All shots are handheld (but checked for camera shake), so this is more of an informal shootout. Furthermore, there were some lighting changes because of the weather.
Camera was E-M1 II, ISO 200.
Special note: The Yi 42.5mm f/1.8 has a special "macro" switch on the side of the lens. This decreases the minimum focusing distance, but increases the maximum aperture from f/1.8 to f/3.5. This can be switched while the camera is running, and works flawlessly.
Comparison shots are taken (1) wide open (2) stopped down by about 1.5 stops. So f/1.8 and f/2.8, and f/3.5 and f/5.6.
100% crops are added after every shot.
Autofocus was set to small spot in the very center of the image for most shots. 100% crops show the focus point.
Conclusions
Build
- Lens weight is identical for all intents and purposes.
- Yi is larger, but still small. Oly is tiny. Oly is almost too small to handle IMHO.
- Yi is cheaper and feels a bit cheaper. Still solid though.
- That fake focussing ring on the Yi is stupid.
- Yi takes larger filters (49mm) than the Oly. This is a very uncommon filter size for m4/3 lenses. Since the front element is not so big, a step-down ring combined with the extremely common 46mm filter size could work without vignetting, but I haven't tried.
Autofocus
- AF performance is equal on both lenses for all intents and purposes (including speed, accuracy, silent operation)
- Minimum focus distance is about equal when the Yi is set to "normal" mode. In "macro" mode, the Yi can get significantly closer.
Image quality
- Yi is significantly sharper wide open than the Oly.
- When stopped down by about 1.5 stops, the lenses are similarly sharp. The oly when stopped down is a hair sharper. But both are excellently sharp.
- There is visible vignetting on the Yi when shooting wide open. It is not corrected in-camera in contrary to the Oly. It goes away completely when stopping down.
- Bokeh & color is comparable on the two lenses, but I did not check in great detail. Bokeh is maybe a touch rounder on the Oly.
- Light transmission seems to be identical on the two lenses.
- Oly has less longitudinal CA.
Which one to get / keep?
I got the Oly for $100 used, and the Yi for $80 new. See here for discussion on the Yi lens deal: https://www.mu-43.com/threads/yi-42-5mm-f-1-8-lens-for-usd-80-free-shipping.104625
Since I will mainly use the lens wide open, and the Yi is much sharper there than my copy of the Oly, I will keep the Yi lens.
I also found the "macro" switch very useful for getting closer than typical "portrait distance". The vignetting wide open doesn't bother me, on the contrary, vignetting is actually pleasing for portraits IMHO.
Images
All images can be downloaded here (including raws): https://drive.google.com/open?id=15VF-YehVg5nE0jUSI5rFOKdonA7FbGOX
Sharpness at portrait distance
Yi wide open / stopped down
Oly
Sharpness at portrait distance 2
Yi
Oly
Camera was E-M1 II, ISO 200.
Special note: The Yi 42.5mm f/1.8 has a special "macro" switch on the side of the lens. This decreases the minimum focusing distance, but increases the maximum aperture from f/1.8 to f/3.5. This can be switched while the camera is running, and works flawlessly.
Comparison shots are taken (1) wide open (2) stopped down by about 1.5 stops. So f/1.8 and f/2.8, and f/3.5 and f/5.6.
100% crops are added after every shot.
Autofocus was set to small spot in the very center of the image for most shots. 100% crops show the focus point.
Conclusions
Build
- Lens weight is identical for all intents and purposes.
- Yi is larger, but still small. Oly is tiny. Oly is almost too small to handle IMHO.
- Yi is cheaper and feels a bit cheaper. Still solid though.
- That fake focussing ring on the Yi is stupid.
- Yi takes larger filters (49mm) than the Oly. This is a very uncommon filter size for m4/3 lenses. Since the front element is not so big, a step-down ring combined with the extremely common 46mm filter size could work without vignetting, but I haven't tried.
Autofocus
- AF performance is equal on both lenses for all intents and purposes (including speed, accuracy, silent operation)
- Minimum focus distance is about equal when the Yi is set to "normal" mode. In "macro" mode, the Yi can get significantly closer.
Image quality
- Yi is significantly sharper wide open than the Oly.
- When stopped down by about 1.5 stops, the lenses are similarly sharp. The oly when stopped down is a hair sharper. But both are excellently sharp.
- There is visible vignetting on the Yi when shooting wide open. It is not corrected in-camera in contrary to the Oly. It goes away completely when stopping down.
- Bokeh & color is comparable on the two lenses, but I did not check in great detail. Bokeh is maybe a touch rounder on the Oly.
- Light transmission seems to be identical on the two lenses.
- Oly has less longitudinal CA.
Which one to get / keep?
I got the Oly for $100 used, and the Yi for $80 new. See here for discussion on the Yi lens deal: https://www.mu-43.com/threads/yi-42-5mm-f-1-8-lens-for-usd-80-free-shipping.104625
Since I will mainly use the lens wide open, and the Yi is much sharper there than my copy of the Oly, I will keep the Yi lens.
I also found the "macro" switch very useful for getting closer than typical "portrait distance". The vignetting wide open doesn't bother me, on the contrary, vignetting is actually pleasing for portraits IMHO.
Images
All images can be downloaded here (including raws): https://drive.google.com/open?id=15VF-YehVg5nE0jUSI5rFOKdonA7FbGOX
Sharpness at portrait distance
Yi wide open / stopped down
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Oly
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Sharpness at portrait distance 2
Yi
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Oly
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Post processed [raw development w/Pop Art filter/crop] in camera. 
