Front/back focusing essay on m4/3?

Cerebus2

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Joined
Oct 12, 2010
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Luminous Landscapes posted a guest essay about sources of front/back focusing errors on digital cameras:

Are your pictures out of focus?

The question I have is that with live-view cameras like m4/3, can this problem even exist? It would seem to me that the use of contrast focus at the image sensor would counter most (all?) sources of focus error in other systems. This in turn would help explain the amazing results we see with many lenses on these cameras.

Of course, I could be wrong.

-- C
 

bogolisk

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Sep 30, 2010
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AFAIK, no! front-back focus is an artifact of the dual sensor system: the imaging sensor and the focus sensor don't "see" the same thing. CDAF (or the Fuji's single sensor PDAF) use a single sensor.
 

Brian S

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Apr 11, 2009
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The only way it could happen on a liveview camera: the lens is focused with the lens aperture wide-open and then stopped down for the exposure after the focus is set. Focus shift can occur when a lens is stopped down.
 

Brian S

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Apr 11, 2009
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On a DSLR, the camera performs autofocus with the lens wide-open. So if you have an F1.4 lens, the AF mechanism will focus with the lens at F1.4 no matter what F-Stop you are actually using. If you use F4 for the exposure, the focus will be behind the point that the AF mechanism focused on. That is focus shift, and is just a property of a lens.

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Focus shift is given in these old Pop Photo tests. A shift of 0.04mm (spherical) means the focus will be ~0.5" behind a point of focus set to 36".
 

joele

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Dec 12, 2010
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Melbourne, Australia
This the one advantage of how the manual lenses stop down on our u4/3 bodies.. It does make it harder as it gets darker in the EVF but it is like a permanent DOF preview and it also avoids focus shift errors which on lenses like my Pentax 50/1.2 where pretty annoying on Pentax bodies.. I wonder if it is also a factor that would have made the Voigtlander 25/0.95 cheaper/easier to design as it is not really an issue if you set your aperture and then focus..
 
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