All this talk about "equivalence" is silly

Started Jun 10, 2011 | Discussions thread
Denabears Regular Member • Posts: 122
once upon a time ...

... some photons were travelling from a subject on their way to becoming a photograph.

Now all of the photons came from the subject, but to gain immortality by making it into the photo which the photographer visualised hanging on her wall filling a space that was just crying out for an image, they had a huge challenge ahead: only those that could make it through the hole within the lens (entrance pupil) had a chance of reaching their ultimate goal, the dark and mysterious pixelated sensor.

There was another challenge too -- life is so very very short and hard for photons -- only the photons that were fortunate enough to arrive within the short interval of time called the exposure time would make it through. The rest would cease to exist in the blackness of the shutter curtains having come too early or fatally too late.

Unknown to the photons (who don't have brains) the photographer had not only selected the angle of view and the distance from which to take the picture, but had also chosen the size of the hole, and preselected the fate of the photons. Choosing between the lucky ones that would make it to the sensor, and those that would miss the entrance aperture and fall into the outer darkness to have their stories lost forever.

Although the photographer probably did not think of it, the angle of view was set by the subject size and distance. The focal length of the lens was then determined by the area of the sensor that was chosen to take the photo. The last important variable, in setting the fate of the photons, was the size of that hole. Now the photographer, following habit, expressed that in her thoughts as a fraction of the focal length, but the absolute size was of more direct interest to the photons.

Of course they found out too late to change their fate which was sealed from the start -- everything is too late for a photon ...

The successful photons carried the information for the image, and how that image became a photograph is another story, involving the sensor performance (but not its size, already fixed), and how the photographer chose to turn the recorded data into a print (of just the right size to fit in that space on the wall).

In another place, a long long way away, some smart photographers understood that, if the angle of view, and subject distance are fixed then this first part of the story is the same (equivalent) for any camera with any sensor size if and only if the hole is the same absolute size, and open for the same time in exactly the same place (in case there is doubt, remember that the angle of view was fixed at the start - the lens does its job). They know this because it lets the very same photons through to take their information to the sensor, and live happily ever after within the recorded image. (Display size affects how we see the photograph, the other 4 factors of equivalence are involved in its formation, the intervening part of the tale affects noise, contrast, colour etc.)

Now most photographers don't need to know this, but the sad part of the bigger story is that some people deny reality for whatever reason or motive.

Perhaps anthropomorphising can be taken a step too far ... and perhaps the equivalence essay is a better read after all.

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