Hasselblad True Exposure - why over exposure compensation?

[mar-ko]

Hi everyone,
digital Hasselblad cameras with leaf shutters have a "true exposure" function, which automatically gives a faster(!) shutter speed (when the lens is stopped down and a fast shutter speed is used) to avoid over exposure.

Stopping down means, that only a small hole for the light to pass is left open by the aperture - and I thought, that the picture may be under exposed, because the shutter would be travelling over large areas the closed aperture (so, although the shutter is "open", no light can pass the areas covered by the stopped down aperture.

But the compensation is not for under- but for over exposure - why is that?
Does the leaf shutter travel a different speeds? There is a PDF from Hasselblad
http://static.hasselblad.com/2015/02/true_exposure_mode.pdf
but I still don't understand - does anyone know of a video or a text to explain to me, why the compensation is for an over exposure?

Thanks, Marko

[Buddy]

the diagram in that pdf pretty much explains it. When stopped down without TE the actual exposure time gets a bit longer (despite the unchanged shutter speed selected e.g. 1/250sec). That is due to the travel time of the shutter blades when opening from the middle of the lens and closing again. Thus the smaller aperture will proportionally longer being fully exposed than a larger aperture setting. TE is set in order to reduce the actual exposure time to the same lenght as it is with the smaller aperture.
se also this:
http://www.hasselbladdigitalforum.com/index.php?topic=5433.0

[mar-ko]

Thank you very much - now I got it: "proportionally longer fully exposed" was the key!