You are correct, I repeated myself too many times. I'll try not to repeat myself anymore on this thread.
I don't know what you mean by intensity of light.
It is correct that for a given subject, under given lighting condition, the wider the aperture is, the more light will hit the media. This is true for both film and digital, both for the entire sensor and for each single photosite.
All the above is correct.
In order to attain less noise (in digital cameras), or less grain (in film cameras), we want to use lower ISO values. This is true for both film and digital cameras.
In order to use lower ISO values, we want enough amount of light to hit the sensor - this is true both for the entire sensor and for each and every photosite in the sensor.
Not exactly, not the way you put it.
The amount of light hitting the sensor and exposure are directly related, however it has nothing to do with the size of the photosites.
The formula for correct exposure includes neither media size nor grain size (in film) or photosites size (in digital cameras). The photosites size affects noise, but it isn't related to exposure.
The exposure formula relates to the entire size of the media (film or sensor).
Again, the size of the photosites is very important for noise considerations, but it's irrelevant to the exposure.
In order to get it, look at the following example:
We are shooting a movie scene in with certain film stock of, say, 180 ISO. The shutter speed is set to 1/48. The DP measures the light on the subject and the light meter says that f/5.6 (or T5.6) is needed.
This is the aperture (f/5.6) that will give the correct exposure (for the given film stock sensitivity and for the given lighting conditions) on all movie cameras, be it 70mm, or 35mm, or 16mm, or 8mm. Thus, the media size (film or sensor) and the grain size (in film), or pohotosites size (in digital cameras), are irrelevant to the exposure.
Also, the focal length of the lens used and the coverage area of the lens are irrelevant to the exposure.
For a given film stock sensitivity (or sensitivity set in digital camera) - and for a given shutter speed - and for given lighting conditions, the one and only one thing that matters for getting correct exposure is the aperture (f stop or T number) used. The media size, film grain size and photosites size aren't considered at all in determining the correct exposure.
We can look at it it this way:
The more light hitting the sensor (both the entire sensor and each and every photosite), the less ISO sensitivity is needed - and hence the less noise will be there.
There are few points to consider here:
(a) In order to attain correct exposure, for a given sensitivity of the media (film or digital sensor), we need that specific amount of light will hit the sensor - no more, no less.
(b) The amount of light hitting the sensor is being determined by: (1) the amount of light being reflected by the subject (which is being determined by the amount of light falling on the subject and the subject's brightness); (2) the shutter speed; (3) the aperture of the lens (f stop or T number).
(c) Since we are dealing with movies here, changing the shutter speed isn't advised, since it will affect adversely the 'filmic' quality of the image.
(d) For outdoors daylight shots, there is too much light as it is. Since we cannot reduce the ISO sensitivity below certain limit - and since we want to maintain wide-enough aperture (for both shallow enough Depth of Field and to avoid sharpness loss caused by diffraction, which is caused by too small apertures [see here: http://www.luminous-landscape.com/tutorials/understanding-series/u-diffraction.shtml ]) - we want to decrease the amount of light entering the lens by using ND filters.
(e) We get into needing more light to hit the media in night shots - and sometimes also in daytime indoors shots. In such cases - when increasing ISO sensitivity isn't desired due to noise - the two options are: (1) use more artificial lights, when possible; (2) use wider aperture. (However, for a given media size, using wider aperture will decrease the DoF [Depth of Field], which at times may be undesirable).
All the above boils down to this:
There are times (but not always, like not in daytime outdoors shoots) when we want more light to hit the sensor. When wanting to keep the ISO sensitivity low enough, our two main means to get more light hitting the media (film or sensor) are adding artificial lights and/or using wider aperture.
You combined and mixed few things, thus you created confusion.
Aperture size (f stop) does affect exposure. The other points you added only add confusion in attempting to get the issue.
I related to SpeedBooster in previous posts, so I'll not repeat myself.
(A hint, it's due to the formula that calculates the f stops and the fact that SpeedBooster changes one of the two parameters in that formula, without changing the second parameter).
The picture may get clearer if we leave SpeedBooster aside for a minute.
I have few Canon cameras, some are film ones, some are digital FF and some are digital APS-C. All my lenses are EF ones, that is, they can be mounted on all cameras, film ones and digital ones of both sensor sizes.
When I'm about to shoot an indoor scene with unchanging lighting, with 2 cameras: 1 digital FF camera and 1 digital APS-C camera. Setting a given ISO on both cameras, say ISO 180, the very same f number will be used on both cameras in order to attain correct exposure.
Therefore the sensor size is irrelevant to the correct exposure.
The only 3 things relevant to the correct exposure are: (1) the amount of light on the subject; (2) the sensitivity (ISO number) used; (3) the aperture (f number or f stop) used.
You yourself wrote in a previous post in this thread:
The only thing that got you into confusion is SpeedBooster.
There is a possibility that this confusion of yours will get cleared should you consider the formula that calculates f stops.
