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04-22-2010, 06:55 AM   #1
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Center AF point accuracy compared to the others

I've read somewhere that the center af point (also for other brands I guess) are more accurate.

Is this because of the cross type sensor at the center? Or are all the cross type af points in the current safox equal in capability?

I am aware of the DOF problem when you focus then recompose, so let's assume that we do not use this method in this case .

tia

04-22-2010, 08:52 AM   #2
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Hi grinmode,

My understanding is that the center focus point is "+" shaped and the 8 surrounding cross type sensors are "T" shaped, with the base of the T pointed down in the top row, up in the bottom row, and inward from the sides. The center point, with a equal length sections in both directions, is a bit larger and more sensitive to borders in both directions than the T shaped ones. The two "l" shaped sensors at the sides of the landscape frame are more sensitive to a horizontal contrast border. The red AF sensor indicators that light up are smaller than the actual sensors, and only estimate the approximate position of the corresponding sensor.

These are assumptions made from informal tests to map out the actual sensor areas. Anyone with better information, please feel free to correct any part of this.

Scott
04-22-2010, 06:27 PM   #3
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thanks snostorm .

I never thought they would be 'T' types. All reviews (where they are mentioned) state 9 cross types.
04-22-2010, 09:18 PM   #4
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QuoteOriginally posted by GrinMode Quote
thanks snostorm .

I never thought they would be 'T' types. All reviews (where they are mentioned) state 9 cross types.
T is cross type.
Any cross type sensor is actually two linear sensors.
Canon's 2.8 sensor is double cross type sensor with enlarged longer linear parts.

04-22-2010, 10:11 PM   #5
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QuoteOriginally posted by Zebooka Quote
T is cross type.
Any cross type sensor is actually two linear sensors.
Canon's 2.8 sensor is double cross type sensor with enlarged longer linear parts.
Oh, ok . I guess I got it too literally.

But does it mean that the center point is more reliable (and/or faster) then?

The reason I ask is for situations where there is no DOF problem in the focus/recompose method, would it be better to just follow that method or select the the appropriate point instead.

Thanks again
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