Olympus OM-D E-M1 low light auto focus performance

Started Jan 16, 2014 | Questions thread
Timur Born
Timur Born Veteran Member • Posts: 4,717
Re: Olympus OM-D E-M1 low light auto focus performance

I wrote about this several time (including in the DPR E-M5 article and my "Focusing explained" forum thread), here is how I interpret these things.

At "Normal" frame-rate the E-M5 and E-M1 drop the frame-rate of Live View to 30 fps in order to get longer exposure on the sensor. You can see this drop in screen/EVF frame-rate with bare eyes, turning on blinkies while panning makes it even more visible. This has nothing to do with final image exposure (not even with the screen frame-rate), but with how many frames the sensor feeds to the processor/screen per second. And of course your Live View image gets darker in bad light, because there is less exposure (less time per single live frame) with higher frame-rates.

The reason why focusing is slowed down by this is that CD AF needs feedback from the sensor after each movement of the lens AF motor. Lens AF motor is moves -> sensor is read -> contrast is measured -> AF motor keeps moving or is stopped -> if AF motor keeps moving -> sensor is read -> contrast is measured -> etc. pp.

At longer sensor exposure = lower sensor read rate = lower frame-rate the processor needs to wait longer for sensor feedback before it can decide whether to stop the AF motor or keep it moving for further CD measurements.

There is one drawback, in *very* dark light it can happen that AF cannot be acquired at "High" frame-rate where "Normal" is still able to focus due to more light hitting the sensor during AF/Live View.

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 Timur Born's gear list:Timur Born's gear list
Panasonic Lumix DMC-LF1 Olympus OM-D E-M5 Nikon D750 Nikon AF-S Nikkor 50mm f/1.4G Olympus M.Zuiko Digital 45mm F1.8 +3 more
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