Why do digital cameras still use a mechanical shutter instead of only an electronic shutter?
Asked 8/8/2024
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I’m trying to understand why many digital cameras still have a mechanical shutter.
With an electronic shutter, the sensor is read line by line, which can cause rolling shutter because the whole sensor is not captured at exactly the same moment. But how does a mechanical shutter help if it doesn’t make the sensor readout faster?
Is the idea that the mechanical shutter blocks light so the exposure can end for the whole frame before the sensor is read, whereas with electronic shutter the sensor may still be receiving light while readout is happening?
I’m also unclear why some cameras show better dynamic range or signal-to-noise performance with mechanical shutter than with electronic shutter.
Originally by matej. Source · Licensed CC BY-SA 4.0
matej
1y ago
2 Answers
5
With electronic shutter each row (or group of rows) cannot be activated until the previous row(s) have been read out; regardless of the shutter speed. Higher SS's are accomplished by energizing the row(s) for ever shorter durations, but the readout speed remains the same. This results in time gaps between adjoining rows (which exaggerates the rolling shutter effect), and a total time gap between first/last rows.
E.g. for a typical camera the readout for the whole sensor may be 1/100s; so the entire sensor exposure requires no less than 1/100s, even if the exposure is only 1/1000s (total time rows are energized/recording). Changing the shutter speed doesn't change the time gap between adjoining rows, or the first/last rows (unless longer than readout).
With the mechanical shutter the entire sensor is energized simultaneously and there is no time gap between the exposure of adjoining rows. However, there is still a time gap between the first and last row; and mechanical shutter is still prone to some rolling shutter effect because of that. With mechanical shutter changing the SS does change the time gap between the first and last rows, so higher SS's reduce the likelihood of the rolling shutter effect being apparent.
Some cameras increase the readout speed for electronic shutter in an attempt to compensate somewhat, which makes it more prone to packet errors (noise). And sometimes the increase is accomplished in conjunction with a reduction in the bit depth (processing accuracy), which further increases the potential for errors (noise). Both potentially result in less dynamic range (signal above noise).
Originally by Steven Kersting. Source · Licensed CC BY-SA 4.0
Steven Kersting
1y ago
0
Generated from our catalog & community — verify before relying on it.
Yes—the key advantage of a mechanical shutter is not faster readout, but better control of when exposure starts and stops across the whole frame.
With electronic shutter, rows are exposed and read sequentially. Even if each row’s exposure is very short, the sensor still takes a finite time to scan from first row to last, so different parts of the image are recorded at different moments. That’s what causes rolling shutter distortion.
With a mechanical shutter, the whole sensor can be put into darkness at the end of the exposure, so the captured charge is effectively “held” before readout. The readout can still take time, but the exposure has already ended for the frame, greatly reducing rolling-shutter effects.
Mechanical shutters can also allow more favorable sensor operation in some cameras. Electronic shutter may use a readout mode that prioritizes speed and can have more noise, less dynamic range, or other tradeoffs versus the camera’s normal mechanical-shutter readout mode. So differences in DR/SNR are often due to how the sensor is operated in each mode, not the shutter curtains themselves.
In short: mechanical shutter mainly helps freeze exposure timing across the frame; electronic shutter mainly trades that for silent operation and less wear.
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