Is the term ISO “totally fake”?

stevelee

FT-QL
Jul 6, 2017
1,239
265
Davidson, NC
+1

I totally agree with you.
I sort of agree. I enjoyed the anticipation waiting a few days for my slides to come in the mail to see my results. I perhaps also could look at the pictures with more objectivity than in the moment.

Now I have a short time of anticipation (except when traveling), the wait between shooting and popping the card into the computer and seeing how the pictures look in Bridge and then in ACR. Looking at pictures on the camera's screen is just a preview. On a long trip, I have looked at pictures on the iPad via Canon software and a wifi connection.
 

Michael Clark

Now we see through a glass, darkly...
Apr 5, 2016
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They are supposed to round up to the nearest 1/3 stop. 321 should be reported as 400. 250 to 400 breaks both the ISO and the industry standard. I wonder if amateur comparisons are done consistency with proper conditions.
I don't think anyone would consider DxO Lab's setup "amateur." They are a bit secretive about their methodology and weighting for the composite scores they give cameras and lenses, but the actual measurements they publish are done under properly controlled conditions.
 

Michael Clark

Now we see through a glass, darkly...
Apr 5, 2016
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"CMOS and CCDs both record the photo-electric charge" - so this is the reason why they are called analog devices? Because charge is a continuous value, not a discrete one (0 or 1).
The sensors in digital cameras are analog devices. That's why the analog measurements they make require an analog-to-digital convertor (ADC) somewhere between the sensels (a/k/a pixel wells, photosites, etc.) and the digital processor.

As we can disregard number-phase uncertainty for photoelectrons, charge is a discrete value (a number of electrons). But counting electrons one by one is too slow for our goals, so the charge is measured by the voltage it creates on a given capacitor, and voltage is a continuous value.
Not really. The exact amount of energy a photon contains varies with the frequency at which it is oscillating. There's no way to measure discrete photons by using the amount of energy absorbed by a piece of silicon, there is only the measurement of the energy that has been released by those photons.

Kit, if i remember well, the exposure meter is sensitive to different colours (measures different values for different colours), but not the sensor. Anyway, if you take photos of a grey-card (18%), this should not matter at all.
That all depends on the exposure meter. Many are monochrome. Just as the earliest emulsions were more sensitive to blue light (and pretty much not sensitive to green-yellow-orange-red light), the earliest light meters were sensitive to the same wavelengths as the current emulsions were. Many current digital cameras have RGB+IR sensitive light meters (DSLRs) if they're not metering directly off the image sensor (MILCs).

At least our modern sensors don't change over time, at least not that I have noticed. Has anyone with a really early camera noticed any changes? We are exposing a silicon surface to light, which does cause changes on the microscopic level. And of course, the shutter could need eventually adjustment, but I haven't had a digital SLR that hasn't warranted upgrading within a short enough period of time (under 10 years so far) such that perhaps there isn't time for deterioration to set in.
Theoretically, the biggest difference would be caused by the Bayer color array fading from exposure to UV, but since the Bayer mask is usually behind the UV filter in the sensor stack, it would take a LOT of exposure to direct sunlight or other sources of UV for this to happen.


Certainly one of the greatest. For me, instant gratification (not having to wait for film developing) would have to be the greatest leap forward of digital photography.
That and the automatic recording of EXIF data without having to stop and write everything down so you can later tell what did, and perhaps more importantly what didn't, work.
 

Kit.

EOS 6D MK II
Apr 25, 2011
1,267
682
Not really. The exact amount of energy a photon contains varies with the frequency at which it is oscillating. There's no way to measure discrete photons by using the amount of energy absorbed by a piece of silicon, there is only the measurement of the energy that has been released by those photons.
The sensor does not measure the energy the absorbed photons release. Most of this energy (for silicon sensors used in photography) is converted into phonons and thus lost with heat.

The sensor measures the number of electrons the absorbed photons excite. Which is usually one per photon, as multi-electron absorption of a photon is a process happening with much lower probability.
 

3kramd5

EOS 5D MK IV
Mar 2, 2012
3,082
404
I don't think anyone would consider DxO Lab's setup "amateur." They are a bit secretive about their methodology and weighting for the composite scores they give cameras and lenses, but the actual measurements they publish are done under properly controlled conditions.
I wasn’t referring to anyone in particular with that comment, merely recognizing that there are myriad outfits offering free test results. Some are undoubtedly better than others when it comes to controls, consistency, and adherence to standards. I could see, for example, some website deciding to round to the nearest 1/3 stop, rather than up as per standard, leading to discrepancies appearing larger.
 
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Michael Clark

Now we see through a glass, darkly...
Apr 5, 2016
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439
The DxOMark data do consistently show that the makers consistently overstate the isos above 100, and understate at iso 50. DxO explains this in https://www.dxomark.com/About/In-depth-measurements/Measurements/ISO-sensitivity where the detail the measuring of iso values. They say manufacturers deliberately overstate the iso to avoid clipping of highlights. But, something smells when different manufacturers have exactly the same deviations from the DxO measurements. Do you know of any other independant measurements?
I've seen quite a few other sources that even measure the 1/3 stop setting (which can be very revealing about how different cameras handle ISO settings between the "whole stop" scale!). They all show the same thing.

