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):
My questions are:
- 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?
- My really basic academic studies of CCD and CMOS sensors have defined the single pixel signal power as:
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?
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.
- 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.