the 28mm is a 28mm and remains a 28mm as long as it exists, no matter which camera, which film size or which sensor size it is used with/on.
The important here is the lens angle, which gives the size of the image circle. This produced image circle will (or will not) cover a certain size or a certain sensor size. The size of the film/sensor is important here. And since we are speaking digital, the current size of the Kodak sensors are 36.7x49mm. And this is NOT the same size as the 4.5x6 flim size (41.5x56mm).
Important is the effect of the lens and compare what can be compared: this particular 28mm will have a certain crop (framing) which corresponds to the same crop with a 31mm (according to Willems calculation) on a (film) 4.5x6.0 (exactly 41.5x 56mm).
Thanks Thierry, that's what I was looking for...
The bit about a lens being a certain mm. no matter for what format, I have understood after my first foray into medium and large format many years ago.
It seems where I got confused by HB "marketing speak" is where one starts to compare a 28mm for the 36.7x49 format (22/39mpix chip) with a 35mm for the 41.5x56 format. This is like comparing 35 with 645 and a different format altogether.
If one disregards the image circle bit for the 28, it needs a conversion factor of 1.1 to be compared to all other HC lenses, and to be treated as a 30,8mm when being compared to both 645 and 35mm for easy understading of this lens in relation to other formats.
What is clear to me though, is that this lens provides a 95 degree diagonal viewing angle "on chip" for the 22/39 mpix kodak chips, making it almost directly comparable to a 20mm on a FF canon (94 degree diagonal viewing angle, so maybe more like a 19,5mm on the canon...) if one needs to easily grasp what this lens offers in terms of wide angle. (If this is not true, and HB gives the viewing angle for a 41.5x56 format in its documentation about this lens, then someone at their marketing department need to be fired as it is seriously misleading!)
Anyways, thanks for the clarification, the 28mm does indeed have a 1.1 conversion factor when being compared to other formats using 41.5x56 film/sensor size as the base for the comparison.
(after re-reading this post, I find that my terminology is'nt at its greatest here, but I hope I have understood, and that what I just wrote is understandable )