'ISO-free' test: How far can the A7 files be pushed?

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Nordstjernen
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'ISO-free' test: How far can the A7 files be pushed?
3 months ago

Played around with my A7 last night, and made some exposures from ISO 100 to 25600 just to find out how much the files can be pushed to keep highlight detail and still keep acceptable shadow detail. This is essential for high dynamic range/high contrast scenes when not usiong HDR technique. I did this to find out if I can handle the A7 as an "ISO-free" camera or not.

I did expose at ISO 100 wit the same shutter speed and aperture as correctly exposed ISO 400, ISO 1600, ISO 6400 and ISO 25600 files, like this:

  • ISO 100: 2 sec at f:8
  • ISO 400: 1/2 sec at f:8 and ISO 100 with the same exposure (-2 stop)
  • ISO 1600: 1/8 sec at f:8 and ISO 100 with the same exposure (-4 stop)
  • ISO 6400: 1/30 sec at f:8 and ISO 100 with the same exposure (-6 stop)
  • ISO 25600: 1/125 sec at f:8 and ISO 100 with the same exposure (-8 stop)

From my finding the exposure can be pushed about 4-6 stops from ISO 100 and still keep good shadow detail. But there is obviously going on a lot of noise reduction and color adjustment before the raw files are written, and also some shadow detail is lots at low ISO. So the sensor for the A7 can probably not be said to be truly "ISO-free - anyway, Sony is doing a phenomenal job with noise reduction and color correction before raw with the high ISO files!

What is impressive though (and expected) is that the pushed ISO 100 files contain almost the same shadow detail as the high ISO files, shown in the extremely pushed examples below (pushed way beond ISO 25600). The pushed files from ISO 100 are much more muddy than the pushed ISO 6400 samples, due to on-chip high ISO noise reduction and obviously also a lot of color corretion. Still the shadow detail is present from the ISO 100 files, which looks completely black when opened in the raw converter (Photoshop CS/ACR).

So what to conclude? The best strategy is to use the needed ISO settings for the A7 to get the cleanest possible shadow data. If I need extra dynamic range to keep maximum highlight detail, like street views at eveninng/night and astrophotography, I can underexpose 3-4 stops from a given ISO setting and then push the files at post processing. But for scenes with normal contrast, I would highly recommend using the appropriate ISO setting and expose so that the highlights are kept, which in most cases should be "correct" exposure.

Here are some example from the ISO 6400 exposure, compared to a pushed ISO 100 exposure (same shutter speed and aperture for both, 1/30 sec at f:8 - the ISO 100 file looked black when opened with the raw converter):

Exposure 1/30 sec at f:8: Guess if this is ISO 6400 or pushed from a heavily underexposed ISO 100 file ...

... or if this is the ISO 6400 exposure or the ISO pushed 100 exposure?

100% crops from correct exposed ISO 6400 file to the left and pushed ISO 100 files to the right. The pushed files are a bit more muddy in the shadows with smeared detail, much more noise and a red color cast.

Extremely pushed files - way beond ISO 25600! 100% crops from the ISO 6400 file to the left and the insanely pushed ISO 100 files to the right. Almost all shadow detail is present in the ISO 100 files but noise and color cast is hard to deal with at processing!

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