"equivalent" aperture and depth of field

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astroste New Member • Posts: 22
"equivalent" aperture and depth of field

I don't understand the term "equivalent aperture" at all. I think using depth of field (e.g., in meters at f/4) (or even the size of a bokeh ball) will be more objective.

Shouldn't the depth of field depend on the lens design (e.g., image circle size, aperture, depth of field, etc which are related but not in a fixed linear relation). That is, what makes a lens be FF or APS-C (and why not in-between)?

Did I miss something?

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Thought experiment 1: If I have a 35mm f/1.4 full frame lens on a full frame body. After the cropping to the center of the image, the depth of field of the image will not magically change.

Similarly, use the same FF f1.4 lens on an APS-C body. I guess no one will say it becomes a f/2 equivalent.

Thought experiment 2: Get a GFX lens on a Fuji X body with some random mount adapters. What's the "equivalent aperture" now?

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