Autofocus can benefit from f/2.8 lenses

[Daniel Browning]

Many cameras have a special autofocus sensor that becomes active when the maximum f-number of the lens is f/2.8 or faster. It has better performance than the normal autofocus sensors because it has a wider baseline, as illustrated in this diagram from the Canon 1D3 manual:

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

This PDF has a great description of how it works:

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

Sometimes it is said that f/2.8 lenses are better for AF because they allow more light for the AF system; actually, the reason is that it allows the the wider baseline of the f/2.8 AF sensors. Also, there is no improvement in going from f/2.8 to f/1.2.

[Chuck Lee]

That's a good one. Your right, I was just trying to keep it simple.

[peety3]

Sometimes it is said that f/2.8 lenses are better for AF because they allow more light for the AF system; actually, the reason is that it allows the the wider baseline of the f/2.8 AF sensors. Also, there is no improvement in going from f/2.8 to f/1.2.
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The AF sensor needs contrast to focus, right? The AF sensor has a range of EVs over which it can focus, right? Wouldn't a "faster" lens allow in more light, possibly shedding enough light on the AF sensor to focus when a "slower" lens might not put enough light on the AF sensor?

[Daniel Browning]

The AF sensor needs contrast to focus, right?


Yes.



The AF sensor has a range of EVs over which it can focus, right?


Yes.



Wouldn't a "faster" lens allow in more light, possibly shedding enough light on the AF sensor to focus when a "slower" lens might not put enough light on the AF sensor?


No. Let me illustrate. Imagine you're in a dark room with no windows. There is only a two-inch pipe that you can peer through to see a tiny circle of the outside world. Now, hold a cardboard tube up to your eye and look through the pipe. You can still only see a tiny bit of the outside world. If the two-inch pipe was replaced with a much bigger pipe, say, one foot in diameter, you would have a much bigger view. But if you are still holding the cardboard tube up to your eye, it wont help. You have to get a bigger pipe *and* a bigger cardboard tube in order to see anything more.


The lens is the pipe and the tube is the autofocus sensor. Canon cameras only have two types of autofocus sensors: f/5.6 (small cardboard tube) and f/2.8 (large cardboard tube). If your lens is f/4, then your pipe is bigger than your cardboard tube. If your lens is f/2.8, then you can finally switch up to the larger cardboard tube. But f/2 and f/1.4 do not help because you are limited by the cardboard tube, not the pipe.


The way autofocus works is that the AF sensor only sees light from one tiny part of the lens, illustrated by the small circles in the diagram above.

[ShutterbugJohan]

Canon cameras only have two types of autofocus sensors: f/5.6 (small cardboard tube) and f/2.8 (large cardboard tube). If your lens is f/4, then your pipe is bigger than your cardboard tube. If your lens is f/2.8, then you can finally switch up to the larger cardboard tube. But f/2 and f/1.4 do not help because you are limited by the cardboard tube, not the pipe.


So a f/4 lens will not focus faster than a f/5.6; only a jump to a f2.8 will increase the focus speed?



The way autofocus works is that the AF sensor only sees light from one tiny part of the lens, illustrated by the small circles in the diagram above.


In the diagram there are 4 small circles. In actual use, is there one for each AF point?

[ShutterbugJohan]

Another question: what is baseline?

[Daniel Browning]

So a f/4 lens will not focus faster than a f/5.6; only a jump to a f2.8 will increase the focus speed?


Correct. The more important thing is that they are more accurate; in Canon's implementation they are faster, too.



In the diagram there are 4 small circles. In actual use, is there one for each AF point?


Yes, for a "cross type" sensor (e.g. the one central f/2.8 AF point in the 20D, 5D, etc.). Regular f/5.6 points are not "cross type", so they only have two small circles.



Another question: what is baseline?


It's the distance between the small circles. The greater the distance, the higher the accuracy. It's explained in the Split_Prism.pdf link above.

[ShutterbugJohan]

So a f/4 lens will not focus faster
than a f/5.6; only a jump to a f2.8 will increase the focus
speed?


Correct. The more important thing is that they are more accurate; in Canon's implementation they are faster, too.





I'm sorry, but I don't understand your answer. I asked if a f/4 and f/5.6 would be equal in AF speed, and you said. "Correct...in Canon's implementation they are faster, too." Would you mind clarifying your answer? Thanks!







Another question: what is baseline?


It's the distance between the small circles. The greater the
distance, the higher the accuracy. It's explained in the
Split_Prism.pdf link above.





OK, thanks. I've downloaded it and will read it.

[Todd Ovick]

This does not apply to canons' entry level DSLRS such as the rebel series right?

[ShutterbugJohan]

This does not apply to canons' entry level DSLRS such as the rebel series right?


