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f2.8 and centre focus point?

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Thread started 22 Aug 2006 (Tuesday) 12:41   
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steveathome
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I have seen somewhere- but not sure where, about the centre focus point being accurate at f2.8?

I am sorry to be so vague with my question, but can anyone give me more information as to what this means?

If your len's have a smaller aperture than f2.8, is focus no longer accurate?

How accurate are the other focus points that are not in the centre

What's the difference between a cross focus point and otherwise?

I would appreciate it if anyone can shed some light on the above for me,
Many thanks

Post #1, Aug 22, 2006 12:41:10




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05Xrunner
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no with the cameras all but the XT...lens with 2.8 or faster it suppose to kick in a cross hair focus point instead of the regular single line point..this is what I understand. so focus would be much more accurate

Post #2, Aug 22, 2006 13:18:10


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tbrasington
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It is claimed that with the 350D that it isn't as accurate as say the 20D. However I have never had any problems when I had the 50 1.4 and with my 24-70 2.8.

Post #3, Aug 22, 2006 13:18:16


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Big ­ Mike
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05Xrunner wrote:
no with the cameras all but the XT...lens with 2.8 or faster it suppose to kick in a cross hair focus point instead of the regular single line point..this is what I understand. so focus would be much more accurate

This what I understand it to be. The centre AF point has a cross type sensor that is only sensitive enough to be active/useful when the lens has a maximum aperture of F2.8 or larger. Otherwise, the centre point is not a cross type, just like the other points.

As far as why a cross type is better....this is how I think it works (I may be wrong). AF works when it sees something that has contrast. With a single direction AF point...it can only detect contrast along or on the line...with a cross type point, it can detect contrast in both directions.

Bottom line is that a cross type sensor will more easily find focus on something that may be lacking in contrast.

Post #4, Aug 22, 2006 13:34:01 as a reply to 05Xrunner's post 15 minutes earlier.


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f2.8 and centre focus point?
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