It would be good to see the histogram of an image with multiple ISO using the same x axis scale.
The histograms in the screenshots I posted above do that. They’re bog-linear, so everything’s bunched to the left, but there’s enough space to show the differences…
I know yours are, but I was referring to the OP images. I dont know what FastRawViewer does relative to scale.
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Ah, got it. I haven’t used it in a long while, but IIRC FRV will do either linear or log x.
Hello,
considering how a CCD works, there is no such thing as an amplification. The CCD detects the photons according to its quantum efficiency. No way to change that electronically (in contrast to e.g. a photomultiplier). What can be changed by the electronics, is the conversion factor from photons to digital numbers. This is equivalent to scaling during image processing.
This is, what I know from astronomical CCD cameras. I would be astonished, if the basic principle does not hold also for digital cameras. Thus I would assume, that changing the ISO setting, just changes the conversion factor from detected photons to the digital number stored in the raw image.
Hermann-Josef
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As I said, I don’t want to share the images that prompted this post, but here is a set of three raws in an exposure bracketed trio that I just took out my front door. They were shot at f8 and 1/125 and the three ISOs are 250, 125, and 64. Open these in Rawtherapee with a neutral profile and they are identical (for all practical purposes).
EM5B2887.ORF (18.3 MB)
EM5B2888.ORF (18.3 MB)
EM5B2889.ORF (18.3 MB)
Well, there is a conversiongain from electrons in the pixel-well to a voltage. After you have a Voltage, this may again be amplified before AD conversion happens. So there are indeed analog gain stages involved.
Also I vaguely remember that CMOS is not CCD. You don’t create static charges, but a photocurrent in the phototransistor (which is the pixel element). There is a transimpedance amplifier in there somewhere before AD conversion happens. There might be avalanching as well, but I can’t remember if that is by design or unwanted.
(Avalanche photodiodes are used for single-photon counting and operate almost like a photomultiplier just as a semiconductor analogue.)
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I was more interested in screenshots of what FRV shows for the files than the actual images. Can you produce the histograms?
Haven’t looked, but RT camconst.json may have per-iso white point, which may scale the data so they all look the same. It’s why I moved the display check off the black white point tool in rawproc, does a similar thing…
Yes, but in terms of signal-to-noise, it is the quantum efficiency of the CCD (or CMOS) that counts and this is the number of photons detected, whatever you do with them afterwards.
Hermann-Josef
Open these in Rawtherapee with a neutral profile and they are identical (for all practical purposes).
EM5B2887.ORF (18.3 MB)
EM5B2888.ORF (18.3 MB)
EM5B2889.ORF (18.3 MB)
I don’t know RT well enough, so I used dcraw:
%DCRAW% -v -w -6 -T -O e7.tiff EM5B2887.ORF
%DCRAW% -v -w -6 -T -O e8.tiff EM5B2888.ORF
%DCRAW% -v -w -6 -T -O e9.tiff EM5B2889.ORF
The three result images (e7.tiff, e8.tif, e9.tiff) are identical. Nor merely “for all practical purposes”, but every pixel is exactly identical.
I find this suspicious. Perhaps your tripod is rock solid, and there was no wind, not even the shiver of a single leaf. Perhaps the camera did take three photos at fractionally different times and nothing changed.
But it looks to me that the camera took one exposure and made three files, with the only difference being the ISO.
They are all the same in darktable. Every pixel is the same, so something seems strange.
No tripod. This was a hand held snapshot.
But clearly if the difference between the three shots is ISO and that ISO is something the camera does after digitization there is no need for separate exposures. The three images are produced essentially instantaneously; I can’t sense three images being taken
@snibgo
It sounds as if OP’s camera behaves like Fuji’s X-T4:
Select a bracketing amount (±1/3, ±2/3, or ±1). Each time the shutter is released, the camera will take a picture at the current sensitivity and process it to create two additional copies, one with sensitivity raised and the other with sensitivity lowered by the selected amount.
Have fun!
Claes in Lund, Sweden
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So it appears there is no difference in current at the sensors between the ISOs because it is only one capture from the camera.
OP: I think you will get different results if you manually take 3 images at each of the iso, keeping S and A constant (and trying to keep the scene lighting constant).
The DateTimeOriginal tag of all three images is exactly the same, 2023:02:01 13:00:29. What gives?
Oh, never mind, you bracketed ISO…
Still, doesn’t make sense. Why even have the dial on the camera if it doesn’t do what it’s supposed to… ???
Sure, that’s the limiting best case. I meant: real world amplifier electronics is part of the signal chain. And that is the part that has improved in the last 10 to 15 years so much so that you can almost do photon counting with individual pixels on consumer Si-CMOS. The analog circuits after photoconversion apply gain to the signal AND contribute noise. So, not looking at those amplifier stages seemed like an omission. That was the reason I wrote what I wrote. But yes of course, photon counting has \sqrt n noise.
Err, it’s a little bit more complicated than that. Please refer to the Emil Martinec web site above (all of it). You need quite a bit of signal for the pure photon shot noise approximation to be valid.
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The Olympus E-M5 Mark III Instruction Manual at Download Olympus E-M5 Mark III Instruction Manual | ManualsLib , page 133, says:
ISO BKT (ISO bracketing)
The camera varies the sensitivity over three shots while keeping
the shutter speed and aperture fixed. You can select the
bracketing increment from 0.3 EV, 0.7 EV, and 1.0 EV. Each
time the shutter button is pressed, the camera shoots three
frames with the set sensitivity (or if auto sensitivity is selected,
the optimal sensitivity setting) on the first shot, negative
modification on the second shot, and positive modification on
the third shot.
Their words “shots” and “shoots” implies to me that the shutter opens three times. But the camera was hand-held, so I suggest the manual is wrong or misleading, and the shutter opened only once, and saved three files.
I suggest the experiment is repeated. Take a photo at a certain ISO, aperture and shutter speed. Alter the ISO and take another photo. Alter the ISO again and take third photo. Develop these in a very simple way. I think the three resulting images will show different lightnesses.
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