dslr phase detection在body里，为什么lens有可能front/back focus# PhotoGear - 摄影器材
G*Y
1 楼
机身因为focus sensor和ccd不在一起，可能会有calibration问题。
为啥lens也会有front/back focus问题呢？
不太明白。
为啥lens也会有front/back focus问题呢？
不太明白。
m*n
2 楼
http://www.dpreview.com/reviews/nikon-d750/9
这里面讲了些，主要是autofoucs module的安装位置有偏差，DPReview用的两台D750
同样的镜头需要AF fine tune在机身上
“Implications for autofocus accuracy
Given that the problem arises from the placement of the AF module, we were
curious to check for any effects on focus accuracy. Since we were lucky
enough to have two D750 bodies with differences in severity of the flare
issue, we tested both bodies for front/back-focusing issues relative to one
another, with two different prime lenses: both a Sigma 35mm f/1.4 Art lens,
and a Nikkor 35mm f/1.8G (to rule out any possibility that it's just the
lens that's at fault).
...
What's interesting is that body #1 (the 'worse' D750) front focuses
consistently relative to body #2 (the 'better' D750), which you'll see if
you roll back and forth over A (D750 #1) and B (D750 #2) below. The red
arrows indicate the point of optimal focus. You'll note below that both
lenses front-focus with both bodies; however, the body (#1) with the AF
module placed slightly higher front-focuses more, in both cases. You can
also see the optimal planes of focus if you hover over C and D, which show
the results of the more accurate contrast-detection autofocus (CDAF).
Furthermore, we verified that the first D750 consistently front-focused
relative to the second one with two different lenses, so it's unlikely that
it's the particular lens solely at fault here. ”
这里面讲了些，主要是autofoucs module的安装位置有偏差，DPReview用的两台D750
同样的镜头需要AF fine tune在机身上
“Implications for autofocus accuracy
Given that the problem arises from the placement of the AF module, we were
curious to check for any effects on focus accuracy. Since we were lucky
enough to have two D750 bodies with differences in severity of the flare
issue, we tested both bodies for front/back-focusing issues relative to one
another, with two different prime lenses: both a Sigma 35mm f/1.4 Art lens,
and a Nikkor 35mm f/1.8G (to rule out any possibility that it's just the
lens that's at fault).
...
What's interesting is that body #1 (the 'worse' D750) front focuses
consistently relative to body #2 (the 'better' D750), which you'll see if
you roll back and forth over A (D750 #1) and B (D750 #2) below. The red
arrows indicate the point of optimal focus. You'll note below that both
lenses front-focus with both bodies; however, the body (#1) with the AF
module placed slightly higher front-focuses more, in both cases. You can
also see the optimal planes of focus if you hover over C and D, which show
the results of the more accurate contrast-detection autofocus (CDAF).
Furthermore, we verified that the first D750 consistently front-focused
relative to the second one with two different lenses, so it's unlikely that
it's the particular lens solely at fault here. ”
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