M.Zuiko 75-300mm focusing oddity

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MarkLeeds2k5
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M.Zuiko 75-300mm focusing oddity
6 months ago

The following pairs of images (these are 100% center crops) were taken with an omd em10 and the 75-300mm lens, shot just seconds apart, always using the EVF.

The first image in each pair was taken using single point auto focus, set to its smallest size.

The second in each pair was taken using the Zoom Frame auto focus feature at 14x magnification.

Small single point auto focus

Zoom Frame auto focus 14x

Small single point auto focus

Zoom Frame auto focus 14x

Small single point auto focus

Zoom Frame auto focus 14x

Small single point auto focus

Zoom Frame auto focus 14x

Small single frame auto focus

Zoom Frame auto focus 14x

The difference is noticable to say the least. I've taken dozens of shots testing this. At the 75mm end, about 80% of the time using Zoom Frame results in a better focus. The other 20% of the time they are equal quality. Not once did the single point auto focus produce better results than the zoom frame method.

At the 300mm end its more like a 40%/60% split, but that still means four out of ten shots are still improved using the zoom frame feature.

My question is... why?

On page 53 of the em10 manual it discusses the relative sizes of the focus frames and includes this diagram:

However, while the 14x magnification zoom frame is smaller than the standard auto focus frame, I'm using the fine setting which is much smaller than the standard frame size. So I'm puzzled as to why there is such a difference in focus quality.

Has anyone else experienced this with the 75-300mm lens, or indeed any lens comparing the small auto focus frame to the zoom frame?

Olympus M.Zuiko Digital ED 75-300mm 1:4.8-6.7 Olympus OM-D E-M10
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