There have been reports of strong self-heating effects at long exposures with the Sony alpha 7R Mark II (a7RII). I mentally discounted them, because I didn't know of a mechanism that would preferentially heat the sensor with the shutter open and the photodiodes sitting there back-biased and waiting to snag a passing photon.
But the posting of a7RII 30 second dark frame data that was at variance with what I was seeing in my own cameras made me want to do a long exposure self-heating test.
I set the camera's shutter speed to 30 seconds, which is as long as it will go. I set the shutter mode to EFCS, no silent shutter, single shot. I turned the IBIS off. I figured that's the setup you'd probably use for astrophotography. I set the ISO to 3200, which is higher than I'd use myself, but seems to be some kind of astrophotography standard. I hooked the camera to an intervalometer that was set to 1 second, so that would be the greatest time interval between exposures. I turned off lens corrections. I turned off long exposure noise reduction. I stopped the lens down to f/22 and firmly affixed the lens cap. I made 101 exposures.
I measured the standard deviation of a central 400x400 pixel sample. Here's what I saw:
There are a couple of glitches; a big one in the red channel and a little one in the blue channel. Ignoring those, we see a pronounced exponential heating effect with a time constant that looks to be about 70 shots, or 36 minutes. It looks to me like the asymptote is about 6.5 stops, which will be a stop and a half worse than where it started out. The dotted line is a linear regression line to the green channel data.
If you look at the entire sensor, you see something similar. Here's what you get at the beginning:
and at the end:
This is going to complicate long exposure testing. I wonder if it's common among other cameras.
Jim
--
http://blog.kasson.com
But the posting of a7RII 30 second dark frame data that was at variance with what I was seeing in my own cameras made me want to do a long exposure self-heating test.
I set the camera's shutter speed to 30 seconds, which is as long as it will go. I set the shutter mode to EFCS, no silent shutter, single shot. I turned the IBIS off. I figured that's the setup you'd probably use for astrophotography. I set the ISO to 3200, which is higher than I'd use myself, but seems to be some kind of astrophotography standard. I hooked the camera to an intervalometer that was set to 1 second, so that would be the greatest time interval between exposures. I turned off lens corrections. I turned off long exposure noise reduction. I stopped the lens down to f/22 and firmly affixed the lens cap. I made 101 exposures.
I measured the standard deviation of a central 400x400 pixel sample. Here's what I saw:
There are a couple of glitches; a big one in the red channel and a little one in the blue channel. Ignoring those, we see a pronounced exponential heating effect with a time constant that looks to be about 70 shots, or 36 minutes. It looks to me like the asymptote is about 6.5 stops, which will be a stop and a half worse than where it started out. The dotted line is a linear regression line to the green channel data.
If you look at the entire sensor, you see something similar. Here's what you get at the beginning:
and at the end:
This is going to complicate long exposure testing. I wonder if it's common among other cameras.
Jim
--
http://blog.kasson.com
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