Yesterday, I reported that the Sony a7RII with firmware 3.30 performs low-pass spatial filtering at base ISO for exposures longer than 3.2 seconds. Today, I’m going to look at some other ISO settings.
What I will show is spectral analyses of the raw red channel of exposures of the back of the body cap. Shutter mode is single shot. The camera is set to produce uncompressed raw. EFCS is on. Mechanical second curtain is on.
Dark field exposures are images of the read noise of the camera. Read noise is divided into two components: that part that is correlated from frame to frame, and that part that is uncorrelated. The portion that is correlated tends to have higher energy at low frequencies. The part that is uncorrelated should be what engineers call white, that is, have equal energy at all frequencies.
If the read noise were truly white, the charts of the read noise would be flat from the left side of the above graphs to the right. If you look at the top graph, that is the case for the vertical frequency components. Such flatness indicates no spatial filtering on the part of the camera firmware. The horizontal component shows a roller-coaster periodic behavior. I am not entirely sure as to its origin, but I saw similar behavior at some ISO settings with FW 1.0, I believe it is related to differences in the column ADCs and column amplifiers on the a7RII sensor.
This flat frequency response indicates that there is no spatial filtering perfromed by the camera at 3.2 seconds. That is also the case with faster shutter speeds at ISO 400.
However, at 4 seconds, and above, the camera begins to attenuate higher spatial frequencies.
This continues all the way up to 30 seconds, which is the slowest shutter speed that the camera can manage.
At ISO 1600, the situation is much the same:
Likewise at ISO 6400:
Looking at 6 second exposures made using the bulb setting:
The high frequency rolloff is similar to the 4 second timed exposures, but there is a periodic component to the horizontal frequency spectra that varies with ISO. We saw this yesterday at ISO 100, and I’ve seen it in a7RII’s with firmware version 1.0.