f-number equivalent between m43 and FF

Started Mar 25, 2014 | Discussions thread
D Cox Forum Pro • Posts: 20,359
Re: Defining The Signal: part 2

Jack Hogan wrote:

Scott McMorrow wrote:

Jack Hogan wrote:

Light from an illuminant is reflected by the scene, travels through/is transformed by the lens and hits the sensing plane forming a 2D image. In this context I believe that the signal is

1) the 2D image projected on the sensing plane - in photometric/radiometric units;
2) the 2D image in 1) filtered, sampled spatially, converted to e- through the photoelectric effect and integrated by the sensor during the time of exposure - in units of e- per pixel of a given channel.

Mean Signal in a specific area of the image is therefore the average of the photometric/radiometric quantity under the given image area or the average of the photoelectrons generated by a sensing element in a given channel in the same area of the image.


Photons are the "signal". Each photon carries information, and is therefore a signal. A camera sensor records those signals on a sensor that integrates photons/unit area over time. A photographic sensor is just one way to measure the signal.

Yes, I am simply trying to create a shared vocabulary for these discussions. I believe (based on relevant textbooks and articles I have read) that when photographers talk about The Signal we are talking about

1) The light intensity at specific points in The 2D Image projected on the sensing plane during exposure

For photographic purposes we can express light intensity in a number of variations of radiometric and photometric units (including W/m^2/nm, lux, photons - of a given wavelength or not - per second per meter squared etc.).

Note that this definition of what The Signal is does not require that The Signal be varying in any way. A blue sky therefore is a perfectly valid Signal because it creates light intensity on the sensing plane.

I also believe that we also often refer to The Signal as being further downstream, at the output of the sensor in a spatially and temporally sampled version of The Image, in units of photoelectrons. Hence the more caustic, discretized extension in 2)

Changes, integrations, confirmations? Do you (the forum at large) agree? More or less, D Cox?

Well, such debates on terminology can be never-ending.

My opinion is that the signal is the photograph. To be more precise, it is the array of numbers in a digital image that provides information about objects and light.

Now, part of that is information about the random arrival of photons. As photographers, we are not very interested in the statistics of how photons behave, so we try to separate the noise from the image signal.

I don't think a single measurement (one pixel, one exposure) should be called a signal. There has to be a set of measurements (different pixels in photography, samples at different times in audio).

So, in "The light intensity at specific points" the important things is that it is plural. The information (the picture) comes from comparing those points: the signal is the differences between the numbers from the pixels. (In audio, it is the differences in the numbers from sample to sample.)

You may not agree.

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