This one actually compares their measurements to DxO:

http://www.photonstophotos.net/Charts/Measured_ISO.htm#Canon%20EOS%201DX%20Mark%20II,Nikon%20D5,Pentax%20K-1

There is a lot of validity to the highlight preservation thing. Digital sensors have linear response all the way up to full saturation (clipping). Film is different because as the chemicals in the emulsion react to light, there are fewer unaltered grains left to react to even more light. The reduced ISO sensitivity of digital sensors is only apparent when measuring raw values. The camera's internal processor and external raw processing applications know to "push" the exposure (after gamma curves have been applied to give the highlight response a "shoulder") when converting the raw data to counteract the "pull" of exposing for ISO 400 when the data from the sensor is only being amplified equivalent to ISO 250-300. That's one of several reasons why raw processing applications must have codecs for specific cameras before they can process raw image files from a specific camera: to know specifically for each sensor how much the "push" needs to be at each ISO setting.
 
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Michael Clark

Now we see through a glass, darkly...
Apr 5, 2016
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The sensor does not measure the energy the absorbed photons release. Most of this energy (for silicon sensors used in photography) is converted into phonons and thus lost with heat.

The sensor measures the number of electrons the absorbed photons excite. Which is usually one per photon, as multi-electron absorption of a photon is a process happening with much lower probability.
You are, of course, correct with your more precise description of how photons colliding with a silicon sensor excites electrons. I was trying to keep it simple. But the fact remains, as long as the possibility/probability of multi-electron absorption is there, we can't with 100% confidence say that one electron = one photon. That's an even more oversimplified description than my original comment above.
 

AlanF

Canon 5DSR II
Aug 16, 2012
5,663
2,922
I've seen quite a few other sources that even measure the 1/3 stop setting (which can be very revealing about how different cameras handle ISO settings between the "whole stop" scale!). They all show the same thing.

This one actually compares their measurements to DxO:

http://www.photonstophotos.net/Charts/Measured_ISO.htm#Canon EOS 1DX Mark II,Nikon D5,Pentax K-1

There is a lot of validity to the highlight preservation thing. Digital sensors have linear response all the way up to full saturation (clipping). Film is different because as the chemicals in the emulsion react to light, there are fewer unaltered grains left to react to even more light. The reduced ISO sensitivity of digital sensors is only apparent when measuring raw values. The camera's internal processor and external raw processing applications know to "push" the exposure (after gamma curves have been applied to give the highlight response a "shoulder") when converting the raw data to counteract the "pull" of exposing for ISO 400 when the data from the sensor is only being amplified equivalent to ISO 250-300. That's one of several reasons why raw processing applications must have codecs for specific cameras before they can process raw image files from a specific camera: to know specifically for each sensor how much the "push" needs to be at each ISO setting.
Michael
Thank you. But, photonstophotos is a site I had consulted and replotted data from it in this thread, and the site doesn't say why the DxO measurements differ, merely that they are different.
 

Michael Clark

Now we see through a glass, darkly...
Apr 5, 2016
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Michael
Thank you. But, photonstophotos is a site I had consulted and replotted data from it in this thread, and the site doesn't say why the DxO measurements differ, merely that they are different.
I've also seen talk by coders who look at the codecs of various camera's plugins for darktable or rawtherapee and determine how much the "push" is for each camera at each ISO setting when converting the raw data. It's a very real thing.
 

TAF

EOS RP
Feb 26, 2012
341
30
Certainly one of the greatest. For me, instant gratification (not having to wait for film developing) would have to be the greatest leap forward of digital photography.
Not only the instant gratification, but the cost savings! I've taken 10x as many photos since getting digital (on an annual basis) as I did with film. Since the 'keeper' rate is about the same, that means I have many more photos I really like than I used to get.

I've also captured things I would never have considered before, since I hated to waste film - one might need that frame for something really important later.

And of course there is the 'essentially no limit' to how many shots in a sequence. 36 frames always seemed limiting (and yes, I used those rare 72 frame rolls, and I even had a 250 frame film back for my Canon F-1 - hated that thing, it was so unwieldy)

We are indeed in the golden age of photography.
 

stevelee

FT-QL
Jul 6, 2017
1,239
265
Davidson, NC
It was good for me to have spent years shooting color slides. Everything had to be right. There was a group of classmates in college who insisted on viewing my roll of slides when they came in, before I had seen them or edited anything out. They were both fans of my work and merciless critics. It inspired a level of discipline when I was shooting and a good education.

And money was a factor, of course, especially while I was still in school.
 

