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Sony a7c: readout speed (photo mode)

I wanted to measure the readout time of the Sony a7c (24MP FF) sensor in the Silent Shooting (fully electronic shutter) mode. I had already done this with the Samsung NX500 (for photofor video) using guitar strings and a 100Hz (50Hz powered) strobe.

This time for the Sony a7c I decided to use a fast blinking LED with an NE555 pulse generator.

The frequency was set to about 500Hz. There is some inaccuracy, but at this frequency it is not that important.

The period (LED off and LED on) is about 2ms. Since the readout of the sensor is much longer, there will be multiple pairs of lines (dark line and light line) visible in the image. The number of line pairs must be multiplied by 2ms to calculate the readout time.

To measure this, I used a tripod to photograph (in ARW) an out-of-focus reflection of the LED at a shutter speed of 1/640s in complete darkness. I did not test the video recording this time (maybe later).

A total of 4 photo shooting modes that affect the readout time in the Silent Shooting mode (fully electronic shutter) of the Sony a7c:

1. Full-frame 14-bit mode

Main (for ARW) but slowest reading mode.

About 31 pairs, 62ms, 1/16s. Very slow.

2. APS-C 14-bit mode

About 20.4 pairs, 40.8ms, 1/25s. Smaller area means faster reading.

3. Full-frame 12-bit mode

There is no explicit setting to switch between 14-bit and 12-bit mode on the Sony a7c. Normally 14-bit is used, but when [lossy] compressed ARW and continuous shooting are enabled together, the camera switches to 12-bit mode.

The 12-bit mode is also used when shooting in JPEG only.

About 17.7 pairs, 35.4ms, 1/28s.

4. APS-C 12-bit mode

About 14.6 pairs, 29.2ms, 1/34s. Fastest reading.


The Silent Shooting mode of the Sony a7c has a rather long readout time:

FF 14-bit: about 1/16s (62ms)
APS-C 14-bit: about 1/25s (40.8ms)
FF 12-bit: about 1/28s (35.4ms)
APS-C 12-bit: about 1/34s (29.2ms)

This is far from the capabilities of a mechanical shutter (x-sync 1/160 for the Sony a7c). Although it is possible to use it (I use it very often), but be aware of potential artifacts from camera or object movement (rolling shutter) and artificial lighting (banding). By the way, image stabilization can help prevent rolling shutter caused by small movements of the camera.

Visual readout time measurement lets to know if 12-bit or 14-bit reading mode is being used. I have checked and made sure that when shooting in [lossy] compressed ARW without a continuous drive, the 14-bit reading mode is used. But when shooting in JPEG only, the 12-bit reading mode is always used.

See also related notes:
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