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Cross-type High-precision f/2.8 Autofocus with Tamron 17-50 f/2.8

Amol Kumar , Feb 01, 2008; 09:12 p.m.

Hey,

I currently own a Rebel XT and Tamrom 17-50 f/2.8.

I understand that the newer Rebels (Xti, Xsi) and the XXD's (20D, 30D, 40D) all feature the "cross-type 9-point f/2.8 high-precision auto focus". Meaning, with lenses that have a f/2.8, the camera will focus faster/"better", preciser etc...

My question is whether the Tamron 17-50 f/2.8 will take advantage of this, if I upgrade to a 30D (or Xti). Will it work with 3rd Party lenses, or does it have to be a Canon brand f/2.8 to take advantage of this feature?

Thanks, Amol

Responses


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Pascal Harvey , Feb 01, 2008; 09:33 p.m.

From what i know, it's only the center one that is cross-type f/2.8 on the Rebels, the others are vertical or horizontal 5.6 only. I'm pretty sure your XT have it too.

However, on the 40D it's all cross-type 5.6 with 2.8 for the center one.

Amol Kumar , Feb 01, 2008; 09:44 p.m.

Yeah, the XT does have cross-type in the center, but not "high-precision with a f/2.8 lens". Well, according to Bob Atkins see here: http://www.bobatkins.com/photography/digital/eos_digital_rebel_xt_vs_20d.html

"The EOS 20D provides full cross-type performance with maximum apertures as small as f5.6, 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. The autofocus system of the Digital Rebel XT is inherited from the film Rebel XT body and has normal precision f5.6 sensors."

So, does it have to be a "Canon" f/2.8 lens, of will the Tamron work.

Mark U , Feb 01, 2008; 10:34 p.m.

f/2.8 is f/2.8 regardless of whether the lens is Canon or Tamron. For cameras that offer extra precision AF measurement with f/2.8 and faster lenses, the aperture, not the manufacturer, decides. Whether the lens is capable of focussing that accurately may be a different matter. For example, it seems that the Tamron 90mm f/2.8 macro can focus more precisely than the Canon 100mm f/2.8 macro.

http://www.nnplus.de/macro/Macro100E.html

Amol Kumar , Feb 01, 2008; 10:52 p.m.

Thanks Mark,

I just wanted to be sure. I had thought, perhaps there is special "Canon" electronics/chip, special programming or maybe USM is required.

Kinda like how a Sigma flash can use only the center auto-focus point, whereas the Canon 430EX can use all 7 points. (on my XT)

Thanks

Chris JB , Feb 02, 2008; 12:20 a.m.

From another review, only the center FP on the 20d is `high precision` I`m sure 30d same but 40d has many more:)

`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 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. `

The Tamron 17 50 2.8 does focus quicker and easier than slower lenses such as Sigma 15 30 f3.5-4.5 and EF28 135 IS but not as quick as primes like EF28 2.8 135 2.8 (less elements to move I guess) still it does work well on my 20d.

Jim Strutz - Anchorage, AK , Feb 02, 2008; 05:17 a.m.

Where did you hear that the XTi or XSi has any high precision AF points? I've not seen this anywhere. The center AF point might be a cross type sensor, but I don't think it's high precision.

On the 20D/30D the center AF point is both cross type and high precision -- others are mono directional and standard precision. As I understand it, these high precision points are actually high precision in only one direction. All the 40D's AF points are cross type, but still only the center one is high precision, and it is a double cross type. It offers high precision in both directions with f/2.8 lenses, and standard precision in both directions with slower lenses. It's a clever design, and one of the reasons the 40D's focusing is more accurate and reliable.

Amol Kumar , Feb 02, 2008; 11:29 a.m.

Jim,

I was going on some of the comments from dpreview.com, for the 400D/XTi:

"Canon has decided to bite the bullet and simply installed the considerably better nine point AF sensor from the EOS 30D in the EOS 400D. This affords both faster, more accurate focusing as well as center point AF which is compatible with F2.8 lenses."

There is some comment about the XSi having "Cross-type F2.8 at center".

So, I guess, my assumption (my interpretation) was that the comments the XTi 9-point being "from the 30D", and the comment about being "compatible with 2.8", meant that it had the "high-precision" feature.

Of course, now that I look at the wording, you are right, they don't mention "high-precision AF points", just the 2.8 thing.

Which then, leads to another question. If the center is not f/2.8 "high-precision" then what does the comment "2.8 compatible" or "Cross-type F2.8 at center" mean. My Rebel Xt is compatible with my Tamron 2.8 too. Why make a reference to 2.8, why not 5.6? Perhaps it's just marketing jargon.

References:

http://www.dpreview.com/reviews/canoneos400d/page3.asp

(in the square labeled "Nine-point Auto Focus sensor"

http://www.dpreview.com/news/0801/08012403canoneos450d.asp

(in the grid specification section, under "Auto-Focus")

Jim Strutz - Anchorage, AK , Feb 02, 2008; 01:56 p.m.

Well, okay then, I had not seen that before for the XSi. What "with f/2.8 cross-type centre point" must mean is that with f/2.8 lenses it becomes cross type, and that would mean high precision in one direction, just like the 20D/30D.

So it looks like the new 450D (XSi) has a high precision cross type sensor in the center, but the 400D (XTi) does not, although the 400D's might still be cross type -- just not high precision.

Amol Kumar , Feb 02, 2008; 02:16 p.m.

Actually, I think, that the Xti (400D) does also have the high-precision AF point (due to the AF points being taken from the 30D). Also, on Bob's page here lists it as a feature.

http://www.bobatkins.com/photography/digital/canon_eos_400D.html

"For smooth operation and flexibility needed for creative expression, the EOS 400D DIGITAL has a 9-point AF sensor that enables fast and high-precision focus detection plus enlarged subject coverage. Convenient cross keys provide fast and easy AF point selection. For more accurate shooting and additional flexibility, focusing points are in a diamond-shaped layout with a central AF point offering cross-type focusing, regardless of lens brightness. In addition, higher precision focusing is possible when using lenses brighter than f/2.8."


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