Khristo

EOS M50
Jan 14, 2012
46
0
So I played with this a bit yesterday. Set up my 5D3 with a highly contrasty scene - inside looking from darker room areas out to bright blue sky and fluffy white clouds. Set a middling manual exposure of aperture and shutter (can't remember exactly what, but say f8 and 60th), then took seven shots in 1 stop increments from 100 ISO through to 3200 ISO, all without changing the manual exposure settings.
Processed RAW images in Lightroom. Without any processing on import, the 100 ISO was very dark, while the 3200 was very bright. Then adjusted the exposure in LR (+3.0 stops for the 100 ISO, +2 stops for the 200 ISO, and so on to -3.0 for the 3200 ISO).
If the whole system (sensor, plus in-camera processing, plus LR processing) was ISO invariant, each image should have looked the same? Well they sure didn't.
The 100 ISO shot had MASSIVE noise in the pushed dark areas while the 1600 and 3200 ISO shots were not noisy in that area. The sky areas in the 100 ISO shot were nice blue sky and fluffy cloud detail. Same areas in the 3200 ISO shot was totally blown white, and almost as much in the 1600 shot. (White balance was also different at the same colour temp setting.)

After reading this thread and some other info, I had expected to see much less difference. And that would have made sense from the idea that the physical sensor's light gathering capability can't change based on what ISO you dial in. In fact, having seen the results of my simple test, I'm now wondering what the hell is going on in-camera to get this difference - even wondering if the camera can evaluate the scene, compare with your Av Tv and ISO settings and maybe adjust the settings a bit (without telling you or your RAW file) on the basis of an assumption of what you're trying to achieve. Seems unlikely, but I really am sceptical that such a difference would be evident just from the difference between the analogue and digital amplification steps.

Anyway, upshot is, if you got a 5D3, don't think you can set any 'ol number on the ISO dial and fix it later. Not quite sure how, but there's a big difference.
 

AlanF

Canon 5DSR II
Aug 16, 2012
5,663
2,922
You missed some key points in the thread. Firstly, the 5DIII doesn’t reach isoinvariance until about iso640 and its DR is relatively flat between 100-640 so pushing dark areas at iso100 is bound to be noisy. Secondly, setting the iso sets-where you begin to clip the highlights so if you have too high an iso you will blow the highlights. What the discussion should have told you is that if your iso setting is in the isoinvariant region then setting too low an iso in RAW, ie you are under exposing, the underexposure can be corrected over quite a range in post processing and you end up withe same noise and DR as you would have had if you had set the “correct” higher iso. For example, if the correct settings should be iso3200 and 1/200s, then setting manually iso1600 and 1/200 and increasing exposure by 1ev or setting at iso800 and 1/200s and increasing by 2ev in Photoshop etc should give very similar DR and noise to what you would have had at iso3200. You can’t do it the other way around by overexposing and decreasing exposure in post.
 

Michael Clark

Now we see through a glass, darkly...
Apr 5, 2016
918
439
So I played with this a bit yesterday. Set up my 5D3 with a highly contrasty scene - inside looking from darker room areas out to bright blue sky and fluffy white clouds. Set a middling manual exposure of aperture and shutter (can't remember exactly what, but say f8 and 60th), then took seven shots in 1 stop increments from 100 ISO through to 3200 ISO, all without changing the manual exposure settings.
Processed RAW images in Lightroom. Without any processing on import, the 100 ISO was very dark, while the 3200 was very bright. Then adjusted the exposure in LR (+3.0 stops for the 100 ISO, +2 stops for the 200 ISO, and so on to -3.0 for the 3200 ISO).
If the whole system (sensor, plus in-camera processing, plus LR processing) was ISO invariant, each image should have looked the same? Well they sure didn't.
The 100 ISO shot had MASSIVE noise in the pushed dark areas while the 1600 and 3200 ISO shots were not noisy in that area. The sky areas in the 100 ISO shot were nice blue sky and fluffy cloud detail. Same areas in the 3200 ISO shot was totally blown white, and almost as much in the 1600 shot. (White balance was also different at the same colour temp setting.)

After reading this thread and some other info, I had expected to see much less difference. And that would have made sense from the idea that the physical sensor's light gathering capability can't change based on what ISO you dial in. In fact, having seen the results of my simple test, I'm now wondering what the hell is going on in-camera to get this difference - even wondering if the camera can evaluate the scene, compare with your Av Tv and ISO settings and maybe adjust the settings a bit (without telling you or your RAW file) on the basis of an assumption of what you're trying to achieve. Seems unlikely, but I really am sceptical that such a difference would be evident just from the difference between the analogue and digital amplification steps.

Anyway, upshot is, if you got a 5D3, don't think you can set any 'ol number on the ISO dial and fix it later. Not quite sure how, but there's a big difference.
The difference is that dynamic range is always relative, never absolute. It's saying the same thing in a different way as when we say "noise": what we really mean is the Signal-to-Noise Ratio. You can have three times as much noise in an image, but if you have nine times the signal (and have not blown the highlights) then you have a cleaner image.

When you underexpose by three stops at ISO 100, you're packing all of the dynamic range of the scene into the lowest 8th of the camera's dynamic range without reducing the camera's fixed pattern and read noise. When you overexpose by three stops, you're placing all of the camera's dynamic range in the dimmest parts of the scene.