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12-29-2014, 06:38 PM   #1
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AF with extremely fast and extremely slow lenses

My K-30's AF struggles with fast and slow lenses when using phase detection. Two examples: my FA 50mm f/1.4 and my FA 28-70 f/4. The DoF when focusing wide open on the FA 50mm is extremely thin and I think the body either overshoots or undershoots the ideal focusing point. If I could only slow down the focusing speed! I get the opposite problem with the FA 28-70mm. The DoF at f/4 is plenty big, especially at 28mm or 35mm. The seem AF stops once it reaches the first allowable focus confirmation. I would rather that it sweep across the entire range and then pick something in the middle.

I have tried fiddling with the AF correction in the configuration menu but the AF results are just as inconsistent because my shooting scenes are changing.

Both lenses focus very well in Live View.

When I use lenses in between these focal ratios, like my FA 28-105mm f/3.2 or my DA 18-135mm f/3.5 I can achieve excellent focus.

Other than collapsing and crying or puffing my chest and smashing the camera against a wall what can I do?

12-29-2014, 06:53 PM   #2
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It certainly sounds like your 50mm is suffering from front or back focus. You should test it with a chart to make sure it's calibrated correctly.
Fixing Front and Back Focus - Introduction - In-Depth Articles

Phase detection, unlike contrast detection, calculates the focus rather than moving the focusing ring back and forth and making measurements, hence the possibility for error.

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