Exposure control in a raw developer: misnomer?

Started Jan 30, 2013 | Discussions thread
Great Bustard Forum Pro • Posts: 41,727
Re: Density of light

Joofa wrote:

Great Bustard wrote:

http://www.merriam-webster.com/dictionary/density

2: the quantity per unit volume, unit area, or unit length

http://en.wikipedia.org/wiki/Light

In common with all types of EMR, visible light is emitted and absorbed in tiny "packets" called photons

Combining the two, we have exposure:

http://www.josephjamesphotography.com/equivalence/index.htm#exposure

The exposure is the density of light (total light per area  -- photons / mm²) that falls on the sensor during the exposure, which is usually expressed as the product of the illuminance of the sensor and the time the shutter is open (lux · seconds, where 1 lux · second = 4.1 billion photons / mm² for green light  -- 555 nm).  The only factors in the exposure are the scene luminance, t-stop (where the f-ratio is often a good approximation for the t-stop), and the shutter speed (note that neither sensor size nor ISO are factors in exposure).

Seems like there's plenty of "proper qualification" going on, and seems even more clear that the proponents of such terms fully understand what they are trying to say.

Under ordinary setups we don't see photons.

This is a complete non-sequitur.

We only have access to some measure of photoelectrons at best.

Photons fall on the sensor, they release electrons, and the resulting signal is measured.

With the usual digital cameras around there is no reliable way of measuring the variation in photon flux, and hence, effectively the so-called "density of light", unless you want to take an image of a uniform brick wall.

Again, a complete non-sequitur.  Who's talking about "variations in photon flux"?  You're trying to divert to your old tired argument that the photon noise is not accurately described by the square root of the signal.  You've been asked over and over to support that claim, and you never deliver.

However, like I said, it's a non-sequitur.  We are talking about the definition of exposure:

http://www.josephjamesphotography.com/equivalence/index.htm#exposure

The exposure is the density of light (total light per area  -- photons / mm²) that falls on the sensor during the exposure, which is usually expressed as the product of the illuminance of the sensor and the time the shutter is open (lux · seconds, where 1 lux · second = 4.1 billion photons / mm² for green light  -- 555 nm).  The only factors in the exposure are the scene luminance, t-stop (where the f-ratio is often a good approximation for the t-stop), and the shutter speed (note that neither sensor size nor ISO are factors in exposure).

Now that I've swallowed your bait, please go start a new thread where you demonstrate that the photon noise is not accurately described by the square root of the signal, where the signal is measured in electrons.  I'm sure Dr. Martinec would love to read all about how you have proved him wrong:

http://theory.uchicago.edu/~ejm/pix/20d/tests/noise/#shotnoise

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