There's really no reason to use full mechanical shutter on the R5. It only makes sense when shooting high speed with very wide apertures.
Otherwise, mechanical shutter means shutter shock and increased chances of blur.
ES @20fps is 12bit
ES only available in single shot or 20fps and 20fps can be a big data storage/workflow issue in post
Can't use flash with ES
ES can be prone to banding with indoor lighting
I haven't done any shooting with EFSC.
I did find the following information from one user which may or may not be accurate. Food for thought though:
* Bokeh is best with Mechanical Shutter Mode with wide aperture lenses.
* Bokeh is slightly reduced with EFCS Mode.
* Mechanical Shutter Mode also eliminates Rolling Shutter distortion effects with fast moving subjects.
* Electronic Shutter may produce odd bokeh under some instances and it will definitely have issues with 'flicker' (horizontal banding) when shooting under many types of artificial lighting (indoor lighting).
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Mechanical Shutter is best for:
* Maximum Bokeh
* Sporting
* Action (airshows, golf, performances etc)
* Portraits where Bokeh is important.
* High Shutter Speeds.
* Better than EFCS and Electronic if Neon Light is in frame (including signage).
* Shooting in artificial light which might cause banding/flicker.
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EFCS (Electronic First Curtain Shutter) is best for:
* Everyday photography.
* Eliminating Shutter Shock potential.
* Shooting in artificial light which might cause banding/flicker.
* Better than Electronic if pulsating Neon lighting is in frame.
* Landscape Photography.
* Portrait Photography.
* Photography where bokeh is not absolutely critical.
* Slower Shutter Speeds.
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Electronic Shutter is best for:
* Silent Shutter (it's absolutely silent).
* Slower Shutter Speeds (there's no shutter vibration).
* Eliminating Shutter Shock completely.
* Rapid 20 fps continuous shooting.
* When maximums frame-rates are essential to capture a moment.