"Equivalence" demonstrated: Canon 5D and Panasonic GX1

Started Apr 27, 2013 | Discussions thread
tko Forum Pro • Posts: 12,375
Re: What you have demonstrated is not what they mean by equivalence

1. Equivalence means what they say it means. What is equivalent is perspective (you got that right), framing, depth of field, shutter speed and displayed size. Nothing more.

And noise. You forgot noise. Shutter speed is not included. That's an input variable. Display size is a constant. Noise, DOF, and framing. Only three.

2. It is not about equivalent exposure (light per unit area), it is about total light reaching the sensor. The goal is to measure efficiency of different sensors.

It has nothing to do with efficiency of sensor as the term is commonly used (photons in, electrons out.) It assumes are sensors are approximately equal.

In your pictures you made exposure the same, and (therefore) depth of field was different. That is meaningful, maybe even more intuitive, but it is not what the people you are aiming at mean by equivalence. Once I understood that I was more happy to let them mean whatever they want. I am not yet convinced that thinking that way will be very useful, but it's not totally bogus. They just defined equivalence as they wanted to serve that particular purpose.

Equivalence only has one definition. What settings should you adjust to make the images from two differently size sensors look as much as possible the same. That is where the name--equivalence--comes from.

If you want to introduce your own theory, give it a new name.

While I'm writing:
3. Exposure only means total light hitting the sensor. Everything after that is "brightening". I a really like that, the only problem is general usage. Even the top slider in ACR is labelled "exposure" when it's clearly iso adjust, or brightening.

Actually, I agree with you, but one must be careful in renaming industry terms -- even if they are wrong. For example, most people would think of changing ISO as adjusting exposure, but under your definition it doesn't. The exposure triangle would collapse into the exposure line. Something about letting sleeping dogs lie. The problem is ISO no longer measures sensor light sensitivity. It is how much gain there is afterwards.

4. 4/3 needs two stops more open than FF to have same total light on the sensor.


I'd better stop here. There is tons more to read, some of it quite mathematical, to see where the definition they use for "equivalence" came from and how it can be useful. My intent here is just to ask you to allow them their definition, and if it's not what you care about that is OK.

Mathematics is only useful when you have the basic concepts and big picture down. Equivalence is so simple you can derive all the formulas in a few minutes, so simple you can state the premise in one sentence.

What camera settings make the images look most similar in terms of noise, framing, and DOF?

I find pages of math are frequently used to obscure simple concepts. You don't need complicated math here!

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