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There are many questions about ISO (such as 1 and 2), but none of them answers my precise question. My doubt starts from the exposure formula (for instance let's consider it in linear scale):

enter image description here

My questions are:

  1. There is a unit of measurement misunderstanding in my formula, and I do not understand the solution. As written there,

ISO speed ratings of a digital camera are based on the properties of the sensor and the image processing done in the camera, and are expressed in terms of the luminous exposure H (in lux seconds) arriving at the sensor.

it appears to be that ISO is expressed as lux*seconds.

But if it were, the exposure unit of measurement would be, by the previous formula, (lux*s)^2! What I'm missing?

  1. My really basic academic studies of CCD and CMOS sensors have defined the single pixel signal power as:

enter image description here

Such a signal power corresponds to a lumen value through the Watt-Lumen relationship. Such a signal is multiplied by the gain of the sensor output amplifier. This gain is tunable and it's the quantity we modify when we change the ISO setting?

enter image description here

So, what is exposure in this formula? Is the signal power converted in lumen? It does not appear to be that. As far as I'm concerned, exposure is measured in lux ( = lumen/m^2), not lumen.

  1. To increase the overall confusion, there is the following statement about Base ISO:

The terms "Base ISO" refers to the unamplified sensitivity of the camera.

So, what is the unamplified sensitivity? Let's consider the exposure equation, and let's imagine the Base ISO being 100 (the same value of the reference value). It means ISO/100 = 1 in such an equation. Exposure would equal the subject luminance times the shutter time divided by the f number squared.

The overall sensor sensitivity would appear to be 1, independently on the quantum efficiency and pixel size. It looks like so strange.

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2 Answers 2

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The basic exposure formula is fine/simple... exposure = sensor illuminance x sensitivity ÷ 100.

But digital exposure is a ratiometric value.

First the sensor is given an electrical charge (like a static charge). Then when a photon of visible light hits the sensor it frees an electron from that charge. That electron is then stored in the photodiode (acting as a capacitor). The stored electrons are the conversion gain, and generally described in terms of photo electrons (photons converted into freed electrons). And the photosite's exposure is the conversion gain (voltage) achieved as a ratio of what the photodiode (capacitor) could store at maximum exposure (full well capacity). Strictly in terms of size/area (m²) a larger photosite requires an equally larger conversion gain in order to be considered equally exposed.

So base ISO is sensor fill efficiency (microlenses, color filters, circuitry, etc), combined with the photosite conversion gain requirement, and limited by photosite read (packet) error and noise (switching).

At its' base, 1 photon hitting the sensor results in one freed electron of conversion gain. And since a photosite of 4x area (2x LxW) would theoretically receive 4x as many photons in a given period of time, the larger photosite remains equally charged/exposed; base ISO is effectively 1 irrespective of photosite/sensor area... one would think it should be the same for all sensors.

But it doesn't always work that way. e.g. a smaller photosite's lower conversion gain requirement can result in a lower read error/noise making the smaller photosite effectively more sensitive... that's why many modern sensors are using smaller photosites of dual gain design (secondary capacitors/dual ISO); and with better fill efficiencies (e.g. back side illuminated to avoid the circuitry limitation). And if you can combine lower conversion gain requirements with lower read noise you can generate fewer ADC errors (write noise) and a much higher overall QE (results in useful ISO invariance).

However ISO/exposure is not exactly standardized... especially the Recommended Exposure Index (REI) method used by most DSLR's and larger format cameras. But the recommended exposure (rated ISO) isn't usually more than 1/2 stop offset (reduced) from a more rigid ISO rating method (such as SOS)... the CIPA standard does say that the camera's ISO should result in an exposure similar to the result given by using a handheld/external exposure meter (and those sensitivities/exposures aren't exactly standardized either).

Any analog voltage gain added to the conversion gain voltage prior to ADC conversion is analog ISO amplification above base ISO (or base ISO's in dual gain designs). Digital ISO (after ADC) is also possible, but that could be just about any type of exposure manipulation.

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  • \$\begingroup\$ Thanks for your answer. So, ISO base embodies the pixel sensitivity, and any value above it embodies an additional amplification? If iso base is 100 and I use iso 800, their ratio will be 8. Does this mean the exposure is amplified with gain 3x? \$\endgroup\$
    – Kinka-Byo
    Sep 7, 2022 at 5:40
  • \$\begingroup\$ If iso base = 100, does it mean the sensor output is 100x the input power? Or is it just a conventional number? \$\endgroup\$
    – Kinka-Byo
    Sep 7, 2022 at 5:55
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    \$\begingroup\$ ISO 100 is convention... Light value (LV, subject luminance) is equal to the exposure value (EV) at ISO 100. E.g. some handheld meters measure in LV, and a camera's low light sensitivity is rated in LV. \$\endgroup\$ Sep 7, 2022 at 12:59
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    \$\begingroup\$ ISO 800 should be an 8x amplification of the ISO 100 base voltage (equates to 3 stops more light)... \$\endgroup\$ Sep 7, 2022 at 13:03
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If the base ISO is 100 the digital values are directly converted from analogue signal from sensor. think about schematic like this:

sensor->amplifier->ADC

So in case of base ISO you have no amplification, amplification=1

P.S. Some amplification (multiplication) may be done after the DAC, but this will require DAC with bigger resolution than RAW files of the camera.

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    \$\begingroup\$ @Kinka-Byo, by my understanding they are absent there because they are constant (sensor sensitivity and ADC conversion). \$\endgroup\$ Sep 6, 2022 at 7:35
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    \$\begingroup\$ It makes sense, they are something the photographer cannot focus on or control. So, do you think ISO/ISObase would merely represent the amplification factor (and so not exactly the sensor sensitivity)? \$\endgroup\$
    – Kinka-Byo
    Sep 6, 2022 at 7:59
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    \$\begingroup\$ @Kinka-Byo, yes, the ISO represent amplification factor. The sensitivity of sensor is constant value. (very rough example) 1000 photons = 1mvolt, 1000000 photons = 1 volt. \$\endgroup\$ Sep 6, 2022 at 8:07
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    \$\begingroup\$ @Kinka-Byo, the idea of SE is one question, one or more answers. So will be better if you ask new question. photo.stackexchange.com/questions/ask \$\endgroup\$ Sep 6, 2022 at 16:35
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    \$\begingroup\$ You are absolutely right \$\endgroup\$
    – Kinka-Byo
    Sep 6, 2022 at 16:58

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