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ISO Invariance

Way back in March 2017, Petapixel had a fascinating article on ISO invariance in digital camera sensors. The article details how ISO affects exposure, and how to ascertain the best ISO for low-light work - in particular, for astrophotography. For one of their tests, a Sony A7S was used (Canon 700D and Fuji XT1 for others), and I doubt you can get a better stills low-light performer than that!

Reading the article got me thinking - how would my Micro Four Thirds camera function in a similar test? Bare in mind the Four Thirds sensor is half the size of the full-frame used in the A7S, and 16 megapixels versus 12.2 megapixels in the Sony. So while the sensor is small, the pixel-count is higher, meaning the photosites are smaller in the m43 camera as well.

Anyway, I'm getting into too much detail in the technical comparisons! Let's look at how we test ISO invariance. Take a series of shots in RAW, each at whole-stop ISO and  match the exposure brightness in post across the range.

For the shots, I followed the article's instructions:
  • Shoot at night
  • Shoot in RAW
  • Manual exposure mode
  • Set white balance to daylight
  • Disabled all noise reduction (including Long Exposure NR)
  • Shoot one exposure at each whole stop ISO (200, 400, 800, 1600, 3200, 6400)

The following is using my Lumix GH4 set-up on a tripod in the back garden. Manual-mode, WB set to Daylight, 1 second shutter speed, aperture at @ f/2, and the Voigtlander 17.5mm in front of it all.

I then ISO incremented by full stops, starting with ISO6400. Gradually reducing by full-ISO-stops, but keeping the same shutter speed and aperture.
Oh, and no noise-reduction reduction in camera. RAW files were imported to LR with no sharpening or NR applied, but all exposures normalised. So ISO 3200 image increased by +1EV to get to same exposure as ISO 6400 etc.



Picture info: Lumix GH4, Voigtlander f/0.95 17.5mm, ISO200-6400, f/2, 1 sec
Best viewed full-screen

So, what does it say? Well, to me there is no discernible difference between 6400 and 3200 (+1EV). And I don't think there's too much difference between ISO3200 (+1EV) and ISO1600 (+2EV).
But it gets very noisy after that. ISO400 has a weird colour cast for some reason (bare in mind same white-balance throughout). I think I'd like to reshoot this, just to make sure that the ISO400 result isn't just a blip. 
The only difference between my test and theirs was my starting at ISO6400 as the base. I believe in the Petapixel article, they use ISO3200 as the base, then apply a -1 exposure compensation.

Looking at the image above, I think I'd be happy shooting astrophotography at 1600 or 3200. Anything lower looks like it will increase noise horribly. For low-light shots around town, I'll remain happy shooting at 800 - I don't tend to boost the exposure much for street, just a slight adjustment to shadows (and a lot of that noise can be mitigated through masking).
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