There are downsides to Electronic First Curtain cameras, but you’ll only see them at the extremes
The Canon EOS R8 II is the latest camera to keep size down by adopting an electronic first curtain shutter design. This isn’t the same thing as full electronic shutter and it has very few downsides, so here’s how it works and what those downsides are.
Perhaps an easier way to understand EFCS is to think of it as Mechanical Second Curtain Shutter. The exposure is started electronically but a normal mechanical shutter then comes down to end the exposure, meaning that, unlike full electronic shutter, you can end your exposure very quickly, rather than being limited by readout speed of your sensor.
This means EFCS doesn’t have the significant rolling shutter artifacts (banding under flickering light and distorted moving subjects) that all but the fastest sensors can exhibit. The fast end to the exposure also means it can usually sync with flashes up to the same speed as a fully mechanical focal plane shutter.
In fact, it can have a benefit over full mechanical shutters, in one regard: because there’s no physical shutter snapping open at the start of the exposure, it eliminates a key source of vibration that can cause shutter shock. Some cameras with full mechanical shutters switch across to their EFCS mode within certain shutter ranges for exactly this reason.
This is the reason we’ve seen an increasing number of cameras, particularly the ones where the manufacturer has focused on downsizing and on cost reduction. In most instances, EFCS lets you use a smaller, mechanically simpler mechanism, with very little disadvantage.
The downside
There is one downside, though. At very high shutter speeds, where the period during which the shutter is closing makes up a significant proportion of the exposure, you can experience bokeh truncation: part of the bokeh pattern getting chopped off.
This happens because the physical shutter used to end the exposure sits some distance in front of the sensor, so light can creep around the edge of the shutter blades as it closes. But the electronic first curtain (progressively turning the rows of the sensor on to start the exposure) doesn’t have that gap, so the start of the exposure didn’t have that light bleeding around the edge of the ‘shutter,’ giving one side of the bokeh a hard cut-off. This results in out-of-focus regions that aren’t as blurred and may not have such attractive bokeh.
However, this only becomes visible at very high shutter speeds when using lenses with fast enough apertures to significantly blur the background. In every other respect, including flash sync, electronic first curtain (/mechanical second curtain) shutter, is every bit as good as full mechanical shutter.
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