"Equivalence" demonstrated: Canon 5D and Panasonic GX1

Started Apr 27, 2013 | Discussions thread
Yohan Pamudji Senior Member • Posts: 2,874
One last stab


I'm going to take one last stab at this in the hope that there's some common ground to be had.

Ignore all the physics mumbo jumbo for now.  None of that "total light" vs. "light per unit of area", etc.  They're important in a way and govern the principles that we live by when making photographs, but it's not necessary to understand all the intricate physics and math to get the concept of equivalence.

The basic idea behind equivalence is simply this question: "What does it take to make the same photograph on 2 different sensor sizes?"  That is the heart of the matter.  What are the basic characteristics of a photograph?  I don't think it's radical to say that these are the basics:

Depth of field
Motion blur or lack thereof
Color or lack thereof

Notice that none of those are actual in-camera settings.  That is, we can set our camera to get certain effects in each category but the direct in-camera settings aren't what we're trying to equalize.  In fact sensor size differences force us to use different in-camera settings to achieve the particular photographic look we're after.

What you are talking about is using the same exposure settings on 2 different formats--aperture, shutter, ISO--which will result in the same brightness but definitely not the same image because DOF is different.  While it is worth knowing that using the same exposure settings on different formats will result in the same brightness, I would hope that's something so basic that it's well-understood.  What equivalence addresses is the fact that the same exposure settings don't actually give you the same photograph if you're comparing 2 photos taken with different sensor sizes.  And isn't DOF a crucial element in a photo?  That's not a radical or narrow definition, is it?

That's really all there is to it.  Once you agree with those statements you can then start unpacking the rest of the argument, i.e.:

  • What aperture do you have to use to get the same DOF on different sensor sizes?
  • When you change the aperture to get the same DOF how do you have to change the ISO to maintain the same brightness?
  • And finally when you change the ISO are you still getting the same amount of noise in the photo and how does the higher/lower ISO affect color fidelity, etc.?

That's the thought progression.  Hope that helps.

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