Autofocus can benefit from f/2.8 lenses

[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.