an f/2 or f/1.4 lens makes absolutely no difference in AF performance. But of course, that's tangential to the OP's post, since he's talking about adding a new f/2 AF sensor.


Similarly, the centre AF point can have added sensitivity - but in order to take advantage of it, you'll need an extra 2 stops of light - hence f/2.8.


It's not about light, but angles.

I hope this post is helpful.

Kind regards,

[joe mama]

There is a lot of misinformation and many misconceptions in this thread. I suggest the following reading material for a quick introduction about how autofocus *really* works...I hope this post is helpful.


*Extremely* helpful -- thanks! So, if I can summarize my understanding of your post. Yes, f/2 AF sensors would improve focus accuracy (but what about speed?) for lenses that are f/2 and faster. This was the impetus of my post

But, as the "speed" of the AF sensor is all about geometry, it would seem that f/2 AF sensors could the coexist with the current f/2.8 and f/5.6 sensors -- it is not a matter of one or the other.

However, you feel that the additional cost of implementing such sensors would not go well since so few people have such lenses. Here, I have to disagree. I would argue that the number of people with f/2 or faster primes that could make good use of such sensors is quite large. So long as the f/2 AF sensors could coexist with the current f/2.8 and f/5.6 AF sensors, I think they would be *very* welcome. The issue, of course, is cost.

I don't have the slightest idea how much more it would cost to add f/2 AF sensors to the camera, but given how many buy L primes, I would think that, at the very least, it's certainly viable as an alternative model, such as a 5DIIX.

[Daniel Browning]

You're welcome.

Yes, f/2 AF sensors would improve focus accuracy (but what about speed?) for lenses that are f/2 and faster.


The only possible improvement in focus speed would be due to the increase in accuracy. For example, the software algorithm may choose to only repeat the focus check / lens drive sequence three times instead of four.


However, you feel that the additional cost of implementing such sensors would not go well since so few people have such lenses. Here, I have to disagree. I would argue that the number of people with f/2 or faster primes that could make good use of such sensors is quite large.

I hope I am wrong, because I would love it.

[EOS_JD]

Thanks... :D i still dont get why it is still an issue for the XS not being f2.8 sensitive.

THe XS doesn't have the extra sensitivity of the 2.8 sensor mainly because it's an entry level camera. Basically what it's doing is not providing the extra sensitivity for 2.8 and faster lenses because it's assumed those buying the entry level model won't need it or because it's assumed they won't buy fast lenses. Probably not the case for many but that's what you pay for.

[Duncan Frenz]

It's not a matter of "sensitivity". The f/2.8 sensors physically point to completely different parts of the lens. An f/4 AF sensor, no matter *what* the technology, will never measure the phase shift of the f/2.8 af sensor because it doesn't accept light from that angle. While f/4 AF sensors would be better than f/5.6, and I can understand your desire for them, but they'll never be as accurate as f/2.8.


I probably worded my response poorly, but was only trying to show the backward thinking of limiting AF focus to faster lenses. The point with my example between the 70-200 f/2.8 and f/4 was only to illustrate this, even though it may not be possible.

To say that f/4 sensors will never be as accurate as f/2.8 sensors is true in the sense that whatever improvements made to an f/4 sensor would presumably be made to an f/2.8 sensor, therefore the f/2.8 will always be more accurate. However, future advancements may make f/4 sensors(and others) as sensitive/accurate as f/2.8 sensors are currently.

[Pixel9ine]

It's not about light, but angles.

I hope this post is helpful.Very helpful - thank you for explaining AF so clearly!

[johnj2803]

thanks for the clarifications :D

based on this, if I was just an enthusiast with an XS, the absence of a f/2.8 sensitivity in my AF sensors would really wont matter much, since i can still use those said lenses :D

right? :D