About high ISO: I can't get my head around this completely ;-)

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deednets Forum Pro • Posts: 11,595
About high ISO: I can't get my head around this completely ;-)

I must admit that I don't always follow when people talk about "dual gain" ISO invariance, ISO-less sensors etc. Meaning I kind of understand, but then I don't. And here is a little question I can't get my head around:

I currently use the A7C with some lenses. My first digital camera was a Mavica (yeah, the one with the floppy drive) and have used RX1s (the lot!) plus the A7III (didn't like it!). When I tried some high ISO shooting I felt like there was a lot of noise in those shots. My theory was that since the noise appeared mainly in the dark part, the actual noise was more like 2-stops under. Again my theory was that if I had added 2 stops to the dark parts then I would have effectively shot at ISO 51.200. A bit like shooting at ISO 100 and then compensating for the rather dark picture in post, instead of using ISO 51.200 in the first place.

Not sure how well those SONY sensors follow my theory here.

And then I failed to notice yesterday arvo that an ND1000 was still sitting on the lens. Since I only used the EVF I actually didn't notice. Maybe I would have noticed on the A7IV or A1 but on the A7C I didn't.

When I then ran the images through Capture One I noticed some noise in the shadows on some shots. And that's when I noticed that the camera had been shooting at ISO 12.800. Here is a shot, for simplicity reduced to 1800px at the long end:

ISO 12.800

And ND1000 means a 10-stop loss of light, if I remember correctly, so why is this shot then so clean? When I tried this on the A7III in a "proper" night scene, the noise wasn't nearly as nice:

ISO 12.800

So here my question: is there some voodoo in all this? I mean when you shoot a proper daylight scene with an ND filter taking you to ISO 12.800 is that any different from taking a shot of a darkish scene also at ISO 12.800?

My first guess was that the nightshot was underexposed. And it was. By 0.7 EV. SO I checked ISO 8.000 on that evening shot:

ISO 8.000

and then - and why not - ISO 3.200:

ISO 3.200

Still, no cigar. ISO 100 maybe?

ISO 100

Still, not quite the same clean-ish shot when compared to the daylight shot. The histogram is flatter in the dog shot, but sits firmly on the left.

Is there maybe more to all this that I am simply not seeing?



 deednets's gear list:deednets's gear list
Sony RX1R II Fujifilm X-Pro3 Sony a7C Fujifilm XF 35mm F1.4 R Fujifilm XF 14mm F2.8 R +9 more
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