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:



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]