Review of the Sony A7S

I bought my Sony A7S in April 2015 after reading reviews of the the incredible low-light performance such as Lonely Speck Review. This was before reports began to emerge about the notorious "star eater" issue and it was the beginning of a love hate relationship with this camera.
It is only fair at this point to mention the existence of a later Lonely Speck article Lonely Speck: Why I no Longer Recommend Sony Cameras for Astrophotography

I don't intend this to be a full review but a summary of my conclusions after extensively using the Sony A7S for deep sky astrophotography.

The main advantages

• Compact and light body
  Compared with DSLRs (especially full-frame DSLRs) the camera body is very compact and light in weight. So it doesn't put much strain on a telescope focusing tube.
   
• High QE
  The Sensorgen website reported the Quantum Efficiency of the sensor as 65%.
   
• Low read noise
  At ISO 2000 and above, read noise is 1.3e and falls to well under 1e at very high ISOs.
  Thierry Legault has done a very useful analysis of read noise here: Sony A7S measures. I have also verified these figures with my own experiments. It is believed that the sensor has 2 modes for analogue to digital conversion and it switches to the low noise mode at ISO 2000 and above - hence the step in the curve.
   
• Low thermal noise
  Very low thermal noise means that for astrophotography in the UK (i.e. the complete absence of warm nights!) even on a fairly slow scope such as F6, skyglow will always dominate thermal noise (unless you are using narrowband filters). This opens up the possibility of using the A7S on a whole range of scopes that previously were not that suitable for DLSR or mirrorless cameras. My own experiments indicate a dark current of 0.2e/pixel/sec after 2 hours of solid imaging at an ambient temperature of 20C. At around 8C this drops to 0.05e/pixel/sec. So at 8C ambient temperature, even a long 20 minute exposure would have a thermal noise of 8e which is similar to the read noise of a typical cooled astro-CCD
   
• Night-time sensitivity
  It is probably the most sensitive low-light consumer camera in existence at the current time. But if you know of a better one, please tell me!
   
• Liveview
  Liveview on this camera is incredible. Under dark skies, nebulas and galaxies can be seen in real time using ISO 102400 and 4 frames/sec. It makes focusing and framing the shot so simple!
   
• Night-time video
  For the same reasons as liveview, night-time video is astonishing. With a fast lens, the camera can be used for viewing the Milky Way and the Aurorae in real time. For example see this real time video of mine: Real time tour of the night sky
   
• Mirrorless
  There is no mirror box to cast shadows and cause diffraction spikes in the image. This was always a problem when using a DSLR on a fast scope like the Takahashi Epsilon or Hyperstar.
   
• Electronic/Silent Shutter
  In silent mode the shutter is fully electronic: the physical shutter is not used which means no telescope vibrations nor shutter wear and tear. This is very useful for taking high magnifaction images of the moon or planets. Note however that the camera drops into 12bit mode when using the electronic (i.e. silent) shutter.

The main disadvantages

• Bulb mode "star eater"
  In Bulb mode, the Sony firmware automatically implements a spatial filtering algorithm to reduce hot pixel noise. Unfortunately, this can also remove small stars from the image, treating them as hot pixels. More details here: Sony "Star Eater". As a workaround 30sec exposures can be used instead of Bulb mode. Read noise at ISO 2000 and above is so low that 30sec exposures are quite practical in most circumstances.
   
• Narrow lens fitting
  Although it works fine for an E-mount lens, the narrow throat of the E-mount lens fitting means that very severe vignetting can occur at the corners of the sensor on a slow F-ratio scope. This is not such a problem for fast scopes and not a problem for lenses. However, on a C11 with F8 flattener the extreme corners were actually in complete shadow.
   
• Lack of software support
  Computer software support for tethered control using a laptop is woefully lacking. Neither APT nor Backyard support Sony cameras. Sony appears to be concentrating on "PlayMemories" camera apps and Wi-Fi control using smart phone apps instead of producing a decent Windows SDK (software development kit)
   
• Split sensor and horizontal bands
  The full frame sensor appears to be split down the middle with each half acting semi-independently. This can cause a visible discontinuities down the centre of the image which seed coloured horizontal bands on the left-hand side of the sensor. These are not visible during normal usage but can become obvious when performing calibration with flats, subtracting the sky glow and then stretching. The workaround is to choose an ISO and exposure length to make sure the back of camera histogram is in the middle of the back of camera display - this is much further to the right than the usual practice for DSLR cameras. I will devote a page to this in the near future but meanwhile here is some useful information on the issue:
  Cloudy Nights: A7S split sensor and banding
  DPReview: A7S left/right sensor split
  A technique for revealing the bands in your own camera can be found here: Cloudy Nights: Which full-frame sensors are split?
   
• Concentric coloured banding
  The Sony firmware appears to apply a scaling to the digital data coming off the sensor. This creates gaps in the histogram and can cause concentric coloured bands to appear in an image that is calibrated with flats, skyglow subtracted and stretched. I will devote a page to this in the near future but meanwhile here is some useful information on the issue:
  DPReview: A7S Concentric Banding and Histogram Gaps
  Cloudy Nights: Concentric coloured rings caused by flats
  Similarly to the split sensor issue, the workaround is to choose the ISO and exposure length so the back of camera histogram is in the middle of the back of camera display - this is much further to the right than the usual practice for DSLR cameras.
   
• Lossy compression
  Sony A7S raw files have lossy compression applied. This caused a huge number of complaints which forced Sony to introduce a firmware update to allow uncompressed raw files but this was only for the Mark 2 models (i.e. A7SII, A7RII) I have never seen any problems caused by this in my own deep sky imaging but it can affect startrail images. The compression squeezes 13 bits of data into 11 bits without introducing artifacts but then performs a delta modulation which can produce artifacts in high contrast regions such as startrails. For more technical details see Rawdigger Sony Posterization.

Other relevant information

• With Wi-Fi switched off (airplane mode) and rear LCD deactivated (FINDER/MONITOR set to Viewfinder) battery life is 6 hours for an imaging session of continuous long exposures. Low power consumption = low generated heat.
   
• Amp glow becomes apparent in liveview mode at high ISOs but for deep sky astrophotography it calibrates out well.
   
• The full frame sensor means you need a large image circle free of optical aberrations on your imaging equipment unless you are prepared to crop the image
   
• The large pixel size leads to some people suggesting it is suitable only for longer focal lengths. Personally I see no reason not to use it with a wide angle lens. After all, this is what DSLR and mirrorless cameras are good at!
   
• Most software (DSS, IRIS, PixInsight) will only extract 12 bits from the raw files (Sony ARW files) instead of the usual Canon/Nikon 14 bits. Actually 13 bits are encoded in the raw file for most modes (but bulb mode and silent mode are 12 bit) but DCRaw used by DSS, IRIS and PixInsight throws away the least significant bit.
   
• To use bulb mode, silent shutter mode must be switched off (I really don't understand why)