I would think that it does, because the XTi, XSi, and T1i use the AF system from the 20D and 30D. I do not think that it applies to the XS, XT, 10D, and earlier non-pro models, but I'm not sure. Daniel Browning would know. [:#] [H]

[Todd Ovick]

Way cool Stuff!!!!!

[Daniel Browning]

I'm sorry, but I don't understand your answer. I asked if a f/4 and f/5.6 would be equal in AF speed, and you said. "Correct...in Canon's implementation they are faster, too." Would you mind clarifying your answer? Thanks!


What I meant is:

f/4 will not be as accurate as f/2.8. (f/4 and f/5.6 will have the same accuracy.)

f/2.8 will be more accurate, but it may or may not be faster. The speed will depend on specific implementation details, and I think on Canon's system f/2.8 is indeed faster than f/5.6, but the more important thing is that it is more accurate.




This does not apply to canons' entry level DSLRS such as the rebel series right?


Here's the breakdown:

The 1000D does not have any f/2.8 AF sensors, AFAICT.

The 450D, 500D, 30D, and 5D2 all have just one f/2.8 AF sensor.
The 40D and 50D have 9 f/2.8 AF sensors.
The 1D3 and 1Ds3 have 18 f/2.8 AF sensors.

[blabarspaj]

What about the 400D? Does it have the same sensor as the 40D? And why dont the make another dont they makean even larger baseline so one can have the advantage of say f1.4?

[Daniel Browning]

What about the 400D? Does it have the same sensor as the 40D?


No, just a single f/2.8 in the center.



And why dont the make another dont they makean even larger baseline so one can have the advantage of say f1.4?


I don't know, but here are my guesses:



* Conflict with existing AF sensors.
* Lack of space would require a larger camera body.
* Parts cost: new and larger secondary imaging forming lens.
* Manufacturing cost - yet another alignment / calibration.
* Benefits only a small demographic - very few f/1.4 AF lens users.
* Accuracy improvement is not big enough.
* Lens aberrations could make performance worse than f/2.8.

[ShutterbugJohan]

OK, thanks.

[Keith B]

Here's the breakdown:

The 1000D does not have any f/2.8 AF sensors, AFAICT.

The 450D, 500D, 30D, and 5D2 all have just one f/2.8 AF sensor.
The 40D and 50D have 9 f/2.8 AF sensors.
The 1D3 and 1Ds3 have 18 f/2.8 AF sensors.







Why is the 5DmkII the red-headed stepchild of autofocus?! I know this bites me in the rear all the time. The outer focus points drive me crazy daily.

[Yves]

The 5D1 havs just one f/2.8 AF sensor, too.

[apersson850]

To add some information, and correct some:

The 1000D has6 linear normal outer focus points (f/5.6), one normal precision cross type (f/5.6) in the centerand one linear high-precision in the center (f/2.8).
The 400D, 450D, 500D, and 30D have8 linear outerpoints (f/5.6), one normal precision cross type (f/5.6) in the centerandone linear high-precision in the center (f/2.8).
The 5D and 5D Mark II have eight linear outer points (f/5.6), one cross-type normal precision (f/5.6) in the centerand one linear high-precision in the center (f/2.8). They also have six linear assist points around the center, of which two are high-precision (f/2.8). The assist points work only in Servo AF.
The 40D and 50D have nine normalcross-type points (f/5.6) and one cross-type high precision (f/2.8) in the center.
The 1D Mark III/1Ds Mark III have one linear normal precision (f/8) in the center. They have 44 linear normal precison (f/5.6) around the center. They have one linear high-precision in the center at f/4. Finally, they have 18 high-precision (f/2.8)linear points spread among the outer points. Only those points that have one high-precision element are selectable, but the assist points work in all AF modes.
The 7D has 19 cross-type points at f/5.6 and one cross-type high-precision point in the center (f/2.8).



The 1D Mark III, 1Ds Mark III and 7D have a substantial set of custom functions that can be used to fine-tune the performance of Servo AF. The other models have no such tuning ability.


Assist points exist on the 5D, 5D Mark II, 1D Mark III, 1Ds Mark III and 7D. On the 5D and 5D Mark II, they are only in the center and only works with Servo AF. The other models can use assist points in all modes. On the 7D, any point can be the main and any adjacent points the assist points. On the 1D Mark III and 1Ds Mark III, only the selectable 19 points can be main points and only the dedicated assist points can work as such.


Models that have linear high-precision points often combine these with a normal precison point, to make a cross-type point with hybrid sensitivity at f/2.8.


I hope this was understandable and also that it shows the large amount of different solutions that does exist among Canon's cameras.

[Daniel Browning]

To add some information, and correct some:


Thanks! I appreciate the correction and new information.