Accuracy of AF in APS-C and FF


torak

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Sep 4, 2009
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#1
Just wonder.

Say an APS-C DSLR fitted with a 200mm lens vs a FF DSLR with the same 200mm lens.

Assuming that both camera's AF has similar performance and sensitivity.

Will it be easier for the APS-C DSLR to focus on far targets compared to the FF? Since the subject will appear 50% bigger on the APS-C, does this help the AF of the camera? Or is there no difference at all?

Thanks
 

giantcanopy

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Feb 11, 2007
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#2
if u r saying all else equal except sensor size, then it shld be the same. If i am not mistaken, the autofocusing sensor/strips in dslr does not reside in the optical sensor that captures the image.

ryan
 

Octarine

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Jan 3, 2008
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#3
Will it be easier for the APS-C DSLR to focus on far targets compared to the FF? Since the subject will appear 50% bigger on the APS-C, does this help the AF of the camera? Or is there no difference at all?
Who says that a 200mm lens can only be used at 'far targets'?
The AF sensors are not sitting on the image sensor but are an independent component (unless using live view). During focusing the shutter is closed, the sensor is not in use. What impact can the sensor possibly have here?
 

szeping

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Jan 13, 2008
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#4
From my logic, AF depends on the optical signal falls on sensor. So it doesn't matter which focal length you are using.

IMHO, bigger subject does not fasten your AF speed, it's based on phase or contrast detection... Imagine if you are zooming into a plain colour object, the object becomes bigger but AF system will have problem finding the right focal plane.
 

Numnumball

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Mar 6, 2009
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#5
Just wonder.

Say an APS-C DSLR fitted with a 200mm lens vs a FF DSLR with the same 200mm lens.

Assuming that both camera's AF has similar performance and sensitivity.

Will it be easier for the APS-C DSLR to focus on far targets compared to the FF? Since the subject will appear 50% bigger on the APS-C, does this help the AF of the camera? Or is there no difference at all?

Thanks
It shd be the same regardless of sensor size..
And why should the sensor have any impact on the AF since sensor is not in use (closed) at the point of AF before the shot is taken?
 

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Prismatic

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#6
As the good fellas above mentioned, the AF sensor does not reside at the same place as the image sensor. Typically, it lies under the mirror or above in the pentaprism/mirror assembly.

If you are using a APS-C and full frame camera like the Canon 500D vs the 5D to shoot the same scene, I think the image circle from the lens hitting the sensor plane is the same size. It's only because the APS-C sensor captures only a smaller portion of this image circle, that's why it appears to be bigger.
 

wildcat

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Sep 8, 2004
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#7
It shd be the same regardless of sensor size..
And why should the sensor have any impact on the AF since sensor is not in use (closed) at the point of AF before the shot is taken?
But it is also unlikely that the focusing system will be the same for an entry level system vs a full frame which generally will have better focusing system, correct?

Thus, even for the same sensor, you may get different AF from different cameras, right? e.g. focusing on D300s vs D5000.
 

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torak

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Sep 4, 2009
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#8
Thx for the responses.

I wasnt too sure about the technical aspect of the af sensors thus i asked this question. Had the assumption that a larger object will be easier to focus thus this question pops up.

Thx for the clarifications :)
 

Numnumball

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Mar 6, 2009
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#9
But it is also unlikely that the focusing system will be the same for an entry level system vs a full frame which generally will have better focusing system, correct?

Thus, even for the same sensor, you may get different AF from different cameras, right? e.g. focusing on D300s vs D5000.
No bro.. this is OT liao.. :p
We are addressing TS qs on the Sensor size with regards to AF which he raised at the very beginning of this thread.

Of cos the Af system is different lar... U dun have to compare DX and FX..

Just look at D500K and D300 will be enough to validate my point..

11-point AF System vs 51-point AF System
 

#11
This is good reading material on what the auto-focus mechnism is about
http://www.nikon.com/about/technology/core/software/caf/index.htm

The auto-focus mechanism does not rely on the imaging sensor (CCD/CMOS). During autofocus, the reflex mirror does not move and the shutter does not open. This means, focusing accuracy and speed has nothing to do with imaging sensor size, it depends on focusing module built into the camera the lens used.
 

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