Schneider Retina lenses in retro position good for APS-C cameras?

Dear community,

I think about buying a Schneider Retina lens (50 mm and/or 28 mm) and use it either directly (or on a bellow) in retro position on my K5.

Does anyone have experience with this strange kind of lens-camera combination?

These lenses are cheap on ebay and they seem to have an extreme sharpness but bokeh seems to be not so nice and in normal (not retro) postion they have a high closest focus distance.

I would like to know if the lens in retro position will cover the APS-C sensor or if I will have strong vignetting and shaddows at the border of each photo.

Best regards

Holger