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PDAF and Aberration Correction
In reply to TOF guy,
4 months ago
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TOF guy wrote:
Yes it does bias the focus differently with different Whitebalance and in tungsten... its suppost to work that way !
I'm not sure I understand why that would be. The focusing plane may move with the wavelength of light. This is as you most likely know the source of axial chromatic aberration. However the effect is small (or the lens would be useless). And it depends on the lens: in other words the camera cannot correct for it or it would need an internal table of the focus shift for all lens, which seems unlikely. And I don't think it's even necessary. Obviously the camera should focus on the "average" focus plane for all wavelengths of the light coming from the subject. But this is (I think) naturally occuring when the Af tries to match lines of contrast with its sensors. There is no need to do anything different depending on the color of the surrounding light.
One possible exception would be in the auto area AF. Since this mode uses colors (tries to recognize skin color) it may have some correction for the range of colors it's after.
--
Thierry - posted as regular forum member
Actually the light source (color temp) can make a difference with phase detect AF. The imaging lenses are typically corrected as achromats, but I don't believe the small optics in the PDAF unit are that well corrected, so some firmware correction for color may need to be applied.
A canon marketing brochure on there PDAF mentions this. See the lower left hand side of page 4. (Auto focus accuracy under all lighting conditions thanks to light-source detection)
http://www.canon.co.uk/Images/EOS%201DMKIV%20Technology%20Feature%20Guide_tcm14-721276.pdf
Of course whether Nikon does this is speculation. Any of the older RGB sensors would be able to apply a target color correction term. This doesn't require 90K RGB sensor.
The PDAF is corrected for spherical aberration for each lens. The camera doesn't have to carry this information. It's in the lens. Nikon service manuals describe how the calibration is done for some of its lenses.
Steve
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