>>3601764>I measured the front element. It seems to be around 29mm diameter, this would corrospond to f1.4 or so.That's not completely correct, the aperture is calculated based on entrance pupil, and not the physical diameter of the aperture. Those two coincide only on pinhole "lenses". Check picrelated, bottom part and the grey bars. In general, in normal focal lengths (~42mm) and with fairly symmetric designs (double gauss etc.), and by placing the aperture roughly in the middle of the (symmetric) groups, those two are kinda close.
Simply said, the lens elemente between the front of the lens and the aperture, depending on how much they "bend" the rays, can change the entrance pupil and "magnify" it (or reduce it).
>as a sidenote that's how constant zoom lenses work, i.e. as you zoom in, the front groups "rearrange" so the magnify the entrance pupil proportionately to the focal length increase, to keep the aperture constant.So yeah it's entirely possible that an even smaller than 23.5mm physical aperture pupil is enough to give you f/1.7. Or it's possible that you need the whole 29mm diameter to achieve an entrance pupil that gives f/1.7.
In practice, most of the fixed lens rangefinders I've used, have a slightly visible aperture when wide open, as you look into the lens up close.
As another anon said, the only way to know without digging up the lens diagrams and running the calculations, is to shoot wide open. If there are visible polygon shapes in your bokehballs, the aperture didn't open fully. If the bokehballs don't have polygon shapes, the aperture is fully open in a sense that no rays are blocked by it.
