400D 9-pt AF (f2.8 support)

[CasonLyn]

Hmm, noticed this on the advert for 400D, what does 'f2.8 support' means? Does that mean 350D 7-pt AF does not focus properly with f2.8 or bigger aperture?? can't be rite?

 

[Denosha]

The center AF Point is a cross-type sensor which when used with a F2.8 or better lens gives you better AF accuracy since it detects both the horizontal and verticle axes.

 

[blithevincent]

still don't understand...can be more specific?

 

[adamadam]

I was also wondering about that "f/2.8 support" thing, but from Denosha's explanation, it gives better AF accuracy - so it will be more focussed? or faster?. Thanks Denosha

 

[spartacus.ret]

in general, the af sensors w/o f/2.8 support don't work as well as those with f/2.8 support. its just the quality of the af sensor.

 

[Slivester]

Yes, it simply meant that the AF works best with f2.8, especially prime lens, to enhance the usability of auto-focus. In actual sense, it doesn't matter much, kind of a "wow" factor actually.

 

[Denosha]

The extra sensitivity and precision of the cross sensor helps in low-light conditions where AF can be iffy. Here's the quote from the 20D technical report (http://www.canon.com/camera-museum/tech/report/200409/report.html):
"(1) Wide-area, high-speed, high-precision 9-point AF
The EOS 20D features a newly developed 9-point AF sensor (see Fig. 2). The center AF point is a high-precision cross-type sensor compatible with f/2.8 large aperture lenses (vertical line-sensitive at f/2.8, horizontal and vertical line-sensitive at f/5.6). The eight other AF points are sensitive to f/5.6(horizontal line-sensitive at top and bottom focusing points, vertical line-sensitive at the other six points), which is standard sensitivity for the EOS series.
The baselength of the center AF point sensor sensitive to f/2.8 is twice as long as f/5.6 compatible sensors. Also, all sensors sensitive to f/5.6 except for the top and bottom focusing points have a 30% longer baselength than the EOS 10D, assuring more accurate AF. In addition, the EOS 20D offers improved low-light AF performance by extending the focusing range in low light by 1-stop to EV-0.5 to 18. This is one stop better than the EOS 10D."

and the 20D press release:
"The sophisticated 9-point autofocus system has a high-precision cross-type sensor in the center position. This new design seen for the first time in the 20D model provides full cross-type performance with maximum apertures as small as f/5.6, yet it achieves up to 3 times the standard focusing precision when used with EF lenses featuring maximum apertures larger than or equal to f/2.8. A new, convenient Multi-controller provides fast and easy AF point selection."

The 400D is using the same AF system as the 20D btw.

 

[oeyvind]

The 400D is using the same AF system as the 20D btw.

Actually it's same as 30D not 20D, so it's a slighly better AF.

 

[Denosha]

Eh? Don't the 30D and 20D share the same AF system?

 

[oeyvind]

Eh? Don't the 30D and 20D share the same AF system?

almost the same... but not.

Though the 9-point autofocus system and all aspects of its configuration and operation are the same as that of the 20D, the algorithm controlling autofocus in the 30D has been refined (though Canon has not revealed what has been addressed). The 20D's autofocus system is the most capable we've ever encountered in a midrange digital SLR; if you want better, you have to spend a lot more. We hope that Canon's tinkering with autofocus in the 30D only makes it a better performer than the already-capable 20D. Strangely, while many of the refinements in the 30D first appeared in the 5D last fall, the 30D did not get the 5D's 6 additional Assist AF points clustered around the centre.

source: Rob Galbraith DPI and from the white paper

The AFsensor/modes/speed, selection ease, AF-assist beam and other AF-related specifications are the same as the EOS20D’s. With regard to AFprecision, further fine-tuning has been incorporated compared with the EOS20D.

 

[Stoned]

Essentially, translated into practical usage, with an f2.8 or brighter lens, the camera will be more likely to be able to lock focus accurately with the centre AF point, especially in difficult situations like low-light scenarios. You can actually try this out for yourself, use an F2.8 or brighter lens in a fairly dim environment and try out the difference between the centre AF point and one at the side. The centre one will have a higher likelihood of locking focus.