Electronic Shutter Artifacts in Sony A9 and Canon 1DX3 Images
It's easier to measure your electronic shutter speed than I thought. It seems that the LED lights I have around the house switch at higher frequency and have sharper pulses than I expected, which makes accurate measurements easy. Just take your A9 or R5, or whatever you have, and photograph a white sheet of paper that is illuminated by an LED light. I found that the LED strip light that is mounted under the bookshelf above my desk has nice sharp bands in the image that are produced by 240 Hz pulses to the LED, double the frequency I was expecting. The light’s dimmer varies the width of the pulses, so I turn it down to a low illumination level to get a convenient band width. Photographs using the ES for the A9 and A7R4 are shown below.
Just counting bands can be quite inaccurate, especially if the number of bands in the frame is small, as it is for fast shutters. For example, the A9 image below shows 2 bands. Using that and that the LEDs are pulsed at 240 Hz, I compute (2/240) = (1/120) sec for the shutter speed. It is much more accurate to take the image file into Photoshop and use the info panel to measure the vertical distance between the top (or bottom) edges of the furthest separated bands. In this case I measured the distance between the top of the bands to be 2332 pixels. That means there are (4000/2332) = 1.71 bands in the full image. Since the LEDs are pulsed at 240 Hz, that gives (1.71/240) = (1/140) sec for the shutter transit time. That is easy and quite accurate. The measurement is as accurate as you can measure the 2332 pixel band spacing, which I would estimate is better than 1%. In order to get the sharpest edge I used 1/10,000 sec shutter. Even if you have several bands in the frame it is more accurate to measure the band spacing to get the fractional band in the image. The A7R4 image shows 26 bright bands, 25 full dark bands and 2 partial dark bands. I measured the spacing and find there are 25.97 cycles. I measured a few others and find the average to be 25.95. The calculation (25.95/240) = 108 msec = 1/9.25 sec. That is for a single shot, uncompressed 14-bit RAW frame. Using continuous compressed 12-bit frames the result is 60.0 msec = 1/16.7 sec, which is 1.80 times faster (but still pretty slow!). These results are similar to those obtained by Jim Kasson, https://blog.kasson.com/a7riv/how-fast-is-the-sony-a7riv-silent-shutter/
I checked a few other LED lights around the house. I did not find any that pulse at 120 Hz. The bulbs in the ceiling lights were also 240 Hz. I was surprised to find some LED lights are pulsed at much higher frequencies. The reading lamp in my office shows over 14 bands with the A9, which means it pulses at nearly 2 kHz. Another reading lamp showed slightly over 41 bands with the A9, which means nearly 6 kHz! It seems that the lighting engineers are phase locking and pulse width modulating to get flicker free variable intensity lamps, although some of these frequencies seem ridiculously high. Some are such high frequency that the number of bands is too high to be practical for measuring slower electronic shutters.
Since some of the high frequencies are not simple multiples of the line frequency, an absolute measurement of the shutter speed is problematic. But it is easy to measure the ratio of shutter speeds quite accurately. For example, it is easy to measure the ratio of the electronic shutter and the mechanical shutter using a high frequency LED light. Or to measure the ratio of an A9 and R5 (if you have both). With a 240 Hz LED light, it can be done in just a few minutes. Note that it can’t be done with a 120 Hz LED since the A9 will show only one LED pulse.
It is also interesting to note that the 12 row bands are seen on the leading and trailing edges of the LED pulse. These bands are seen due to the rapidly changing light as the LED is pulsed on or off, not the interaction between motion blur and rolling shutter that was discussed at the beginning of this thread.
Hopefully someone with an R6 and R5 can use these methods to give us a measure of their electronic shutter speeds.
Sep 25, 2020 at 02:22 PM
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