"Equivalence" demonstrated: Canon 5D and Panasonic GX1

Started Apr 27, 2013 | Discussions thread
JosephScha Veteran Member • Posts: 5,225
What you have demonstrated is not what they mean by equivalence

I spent a considerable amount of time reading several long posts here and one huge web page on this topic, last night. I am not up to explaining it in one short post. But I can give you some ways that it differs from what you are thinking - which is exactly the way I was thinking.

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.
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.

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.

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.
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.

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