# Do you live in the Wonderland?

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Do you live in the Wonderland?
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“When I use a word,” Humpty Dumpty said, in rather a scornful tone, “it means just what I choose it to mean—neither more nor less."  L.Carrol Alice in the Wonderland

Equivalence is binary relation, which is reflective, symetrical and transitive. Wikipedia.com

Equivalent photos as opposed to "equal" photos, are photos that have the same perspective, framing, DOF, shutter speed, lightness, and display dimensions.

Equivalent lenses, then, are lenses on different formats that produce Equivalent images (same angle of view and same aperture diameter).  Josephjames.com essay Equivalece

The question is,” said Alice, “whether you can make words mean so many different things.”

Let's check it.

Definition of Equivalence - correct, pure math.

Equivalent photos - clean, pure and correct. (Thank you JJ)

Equivalent lenses - looks good, but I need to check it.

So, Canon FF lens 400/5.6 is equivalent to PL MFT 200/2.8 lens? For every photo I can take with the Canon lens is it possible to get equivalent image with the Panasonic lens? Yes, of course. And vice versa?

No, there are some photos (perspectives) with camera to subject distances between 115cm and 350cm I can take with the Panasonic lens but not at all with the Canon lens. Hint, both lenses have very different Minimum Focus Distances. In reality MFD is closely related to physical FL, the longer FL the longer MFD, and vice versa.

Result:  the definition of Equivalent lenses is incorrect. The relation defined is not symetric.

It is a very strong and useful relation, but definitely not Equivalence. And unfortunately it can't be repaired with inclusion of requirement for same perspectives,  because of the real world dependence of MFDs on FLs.

If you don't require symetricity of the Equivalent lenses relation then:

1) the above mentioned lenses are equivalent, but then

2) the PL MFT lens 200/2.8 is equivalent to a Canon FF lens 400/2.8 as well.

It is up to you where do you want to live - in here, or the Wonderland.

“The question is,” said Humpty Dumpty, “which is to be master—that's all.”

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PS. One of the goals of the Equivalence theory is to help people to choose right equipment for their needs. It does help, but only to some extent.

Peter

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