Joseph James: Equivalence

Started Mar 8, 2011 | Discussions thread
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quadrox
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Joseph James: Equivalence
Mar 8, 2011

I have a question directed mainly towards Joseph, but if anyone else wants to join the discussion I'd be more than happy to hear your thoughts.

The reason I'm creating this topic is that I am trying to wrap my head around the equivalence article that Joseph has posted on his website, and I've been trying for a while now.

The article: http://www.josephjamesphotography.com/equivalence/

In my humble opinion (and meaning no disrespect) josephs article is attacking the problem from the wrong end: The article is starting out with a lot of technical details and explains what they mean for the photographer. I would prefer to start talking about what the Photographer wants to do, and then see how to deal with this from a technical perspective.

I would therefore prefer to start out defining equivalent images in terms of how the image looks, instead of the technical aspects. Perhaps they will end up meaning the same as josephs, but from my current understanding they won't.

I will try my best not to redefine terms for which joseph has already provided a definition, so sometimes my language may be less precise than it should be. If you respond to this post, please be aware of which terms you are using and what their stated definition is.

Visually equivalent images are images which share the same framing, perspective, DOF, and brightness, so that comparing two equivalent images none of them will seem to be darker or brighter than the other.

My understanding is now that between two images that are visually equivalent, as defined above, the image that was taken with a FF body will have less noise than the image taken with a crop body, as it will be exposed with a greater amount of total light. Is this correct?

If the above is not correct, what happens if you take perspective out of the definition, would it be more correct then?

Will there other differences, e.g. sharpness? (I know that e.g. sharpness depends heavily on the lens, but if we assume the current canon lens lineup I think we can make some generic conclusions, no?)

What about diffraction - if you take two visually equivalent images with specifically the 5D II and a 7D, wouldn't you be able to stop down the 5D image more before diffraction kicks in compared with the 7d? Or to put it in other words: For two visually equivalent images taken at apertures so small that diffraction kicks in, will the FF image always be sharper than the crop image?

I imagine the latter is not true, unless we take perspective out of the definition?

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