X-T30 II, ISO settings, and some analysis of the RAW file

[Astigmatism]

I've just had the best time trying to understand what the ISO setting does in my new X-T30 II. ISO numbers are modeled after the old ASA numbers for film, and films were truly different to create their speeds (I was a big fan of Kodak Pan, Plus and Tri X at ASA 32, 125 and 400 IIRC).

But I find references online, in the manual, and in books, that seem to confuse all this.

So I bought RawDigger, which is software that reads RAW files including the ones my camera writes, and lets you do statistics on the pixels. Below is the table of data I generated. Filename is just the photo name, Sample_Name describes the central 600X400 pixel rectangle I did my measurements in, Shutter is the Shutter_Value reported in the EXIF (there's a rounded version of that that is more accessible but apparently less accurate), and for Red, Green and Blue we have an average and a standard deviation. This made it clear that the camera is writing 14 bit pixel values.

I took this series of photos with the Fujinon 80 mm macro lens at f/2.8, in aperture priority mode, letting the camera adjust the shutter speed to make my exposures "right" at every ISO. By "right" they seem to mean about a 14% or 16% gray, at least in the green channel which is the one I paid the most attention to. I had the camera on a tripod with the lens a few inches from a sheet of paper, but the lens was manually focused at infinity to blur any texture in the paper. The paper was taped to the inside of our downstairs sliding glass door where it was backlit by our gray day (which wouldn't have any flicker like our indoor lighting would), and I worked quickly so the light wouldn't change much. I kept shifting the ISO and taking a picture.

 Filename   Id   Sample_Name       ISO Shutter   Ravg    Rdev   Gavg   Gdev    Bavg    Bdev
DSCF0190.raf 1 2823:1885-600x400    80 0.023810 2216.86 37.914 4618.88 57.640 3319.76 47.421
DSCF0191.raf 1 2823:1885-600x400   100 0.019231 2235.03 42.159 4653.14 63.400 3344.33 52.997
DSCF0192.raf 1 2823:1885-600x400   125 0.014286 2207.49 46.383 4595.81 69.240 3302.66 57.713
DSCF0193.raf 1 2823:1885-600x400   160 0.011765 1107.19 25.931 2301.03 37.817 1654.98 31.877
DSCF0194.raf 1 2823:1885-600x400   200 0.009091 1107.62 29.042 2302.50 41.715 1656.32 35.520
DSCF0195.raf 1 2823:1885-600x400   250 0.007143 1104.86 32.353 2297.14 46.321 1652.51 39.415
DSCF0196.raf 1 2823:1885-600x400   320 0.005882 1108.27 36.223 2305.09 51.731 1659.00 44.053
DSCF0197.raf 1 2823:1885-600x400   400 0.004545 1105.60 40.168 2300.77 57.615 1655.81 49.440
DSCF0198.raf 1 2823:1885-600x400   500 0.003571 1100.87 45.328 2303.92 64.488 1655.62 55.007
DSCF0199.raf 1 2823:1885-600x400   640 0.002941 1106.97 50.532 2319.02 72.443 1668.62 61.623
DSCF0200.raf 1 2823:1885-600x400   800 0.002381 1102.26 56.709 2310.84 80.642 1663.07 69.028
DSCF0201.raf 1 2823:1885-600x400  1000 0.002000 1115.02 63.675 2338.81 90.478 1684.51 78.111
DSCF0202.raf 1 2823:1885-600x400  1250 0.001471 1103.31 70.903 2315.34 100.94 1666.91 86.671
DSCF0203.raf 1 2823:1885-600x400  1600 0.001176 1103.12 79.595 2315.34 113.06 1668.75 97.235
DSCF0204.raf 1 2823:1885-600x400  2000 0.001000 1137.55 91.134 2390.45 128.58 1722.92 110.99
DSCF0205.raf 1 2823:1885-600x400  2500 0.000714 1132.56 101.96 2378.89 144.38 1713.78 124.72
DSCF0206.raf 1 2823:1885-600x400  3200 0.000588 1152.40 115.72 2419.68 162.58 1740.69 140.23
DSCF0207.raf 1 2823:1885-600x400  4000 0.000455 1154.66 129.77 2421.64 183.12 1741.71 157.47
DSCF0208.raf 1 2823:1885-600x400  5000 0.000370 1145.31 146.93 2399.44 205.13 1723.30 177.25
DSCF0209.raf 1 2823:1885-600x400  6400 0.000312 1152.15 163.63 2413.90 229.57 1729.91 199.96
DSCF0210.raf 1 2823:1885-600x400  8000 0.000227 1198.36 190.44 2532.11 272.33 1806.28 233.41
DSCF0212.raf 1 2823:1885-600x400 10000 0.000185 1167.54 214.59 2465.34 303.63 1755.25 259.34
DSCF0213.raf 1 2823:1885-600x400 12800 0.000143 1203.58 244.62 2535.27 343.70 1800.83 294.57
DSCF0214.raf 1 2823:1885-600x400 25600 0.000071  774.31 236.88 1594.28 295.63 1159.30 279.98
DSCF0215.raf 1 2823:1885-600x400 51200 0.000036  411.89 177.73  838.00 220.37  614.93 210.30

Here's what I learned:

If by "sensor" we mean the sensor chip that includes analog to digital conversion, then over most of its range the ISO setting does change the sensitivity of the sensor by changing the analog amplification before the digitization. This, coupled with the changing automatic shutter speed (which was either mechanical or electronic at different speeds), kept the raw pixel values at around 14% or 15% of the full scale 2^14. At ISO 80, 100, and 125, which are all specially preceded by an "L" in the display, there's something else going on that I haven't figured out, but it would have overexposed areas that were just 2 stops brighter than my sample area, so not much headroom for typical photography. At ISO 25600 and 51200, which are both specially preceded by an "H" in the display, the exposure fell way off, and I think for ISO 12800 the analog amplifier must have already been at maximum gain, so all they could do is encode someplace that the jpg conversion should bump up the brightness for the two highest ISO settings. Note that over most of the range, ISO numbers go up by around 1.25X per step, but these last two are going up 2X.

It is interesting to see that the noise, or at least the standard deviation as a fraction of the average as a proxy, goes up steadily. In those last couple steps this standard deviation over the mean goes way up.

I might be able to clean this up with a more constant light source (which of course must not flicker), and maybe a frosted glass filter threaded into the front of the lens so there's less texture. All the same, I feel now like I know how to use the ISO -- whereas all the references I had left me nearly clueless what to do.

[jerryy]

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https://www.photonstophotos.net/
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