These equivalence debates are stupid - equivalence is trivial.
There are two primary defining characteristics of a lens-camera system - the angle of view the sensor sees and the size of the hole through which the sensor is looking, called the "aperture", not to be confused with the f-stop.
Most people don't easily think in terms of angle-of-view, instead preferring to combine two measurements - sensor size and lens focal length - to describe angle-of-view. That's fine, as it fully describes the angle-of-view (AOV = 2*arctan(sensor size/(focal length*2))).
Equivalence is just about comparing the results from two or more different lens-camera systems on an equal basis by holding these two parameters - angle-of-view and aperture - constant or comparing how different they are between the two systems.
Simple.
To keep these two parameters (angle-of-view and aperture) constant, you do exactly what you do when you add a teleconverter to an existing lens-camera system - multiply both the lens' focal length and f-stop by the teleconverter multiplication ratio. You do the same with cropping, either by reducing sensor size ("crop factor") or by cropping in post processing.
If you are multiplying one or the other but not both, you are doing it wrong. You have to do both or neither. If you only do one, you are changing either angle-of-view or aperture, which is not a fair comparison.
Note that we are not talking about exposure here, just about comparing the imaging results from two or more different lens-camera systems shooting in the same conditions with the same shutter period. Exposure (the multiplication of illuminance and time) can change between the systems, as long as aperture and shutter speed are held the same.
There are two primary defining characteristics of a lens-camera system - the angle of view the sensor sees and the size of the hole through which the sensor is looking, called the "aperture", not to be confused with the f-stop.
Most people don't easily think in terms of angle-of-view, instead preferring to combine two measurements - sensor size and lens focal length - to describe angle-of-view. That's fine, as it fully describes the angle-of-view (AOV = 2*arctan(sensor size/(focal length*2))).
Equivalence is just about comparing the results from two or more different lens-camera systems on an equal basis by holding these two parameters - angle-of-view and aperture - constant or comparing how different they are between the two systems.
Simple.
To keep these two parameters (angle-of-view and aperture) constant, you do exactly what you do when you add a teleconverter to an existing lens-camera system - multiply both the lens' focal length and f-stop by the teleconverter multiplication ratio. You do the same with cropping, either by reducing sensor size ("crop factor") or by cropping in post processing.
If you are multiplying one or the other but not both, you are doing it wrong. You have to do both or neither. If you only do one, you are changing either angle-of-view or aperture, which is not a fair comparison.
Note that we are not talking about exposure here, just about comparing the imaging results from two or more different lens-camera systems shooting in the same conditions with the same shutter period. Exposure (the multiplication of illuminance and time) can change between the systems, as long as aperture and shutter speed are held the same.
